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Unix Shell Programming is a tutorial aimed at helping Unix and

1. shift command 2nd 3rd paste command d option 2nd s option 2nd examples 2nd 3rd syntax pasting lines paste command 2nd 3rd 4th 5th 6th 7th PATH variable 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th 12th pathnames 2nd pathname pathname full pathnames relative pathnames paths CDPATH variable 2nd 3rd 4th FPATH variable PATH variable 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th 12th 13th pattern matching 2nd 3rd 4th amp amp construct 2nd 3rd 4th asterisk 2nd question mark L construct construct 2nd 3rd 4th any character 2nd 3rd beginning of line 2nd end of line 2nd 3rd grep command l option 2nd n option 2n v option 2n examples 2nd 3rd 4th 5th 6th 7th a a syntax parameter substitution contructs 2nd 3rd 4th precise number of characters 2nd 3rd 4th saving matched characters 2nd 3rd 4th specific characters 2nd 3rd summary of regular expressions 2nd zero or more characters 2nd 3rd 4th percent sign 2nd period 2nd 3rd command 2nd 3rd 4th 5th 6th 7th pathname pathname perl command Perl in a Nutshell permissions execute permissions command files 2nd phone book rolodex program design considerations 2nd entries adding 2nd deleting 2nd 3rd 4th 5th 6th 7th 8th 9th displaying 2nd editing 2nd 3rd listing 2n
2. The read Command General Format read vars This command causes the shell to read a line from standard input and assign successive whitespace delimited words from the line to the variables vars If fewer variables are listed than there are words on the line the excess words are stored in the last variable Specifying just one variable has the effect of reading and assigning an entire line to the variable The exit status of read is zero unless an end of file condition is encountered Examples read hours mins 10 19 echo hours mins 10 19 read num rest 39 East 12th Street New York City 10003 echo num n rest 39 East 12th Street New York City 10003 read line Here is an entire line r echo line Here is an entire line r Note in the final example that any leading whitespace characters get eaten by the shell when read You can change IFS if this poses a problem Also note that backslash characters get interpreted by the shell when you read the line and any that make it through double backslashes will get through as a single backslash get interpreted by echo if you display the value of the variable A r option to read says to not treat a character at the end of a line as line continuation The readonly Command General Format readonly vars This command tells the shell that the listed variables cannot be assigned values These variables may be optionally assigned values on the reado
3. rs ca The set Command The shell s set command is a dual purpose command it s used both to set various shell options as well as to reassign the positional parameters 1 2 and so forth The x Option This option turns on trace mode in the shell It does to the current shell what the command sh x ctype a did for the execution of the ctype program in Chapter 8 Decisions Decisions From the point that the set x command is executed all subsequently executed commands will be printed to standard error by the shell after filename variable and command substitution and I O redirection have been performed The traced commands are preceded by plus signs x set x Set command trace option echo x echo add greetings lu rem rolo add greetings lu rem rolo cmd wc cmd wc ls cmd l ls we l 5 You can turn off trace mode at any time simply by executing set with the x option set x set x ls wc l 5 Back to normal You should note that the trace option is not passed down to subshells But you can trace a subshell s execution either by running the shell with the x option followed by the name of the program to be executed as in sh x rolo or you can insert a set x command inside the file itself In fact you can insert any number of set x and set x commands inside your program to turn trace mode on and off as desired set with No Arguments If you don t give any a
4. An advanced Unix programming book The Unix Programming Environment B W Kernighan and R Pike Prentice Hall 1984 ISBN 013937681X An advanced Linux programming book Advanced Linux Programming M Mitchell J Oldham and A Samuel New Riders Publishing 2001 ISBN 0735710430 Fe previous fl wext 4 rrevious SYMBOL A B C D E F G H 1 J K L M N O P Q R S T U V W X Y Z a Previous 4 rrevious SYMBOL A B C D E F G H 1 CL K L M N O P Q R S T U V W X Y Z logical negation operator 2nd l operator pound sign 2nd 3rd 4th 5th command vi construct dollar sign 2nd 3rd 4th 5th command vi 2nd 3rd variable 2nd 3rd variable 2nd 3rd 4th variable 2nd construct 2nd 3rd 4th 5th 6th 7th 8th 9th variable 2nd 3rd 4th variable variable 2nd 3rd 4th 5th 6th 7th variable 2nd 3rd 4th array i construct parameter construct variable construct 2nd array construct array i construct 2nd n variable parameter pattern construct parameter pattern construct parameter pattern construct parameter value construct parameter value construct parameter value construct parameter value construct parameter colon value construc
5. SYMBOL A B C D E F G H 1 CL K L M N O P Q R S T U V W X Y Z i command vi 2nd 3rd command vi i option sh command I O input output 2nd filters 2nd formatted output conversion specification characters 2nd conversion specification modifiers 2nd printing 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th 12th 13th 14th in line input redirection 2nd 3rd 4th 5th 6th 7th 8th 9th creating archives with 2nd 3rd 4th 5th example 2nd 3rd 4th input redirection 2nd 3rd 4th 5th 6th 7th 8th loops 2nd output redirection 2nd 3rd 4th 5th 6th 7th 8th loops 2nd pipes 2nd 3rd 4th pipeline hookup 2nd standard error 2nd 3rd redirecting writing to 2nd standard input 2nd 3rd 4th closing translating characters from 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th 12th standard output 2nd 3rd 4th closing writing sorted data to 2nd stream editor sed d option 2nd n option 2nd examples 2nd 3rd 4th 5th syntax IDs job_ids 2nd PIDs process ids 2nd PPIDs parent process ids if command 2nd 3rd 4th 5th amp amp construct 2nd 3rd 4th construct 2nd 3rd 4th elif construct 2nd 3rd 4th 5th else construct 2nd 3rd 4th exit status variable 2nd 3rd 4th 5th 6th nonzero values zero values syntax 2nd testing conditions in See test command IFS vari
6. slash _ command vi 2nd 3rd equal sign command vi operator question mark filename substitution 2nd 3rd letter command vi italic letter construct L construct 2nd 3rd 4th brackets filename substitution 2nd 3rd colon null command 2nd colon command 2nd count command vi count command vi count command vi count command vi count command vi count b command vi count B command vi count d motion command vi italic motion count e command vi count E command vi count F command vi Lcount fc command vi italic c Lcount Fc command vi italic c count h command vi count j command vi count k command vi count command vi Lcount rc command vi italic c Lcount tc command vi italic c Lcount Tc command vi italic c count v command vi count w command vi count W command vi count x command vi double left angle bracket redirection operator left angle bracket redirection operator left angle bracket amp notation left angle bracket amp redirection operator left angle bracket amp characters left angle bracket amp redirection operator left angle bracket redirection operator escape character escape cha
7. echo firstchar sed is also often used to edit the value stored in a variable Here it is used to extract the last character from the variable file file exec o Lastchar echo file sed s 1 echo lastchar o The sed command says to replace all the characters on the line with the last one The result of the sed is stored in the variable lastchar The single quotes around the sed command are important because they prevent the shell from messing around with the backslashes would double quotes also have worked Finally command substitutions can be nested Suppose that you want to change every occurrence of the first character in a variable to something else In a previous example firstchar echo filename cut c1 gets the first character from filename but how do we use this character to change every occurrence in filename A two step process is one way filename users steve memos firstchar echo filename cut c1 filename echo filename tr firstchar translate to echo filename users steve memos Or a single nested command substitution can perform the same operation filename users steve memos filename echo filename tr echo filename cut c1 echo filename users steve memos If you have trouble understanding this example compare it to the previous one Note how the firstchar variable in the earlier example is replace
8. ex1 l EX2 The Logical AND Operator a The operator a performs a logical AND of two expressions and returns true only if the two joined expressions are both true So f mailfile a r mailfile returns true if the file specified by mailfile is an ordinary file and is readable by you An extra space was placed around the a operator to aid in the expression s readability and obviously has no effect on its execution The command count ge 0 a count lt 10 will be true if the variable count contains an integer value greater than or equal to zero but less than 10 The a operator has lower precedence than the integer comparison operators and the string and file operators for that matter meaning that the preceding expression gets evaluated as count ge 0 a count lt 10 as you would expect Parentheses Incidentally you can use parentheses in a test expression to alter the order of evaluation just make sure that the parentheses are quoted because they have a special meaning to the shell So to translate the preceding example into a test command you would write count ge 0 a count lt 10 J As is typical spaces must surround the parentheses because test expects to see them as separate arguments The Logical OR Operator o The o operator is similar to the a operator only it forms a logical OR of two expressions That is evaluation of the expression will
9. who root console Jul 8 10 37 barney tty03 Jul 8 12 38 fred tty04 Jul 8 13 40 joanne tty07 Jul 8 09 35 tony tty19 Jul 8 08 30 lulu tty23 Jul 8 09 55 on tony We know he s on tony tty19 Jul 8 08 30 Where did this come from tony is logged on on steve We know he s not on on ann Try this one joanne tty07 Jul 8 09 35 ann is logged on We seem to have uncovered a couple of problems with the program When the specified user is logged on the corresponding line from who s output is also displayed This may not be such a bad thing but the program requirements called for only a message to be displayed and nothing else This line is displayed because not only does grep return an exit status in the pipeline who grep user but it also goes about its normal function of writing any matching lines to standard output even though we re really not interested in that We can dispose of grep s output by redirecting it to the system s garbage can dev null This is a special file on the system that anyone can read from and get an immediate end of file or write to When you write to it the bits go to that great bit bucket in the sky who grep user gt dev null The second problem with on appears when the program is executed with the argument ann Even though ann is not logged on grep matches the characters ann for the user joanne What you need here is a more restrictive pattern specification which you learned how
10. Would 1 2 Be Please Read read c echo case in 1 you like to Look someone up Add someone to the phone book Remove someone from the phone book Change an entry in the phone book List all names and numbers in the phone book Exit this program select one of the above 1 6 c and process selection hoice choice echo Enter name to look up c read name if z name then echo Lookup ignored else lu name fit 2 add 3 echo Enter name to remove c read name if z name then echo Removal ignored else rem name Ti 4 echo Enter name to change c read name if z name then echo Change ignored else change name fi 5 listall 6 exit 0 echo Bad choice a esac done Instead of requiring that the user have a phone book file in his or her home directory the program checks on startup to see whether the variable PHONEBOOK has been set If it has it s assumed that it contains the name of the phone book file If it hasn t it s set to HOME phonebook as the default In either case the program then checks to see whether the file exists and if it doesn t instead of immediately exiting asks the user whether he would like to have an initial file created This was added so that first time users of rolo can have an empty phone book file created for them by the program This version of rolo also has a couple
11. and yet another ENDOFDATA Vv 3 Here the shell fed every line typed into the standard input of wc until it encountered the line containing just ENDOFDATA Inline input redirection is a powerful feature when used inside shell programs It lets you specify the standard input to a command directly in the program thus obviating the need to write it into a separate file first or to use echo to get it into the standard input of the command cat mailmsg mail lt lt END OF DATA Attention Our monthly computer users group meeting will take place on Friday October 4 2002 at 8am in Room 1A 308 Please try to attend END OF DATA To execute this program for all members of the group that are contained in the file users list you could write mailmsg cat users list The shell performs parameter substitution for the redirected input data executes back quoted commands and recognizes the backslash character However any other special characters such as and are ignored If you have dollar signs back quotes or backslashes in these lines that you don t want interpreted by the shell you can precede them with a backslash character Alternatively if you want the shell to leave the input lines completely untouched you can precede the word that follows the lt lt with a backslash cat lt lt FOOBAR gt HOME gt RKKKK gt foobar gt date gt FOOBAR Terminates the input users steve KKK k
12. cdh l 0 users pat 1 tmp 2 users pat Well it worked but now users pat shows up twice in the list One of the exercises at the end of this chapter asks you to remedy this Okay the most useful feature of cdh is the n option which causes it to change the current directory to the one specified in the list cdh usr spool uucppublic cdh l 0 users pat 1 tmp 2 users pat 3 usr spool uucppublic cdh 1 pwd tmp cdh 3 pwd usr spool uucppublic We can make cdh replace our cd command by using the fact that alias lookup is performed before built in commands are executed So if we create a cd alias to cdh we can have an enhanced cd In that case we have to quote every use of cd in the cdh function to prevent recursion cat cdh CDHIST 0 PWD initialize CDHIST 0 alias cd cdh cdh typeset i cdlen i if eq 0 default to HOME with no arguments then set HOME fi A A FR YY cdlen CDHIST case in number of elements in CDHIST l print directory list i 0 while i lt cdlen do printf 3d s n i CDHIST i i i 1 done return 0 9 0 9 19 9 i 1 cd to dir in list remove leading ed S CDHIST i1 33 cd Q esac CDHIST cdlen PWD cdh cd tmp cd l 0 users pat 1 tmp cd usr spool cd l 0 users pat cd to new dir Define cdh function and cd alias 1 t
13. commands to background 2nd 3rd set command 2nd 3rd 4th 5th 6th 7th 8th option 2nd 3rd 4th x option 2nd executing without arguments 2nd reassigning positional parameters with 2nd 3rd sh command x option 2nd 3rd 4th shell 2nd 3rd 4th command execution 2nd 3rd 4th defined entering commands in 2nd 3rd environmental control filename substitution 2nd input redirection 2nd 3rd login shell 2nd 3rd 4th getty program 2nd init program 2nd output redirection 2nd 3rd pipeline hookup 2nd shell programming language 2nd comments 2nd 3rd 4th variables 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th 12th 13th 14th 15th 16th 17th 18th 19th 20th 21st variables See variables starting up trace mode disabling enabling 2nd variables assigning values to shell archive files creating 2nd 3rd 4th 5th shell programming language 2nd comments 2nd 3rd 4th pound sign shell programming languages variables variable 2nd 3rd variable 2nd n variable 2nd variable construct italicize variable 2nd arithmetic expansion 2nd 3rd assigning to other variables assigning values to 2nd displaying values of examples 2nd filename substitution 2nd 3rd naming conventions null values 2nd 3rd positional parameters 2nd 3rd 4th 5th variable substitution 2nd
14. count y motion Same as fc followed by h Same as Fc followed by l Repeat the last f F t or T command Reverse of Enter input mode and enter text after the current character Append text to the end of the line same as a Delete current character through character specified by motion and enter input mode if motion is c the entire line is deleted Delete current character through end of line and enter input mode Same as CC Delete current character through the character specified by motion if motion is d the entire line is deleted Delete current character through the end of line same as d Enter input mode and insert text before the current character Enter input mode and insert text before the first word on the line Place the previous text modification before the cursor Place the previous text modification after the cursor Copy current character through character specified by motion into buffer used by p and P if motion is y the entire line is copied count rc count x count X count count _ letter count v Ctrl l Copy current character through the end of line same as y Enter input mode and overwrite characters on the line Replace the current character with c Delete current character Delete preceding character Repeat the previous text modification command Invert the case of the current character and advance the cursor Append the count
15. dchars where chars is one or more characters that will be used to separate the lines pasted together That is the first character listed in chars will be used to separate lines from the first file that are pasted with lines from the second file the second character listed in chars will be used to separate lines from the second file from lines from the third and so on If there are more files than there are characters listed in chars paste wraps around the list of characters and starts again at the beginning In the simplest form of the d option specifying just a single delimiter character causes that character to be used to separate all pasted fields paste d names addresses numbers Tony 55 23 Vine Street Miami 307 555 5356 Emanuel 39 University Place New York 212 555 3456 Lucy 17 E 25th Street New York 212 555 9959 Ralph 38 Chauncey St Bensonhurst 212 555 7741 Fred 17 E 25th Street New York 212 555 0040 It s always safest to enclose the delimiter characters in single quotes The reason why will be explained shortly The s Option The s option tells paste to paste together lines from the same file not from alternate files If just one file is specified the effect is to merge all the lines from the file together separated by tabs or by the delimiter characters specified with the d option paste s names Paste all lines from names Tony Emanuel Lucy Ralph Fred ls paste d s Paste ls
16. 1 What will be matched by the following regular expressions x 0 9 3 xxt 0 9 3 5 x 1 5 0 9 1 3 0 9 3 x 5 Nans x 10 A Za z_ A Za z 0 9 0 9 A Za z0 9 1 V M1 0 9 Begin lt 9 6 3 18 9 NCA NIG 2 What will be the effect of the following commands who grep mary who grep mary grep Uu nix ch ls l sort 4n sed d text gt text out sed s Uu nix 1 TM g text gt text out date cut c12 16 date cut c5 11 25 sed s 0 9 1 2 1 3 Write the commands to Find all logged in users with usernames of at least four characters Find all users on your system whose user ids are greater than 99 Find the number of users on your system whose user ids are greater than 99 List all the files in your directory in decreasing order of file size 4 Previous NEXT Chapter 5 And Away We Go IN THIS CHAPTER Command Files Variables Built in Integer Arithmetic Exercises Based on the discussions in Chapter 3 What Is the Shell you should realize that whenever you type something like who wc l that you are actually programming in the shell That s because the shell is interpreting the command line recognizing the pipe symbol connecting the output of the first command to the input of the second and initiating execution of both commands In this chapter you ll learn how to write your own commands and how to
17. 9 KILL Kill 14 ALRM Alarm timeout 15 TERM Software termination signal sent by kill by default The shell scans commands when the trap command is encountered and again when one of the listed signals is received This means for example that when the shell encounters the command trap echo count lines processed gt gt LOGFILE exit HUP INT TERM it substitutes the value of count at that point and not when one of the signals is received You can get the value of count substituted when one of the signals is received if you instead enclose the commands in single quotes trap echo count lines processed gt gt LOGFILE exit HUP INT TERM The true Command General Format true This command returns a zero exit status The type Command General Format type commands This command prints information about the indicated commands Examples type troff echo troff is usr bin troff echo is a shell builtin The umask Command General Format umask mask umask sets the default file creation mask to mask Files that are subsequently created are ANDed with this mask to determine the mode of the file umask with no arguments prints the current mask The S option says to produce symbolic output Examples umask Print current mask 0002 No write to others umask 022 No write to group either The unalias Command General Format unalias names The alias s names are removed from the alias list The a option say
18. Finally if a is used in place of a number for width or precision the argument preceding the value to be printed must be a number and will be used as the width or precision respectively If a is used in place of both two integer arguments must precede the value being printed and are used for the width and precision printf s s n 12 test one 10 2 test two test one te printf 12s 10 2s n test one test two test one te As you can see the two printfs in this example produce the same results In the first printf 12 is used as the width for the first string 10 as the width for the second string and 2 as the precision for the second string In the second printf these numbers are specified as part of the conversion specification Table 10 3 summarizes the various conversion specification modifiers Table 10 3 printf Conversion Specification Modifiers Modifier Meaning flags Left justify value Precede integer with or Space Precede positive integer with space character Precede octal integer with 0 hexadecimal integer with Ox or 0X width Minimum width of field means use next argument as width precision Minimum number of digits to display for integers maximum number of characters to display for strings means use next argument as precision Here s a simple example that uses printf to align two columns of numbers from a file cat align Align two columns of numbers w
19. HOWE users steve exported variables PATH usr bin steve bin PSi s l login shell BIN usr2 data bin DATA usr2 data rawdata CDOPATH usr2 data usr2 data rpts exported HOWE usr2 data variables PATH bin usr bin users steve bin usr2 data bin PSi DB 1 RPTS usr2 data rpts l I I i i BIN usr2 data bin DATA usre2 data rawdata COPATH usr2 data usr2 data rpts exported HOME usr2 data sain fuser fbin sh PATH bin usr fbin users steve bin fuser2 data bin FS1 DB APTS usr2 data plus The exec Command After you started up the new shell from db you weren t interested in doing anything further after the shell finished as evidenced by the fact that no commands followed usr bin sh in the program Instead of having db wait around for the new shell to finish you can use the exec command to replace the current program db with the new one usr bin sh The general format of exec is exec program where program is the name of the program to be executed Because the exec ed program replaces the current one there s one less process hanging around also startup time of an exec ed program is quicker due to the way the Unix system executes processes To use exec in the db program you simply replace the last line with exec usr bin sh As noted after this gets executed db will be replaced by usr bin sh This means that it s pointless to have any commands follow the exec because t
20. Stopped SIGTSTP troff memo photo bg 1 troff memo photo amp The preceding sequence is one of the most often used with job control sending a job mistakenly started in the foreground to the background If a job running in the background tries to read from the terminal it is stopped and the message n Stopped SIGTTIN sequence is printed It can then be brought to the foreground with the fg command After entering input to the job it can be stopped with the Ctri z and returned to the background until it again requests input from the terminal Output from a background job normally goes directly to the terminal The command stty tostop causes any background job that attempts to write to the terminal to be stopped and the message n Stopped SIGTTOU sequence to be printed Note that Bash generates slightly different messages than the ones shown here The following shows how job control might be used stty tostop rundb Start up data base program find green red Find green and red objects Ctrl z This may take a while 1 Stopped rundb bg So put it in the background 1 rundb amp Do some other stuff jobs 1 Stopped tty output rundb amp fg Bring back to foreground rundb 1973 Ford Mustang 1975 Chevy Monte Carlo 1976 Ford Granada 1980 Buick Century 1983 Chevy Cavalier 2 find blue Ctrttz 1 Stopped rundb bg 1 rundb amp red green green green
21. String comparison echo 1 False x1 eq 5 Integer comparison echo 0 True x2 10 String comparison echo 1 False x2 eq 10 Integer comparison echo 0 True The first test x1 5 uses the string comparison operator to test whether the two strings are identical They re not because the first string is composed of the three characters 005 and the second the single character 5 In the second test the integer comparison operator eq is used Treating the two values as integers 005 is equal to 5 as verified by the exit status returned by test The third and fourth tests are similar only in this case you can see how even a leading space stored in the variable x2 can influence a test made with a string operator versus one made with an integer operator File Operators Virtually every shell program deals with one or more files For this reason a wide assortment of operators is provided by test to enable you to ask various questions about files Each of these operators is unary in nature meaning that they expect a single argument to follow In all cases this argument is the name of a file and that includes a directory file of course Table 8 3 lists the commonly used file operators Table 8 3 Commonly Used test File Operators Operator Returns TRUE exit status of 0 if d file file is a directory e file file exists f file file is an ordinary file r file
22. archive files creating 2nd 3rd 4th 5th args program 2nd 3rd 4th 5th 6th arguments functions passing 2nd 3rd 4th variable 2nd variable 2nd n variable 2nd phonebook file example 2nd 3rd 4th 5th 6th 7th 8th 9th 10th shift command 2nd 3rd positional parameters 2nd defined 2nd left shifting 2nd 3rd reassigning values to 2nd 3rd arithemetic operators 2nd arithmetic integer arithmetic arithmetic bases 2nd 3rd arithmetic expressions 2nd integer types 2nd arithmetic expansion 2nd 3rd arithmetic expressions 2nd arithmetically sorting files 2nd array i val construct arrays 2nd 3rd 4th 5th 6th 7th 8th assigning elements to retrieving elements from subscripts assigning values to keyword parameters 2nd variable values 2nd 3rd to other variables asterisk 2nd 3rd 4th 5th 6th 7th filename substitution 2nd 3rd 4th 5th asynchronous execution automatically loaded functions 2nd awk command AWK Programming Language The 4 rrevious SYMBOL A B C D E F G H L 1 K L M N O P Q R S T U V W X Y Z b b escape character b b escapecharacte b command vi 2nd b conversion specification character b option set command background executing loops in 2nd sending commands to 2nd 3rd backgroun
23. for file in memo 1 4 do run file done And if you wanted to run all the files in your current directory through run you could type for file in do run file done If the file filelist contains a list of the files that you want to run through run you can type files cat filelist for file in files do run file done to run each of the files or more succinctly for file in cat filelist do run file done If you found that you were using the run program often to process several files at once you could go inside the run program and modify it to allow any number of files to be passed as arguments to the program cat run process files through nroff version 2 for file in do tbl file nroff rom Tlp lp done Recall that the special shell variable stands for all the arguments typed on the command line So if you executed the new version of run by typing run memol memo2 memo3 memo4 the in the for s list would be replaced by the four arguments memol1 memo2 memo3 and memo4 Of course you could also type run memo 1 4 to achieve the same results The a Variable While we re on the subject of let s look at it in a bit more detail We ll write a program called args that displays all the arguments typed on the command line one per line cat args echo Number of arguments passed is for arg in do echo arg done Now to try it args abc Numbe
24. gt users steve wb rwWXr xr x 1 pat DP3822 504 Apr 21 18 30 xtr Note that pat is listed as the owner of symwb and the file type is l which indicates a symbolic link The size of the symbolic link is 15 the file actually contains the string users steve wb but if we attempt to access the contents of the file we are presented with the contents of its symbolic link users steve wb wc symwb 5 9 89 symwb The L option to the Ls command can be used with the l option to get a detailed list of information on the file the symbolic link points to ls Ll total 5 rWXr Xr x 1 pat DP3822 1358 Jan 15 11 01 lcat rWXr Xr x 2 steve DP3725 89 Jun 25 13 30 wb rwWXr xr x 1 pat DP3822 504 Apr 21 18 30 xtr Removing the file that a symbolic link points to invalidates the symbolic link because symbolic links are maintained as filenames although the symbolic link continues to stick around rm users steve wb Assume pat can remove this file ls l total 5 rWXr Xr x 1 pat DP3822 1358 Jan 15 11 01 lcat lrwxr xr x 1 pat DP3822 15 Jul 20 15 22 wb gt users steve wb rwWXr xr x 1 pat DP3822 504 Apr 21 18 30 xtr wc wb Cannot open wb No such file or directory This type of file is called a dangling symbolic link and should be removed unless you have a specific reason to keep it around for example if you intend to replace the removed file One last note before leaving this discussion The Ln command follow
25. shell variables See variables shells Bash compatibility summary 2nd Korn restricted shell rsh 2nd 3rd 4th selecting 2nd shell modes 2nd subshells 2nd 3rd 4th 5th shift command 2nd 3rd 4th 5th signals ignoring 2nd sending to programs signal numbers 2nd 3rd trapping 2nd 3rd 4th 5th 6th 7th 8th single characters matching 2nd 3rd single quote 2nd 3rd 4th 5th 6th 7th sizing command history command history file 2nd skipping commands in loops 2nd 3rd 4th skipping fields 2nd slash 2nd 3rd sort command 2nd In option 2nd n option 2nd 0 option 2nd r option 2nd toption 2nd 3rd u option 2nd sorting files 2nd arithmetic sorts 2nd delimiter characters 2nd 3rd duplicate lines 2nd reversing sort order 2nd skipped fields 2nd writing sorted data to standard output 2nd source files special files special variables See variables standard error 2nd 3rd redirecting writing to 2nd standard input 2nd 3rd 4th closing translating characters from 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th 12th standard output 2nd 3rd 4th closing writing sorted data to 2nd starting shell startup status exit status 2nd 3rd variable 2nd 3rd 4th 5th 6th nonzero values read command 2nd 3rd 4th 5th zero of jobs status of processes returning 2nd stoppi
26. A subshell has no knowledge of local variables that were assigned values by the login shell the parent shell Furthermore a subshell cannot change the value of a variable in the parent Shell as evidenced by vartest2 Let s review the process that goes on here Before executing vartest2 your login shell has a variable called x that has been assigned the value 100 assume for now that this is the only variable defined in the Shell This is depicted in Figure 11 1 Figure 11 1 Login shell with x 100 local variables When you ask to have vartest2 executed your login shell starts up a subshell to run it giving it an empty list of local variables to start with see Figure 11 2 Figure 11 2 Login shell executes vartest2 local variables variesi2 Bq gt After the first command in vartest2 is executed that assigns 50 to x the local variable x that exists in the subshell s environment will have the value 50 see Figure 11 3 Note that this has no relation whatsoever to the variable x that still maintains its value of 100 in the login shell Figure 11 3 vartest2 executes x 50 local variables local variables subshell When vartest2 finishes execution the subshell goes away together with any variables assigned values rs ca Exported Variables There is a way to make the value of a variable known to a subshell and that s by exporting it with the export command The format of this command
27. B C D E F G H 1 CL K L M N O P Q R S T U V W X Y Z k command vi 2nd 3rd kernel defined 2nd keys Break Delete 2nd keyword parameters 2nd assigning values to 2nd 3rd 4th kill command 2nd 3rd 4th kill command vi KILL signal killing jobs 2nd 3rd 4th Korn shell Korn shell Web site 2nd Korn David 2nd Fe Previous 4 rrevious SYMBOL A B C D E F G H 1 J K L M N O P Q R S T U V W X Y Z L L operator command vi 2nd 3rd L command vi option grep command 2nd wc command languages See shell programming language shell programming language comments shell programming languages variables 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th 12th 13th 14th 15th 16th 17th 18th 19th 20th 21st le le operator Learning Pearl Learning Perl left shifting positional parameters 2nd 3rd line continuation character 2nd line numbering program 2nd LINENO variable lines of text deleting sed command 2nd duplicate lines deleting 2nd duplicating 2nd 3rd 4th 5th 6th 7th 8th line numbering program 2nd pasting together paste command 2nd 3rd 4th 5th 6th 7th pattern matching beginning of line 2nd end of line 2nd 3rd sorting 2nd arithmetic sorts 2nd delimiter characters 2nd 3rd
28. I I I I l i Unix l system I kernel I I I I l l I According to the other entries from etc passwd shown previously pat gets the program ksh stored in usr bin this is the Korn shell and bob gets the program data_entry see Figure 3 5 Figure 3 5 Three users logged in login sue Password Jusr bin sh login pat Unix Password system usr bin ksh kernel usr data bin IT login bob data_entry Password The init program starts up other programs similar to getty for networked connections For example sshd telnetd and rlogind are started to service logins via ssh telnet and rlogin respectively Instead of being tied directly to a specific physical terminal or modem line these programs connect users shells to pseudo ttys These are devices that emulate terminals over network connections You can see this whether you re logged in to your system over a network or on an X Windows screen who phw pts 0 Jut 20 17 37 Logged in with rlogin rrevious next Typing Commands to the Shell When the shell starts up it displays a command prompt typically a dollar sign at your terminal and then waits for you to type in a command see Figure 3 6 Steps 1 and 2 Each time you type ina command and press the Enter key Step 3 the shell analyzes the line you typed and then proceeds to carry out your request Step 4 If you ask it to execute a particular program th
29. Modify Lu so that it ignores case when doing the lookup What happens if you forget to supply an argument to the Lu program What happens if the argument is null as in Lu The program ison from this chapter has a shortcoming as shown in the following example ison ed fred tty03 Sep 4 14 53 The output indicates that fred is logged on while we were checking to see whether ed was logged on Modify ison to correct this problem Write a program called twice that takes a single integer argument and doubles its value twice 15 30 twice 0 0 What happens if a noninteger value is typed What if the argument is omitted 5 Write a program called home that takes the name of a user as its single argument and prints that user s home directory So home steve would print users steve if users steve is steve s home directory Hint Recall that the home directory is the sixth field stored in the file etc passwd 6 Write a program called suffix that renames a file by adding the characters given as the second argument to the end of the name of the file given as the first argument So suffix memol sv should rename memol to memol sv 7 Write a program called unsuffix that removes the characters given as the second argument from the end of the name of the file given as the first argument So unsuffix memol sv sv should rename memol sv to memol Be sure that the characters are removed from the end so un
30. Table A 9 summarizes the options that can be selected Table A 9 set Options Meaning Option Don t treat subsequent args preceded by a as options If there are no arguments the positional parameters are unset Automatically export all variables that are subsequently defined or modified If supported by the implementation cause the shell to notify you when background jobs finish Don t allow output redirection to overwrite existing files gt can still be used to force individual files to be overwritten even if this option is selected Exit if any command that gets executed fails or has a nonzero exit status Disable filename generation Add commands inside functions to the hash list as they are defined and not as they are executed Turn on the job monitor Read commands without executing them useful for checking for balanced do dones and if fis 0 Write current option mode settings in command format Turn on option mode m see Table A 10 Issue an error if a variable is referenced without having been assigned a value or if a positional parameter is referenced without having been set Print each shell command line as it is read Print each command and its arguments as it is executed preceded by a Shell modes are turned on or off by using the o and o options respectively followed by an option name These options are summarized in Table A 10 Mode allexport errexit ignoreeof mon
31. a b c Try quotes 1 arguments passed arg 1 a bc arg2 arg3 ls x See what files start with x xact xtra args x Try file name substitution 2 arguments passed arg 1 xact arg 2 xtra arg 3 my bin users steve bin args my bin And variable substitution 1 arguments passed arg 1 users steve bin arg 2 arg 3 args cat names Pass the contents of names 7 arguments passed arg 1 Charlie arg 2 Emanuel arg3 Fred As you can see the shell does its normal command line processing even when it s executing your shell programs This means that you can take advantage of the normal niceties such as filename substitution and variable substitution when specifying arguments to your programs fe previous next gt lt Previous fl next gt The s Variable The special variable references all the arguments passed to the program This is often useful in programs that take an indeterminate or variable number of arguments You ll see some more practical examples later Here s a program that illustrates its use cat args2 echo arguments passed echo they are args2 abc 3 arguments passed they are a bc args2 one two 2 arguments passed they are one two args2 0 arguments passed they are args2 8 arguments passed they are args args2 names nu phonebook stat xact xtra ns Ga ns Ga A Program to Look Up Someone in the Phone Book
32. defined parent processes PIDs process ids 2nd pipeline processes 2nd 3rd PPI Ds parent process ids returning status of 2nd profile profile file 2nd 3rd program loops See loops Programming Pearl Programming Perl programs See also command files commands add 2nd 3rd 4th 5th addi 2nd args 2nd 3rd 4th 5th 6th cdtest change 2nd 3rd display 2nd exit status 2nd variable 2nd 3rd 4th 5th 6th nonzero values zero exiting 2nd getty 2nd init 2nd interrupt signals ignoring 2nd sending signal numbers 2nd trapping 2nd 3rd 4th 5th ison 2nd lu 2nd 3rd 4th 5th mon 2nd 3rd 4th 5th 6th 7th 8th 9th mycp destination files checking 2nd 3rd echo escape characters 2nd final code listing 2nd 3rd 4th 5th 6th initial code listing 2nd 3rd read command number 2nd number2 2nd passing arguments to 2nd 3rd 4th variable 2nd variable 2nd n variable 2nd phonebook file example 2nd 3rd 4th 5th 6th 7th 8th 9th 10th shift command 2nd 3rd rem 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th 12th 13th replacing current program with new program 2nd 3rd reverse rolo 2nd 3rd 4th 5th 6th 7th 8th 9th case statement echo command sample runs 2nd rolodex add program 2nd change program 2nd 3rd design considerations 2nd display program 2nd l
33. directory users Similarly the pathname users steve documents references the directory documents as contained in the directory steve under users As a final example the pathname users steve documents letters AMG reply identifies the file AMG reply contained along the appropriate directory path To help reduce some of the typing that would otherwise be required Unix provides certain notational conveniences Pathnames that do not begin with a slash character are known as relative pathnames The path is relative to your current working directory For example if you just logged in to the system and were placed into your home directory users steve you could directly reference the directory documents simply by typing documents Similarly the relative pathname programs mon could be typed to access the file mon contained inside your programs directory By convention the directory name always references the directory that is one level higher For example after logging in and being placed into your home directory users steve the pathname would reference the directory users And if you had issued the appropriate command to change your working directory to documents letters the pathname would reference the documents directory would reference the directory steve and proposals new hire would reference the file new hire contained in the proposals directory Note that in this case as in most cases there is usually more than one wa
34. do display line done lt tmp matches fi rm tmp matches This is the program to look up an entry in the phone book The matching entries are written to the file tmp matches If the size of this file is zero no match was found Otherwise the program enters a loop to read each line from the file remember an entry is stored as a single line in the file and then display it at the terminal A program called display is used for this purpose This program is also used by the rem and change programs to display entries at the terminal display Display entry from the phonebook entry 1 IFS set entry for line in ee So 3 4 gg 6 do printf 34 34s n line done echo 0 o echo n echo As noted this program displays an entry passed as its argument To make the output more aesthetically pleasing the program actually draws a rolodex card So typical output from display would look like this Steve s Ice Cream 444 6th Avenue New York City 10003 212 555 3021 After skipping a line and then displaying the top of the card display changes IFS to and then executes the set command to assign each line to a different positional parameter For example if entry is equal to Steve s Ice Cream 444 6th Avenue New York City 10003 212 555 3021 executing the set command assigns Steve s Ice Cream to 1 444 6th Avenue to 2 New York
35. e suspend kill STOP Note from the preceding example that r is actually an alias for the fc command with the e option and history is an alias for fc l Bash doesn t automatically define any aliases by default Removing Aliases The unalias command is used to remove aliases from the alias list The format is unalias name which removes the alias name and unalias a which removes all aliases This concludes this section on aliases If you develop a set of alias definitions that you like to use during your login sessions you may want to define them inside your ENV file so that they will always be available for you to use ns Ga os fa Arrays The Korn shell and Bash provide a limited array capability arrays are not a part of the POSIX standard Shell Bash arrays may contain an unlimited number of elements subject to memory limitations Korn shell arrays are limited to 4096 elements Array indexing in both shells starts at zero An array element is accessed with a subscript which is an integer valued expression enclosed inside a pair of brackets You don t declare the maximum size of a shell array you simply assign values to elements as you need them The values that you can assign are the same as for ordinary variables arr O hello arr 1 some text arr 2 users steve memos To retrieve an element from an array you write the array name followed by the element number enclosed inside a pair of brac
36. foobar Wed Oct 2 15 23 15 EDT 2002 Here the shell supplies all the lines up to FOOBAR as the input to cat It substitutes the value for HOME but not for foobar because it s preceded by a backslash The date command is also executed because back quotes are interpreted cat lt lt FOOBAR ANW gt date gt HOME gt FOOBAR AANA date HOME The backslash before FOOBAR tells the shell to leave the following lines alone So it ignores the dollar signs backslashes and back quotes Use care when selecting the word that follows the lt lt Generally just make sure that it s weird enough so that the chances of it accidentally appearing in the following lines are remote If the first character that follows the lt lt is a dash leading tab characters in the input will be removed by the shell This is useful for visually indenting the redirected text cat lt lt END gt Indented lines gt So there you have it gt END Indented lines So there you have it Shell Archives One of the best uses of the inline input redirection feature is for creating shell archive files With this technique one or more related shell programs can be put into a single file and then shipped to someone else using the standard Unix mail commands When the archive is received it can be easily unpacked by simply running the shell on it For example here s an archived version of the lu add and rem programs used
37. i hello ksh i bad number Bash simply ignores any strings that don t contain numeric values and generates an error for anything that contains both numbers and other characters typeset i i i hello echo i 0 i lhello bash lhello value too great for base error token is lhello i1 10 15 echo i 25 The preceding example shows that integer valued expressions can be assigned to an integer variable without even having to use the construct This holds true for both Bash and the Korn shell Numbers in Different Bases The Korn shell and Bash allow you to perform arithmetic in different bases To write a number in a different base with these shells you use the notation base number For example to express the value 100 in base 8 octal you write 8 100 You can write constants in different bases anywhere an integer value is permitted To assign octal 100 to the integer variable i you can write typeset i i 8 100 Note that with the Korn shell the base of the first value assigned to an integer variable fixes the base of all subsequent substitutions of that variable In other words if the first value you assign to the integer variable i is an octal number each time you subsequently substitute the value of i on the command line the Korn shell substitutes the value as an octal number using the notation 8 value typeset i i 8 100 echo i 8 100 i 50 echo i 8 62 i
38. readonly command 2nd 3rd 4th reassigning values to positional parameters 2nd 3rd recommended reading See books recursive functions 2nd Red Hat Linux 8 Unleashed redirecting I O input output loops 2nd redirection in line input redirection 2nd 3rd 4th 5th 6th 7th 8th 9th creating archives with 2nd 3rd 4th 5th example 2nd 3rd 4th input redirection 2nd 3rd 4th 5th 6th 7th 8th output redirection 2nd 3rd 4th 5th 6th 7th 8th standard error regular expressions 2nd construct 2nd 3rd 4th construct 2nd 3rd construct 2nd 3rd 4th asterisk 2nd 3rd 4th caret 2nd dollar sign 2nd 3rd grep command l option 2nd n option 2nd v option 2n examples 2nd 3rd 4th 5th 6th 7th gt n syntax period 2nd 3rd summary of 2nd relative pathnames rem program 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th 12th 13th removing aliases removing See deleting renaming files mv command 2nd 3rd repeating characters echo command 2nd replacing current program with new program 2nd 3rd resetting traps 2nd restricted shell rsh 2nd 3rd 4th return command 2nd 3rd 4th reverse program reversing sort order 2nd rm command 2nd rmdir command 2nd rolo program 2nd 3rd 4th 5th 6th 7th 8th 9th case statement echo command sample runs 2nd rolodex program
39. test argument expected The operator has higher precedence than the z operator so test expects an argument to follow To avoid this sort of problem you can write your command as test X symboLl X which will be true if symbol is null and false if it s not The X in front of symbol prevents test from interpreting the characters stored in symbol as an operator An Alternative Format for test The test command is used so often by shell programmers that an alternative format of the command is recognized This format improves the readability of the command especially when used in if commands You ll recall that the general format of the test command is test expression This can also be expressed in the alternative format as expression The is actually the name of the command who said anything about command names having to be alphanumeric characters It still initiates execution of the same test command only in this format test expects to see a closing at the end of the expression Naturally spaces must appear after the and before the You can rewrite the test command shown in a previous example with this alternative format as shown z nonnullvar echo 1 When used in an if command this alternative format looks like this if name julio then echo Would you like to play a game fi Which format of the if command you use is up to you we prefer the format so tha
40. 1 Figure 3 1 The Unix system Utilities disks l ee Memory The kernel is the heart of the Unix system and resides in the computer s memory from the time the computer is turned on and booted until the time it is shut down The utilities on the other hand reside on the computer s disk and are only brought into memory as requested Virtually every command you know under the Unix system is classified as a utility therefore the program resides on the disk and is brought into memory only when you request that the command be executed So for example when you execute the date command the Unix system loads the program called date from the computer s disk into memory and initiates its execution The shell too is a utility program It is loaded into memory for execution whenever you log in to the system In fact it s worth learning the precise sequence of events that occurs when the first shell on a terminal or window starts up Team LiB EA The Login Shell A terminal is connected to a Unix system through a direct wire modem or network In the first case as soon as you turn on the terminal and press the Enter key a couple of times if necessary you should get a login message on your screen In the second case you must first dial the computer s number and get connected before the Login message appears In the last case you may connect over the network via a program such as ssh telnet or rlogin or yo
41. 1 Cron Daemon for periodic tasks bin 3 3 The owner of system files uucp 5 5 usr spool uucppublic usr lib uucp uucico asg 6 6 The Owner of Assignable Devices sysinfo 10 10 Access to System Information usr bin sh george 75 75 users george usr lib rsh steve 203 100 users steve usr bin ksh pat 300 300 users pat usr bin ksh mail 301 301 usr mail Here we ve emboldened the third field of each line so that you can easily verify that the file was sorted correctly by user id Other Options Other options to sort enable you to skip characters within a field specify the field to end the sort on merge sorted input files and sort in dictionary order only letters numbers and spaces are used for the comparison For more details on these options look under sort in your Unix User s Manual Fe previous Next rs ci uniq The uniq command is useful when you need to find duplicate lines in a file The basic format of the command is uniq in file out file In this format uniq copies in_file to out_file removing any duplicate lines in the process uniq s definition of duplicated lines are consecutive occurring lines that match exactly If out_file is not specified the results will be written to standard output If in_file is also not specified uniq acts as a filter and reads its input from standard input Here are some examples to see how unig works Suppose that you have a file called na
42. 16 a5 16 120 echo i 8 705 Because the first value assigned to i in this example is an octal number 8 100 all further substitutions of i will be in octal When the base 10 value of 50 is next assigned to i and then i is subsequently displayed we get the value 8 62 which is the octal equivalent of 50 in base 10 In the preceding example the construct is used to add together the two hexadecimal values a5 and 120 The result is then displayed once again in octal Bash uses both the base number syntax for arbitrary bases and the C language syntax for octal and hexadecimal numbers octal numbers are preceded by 0 zero and hexadecimal numbers are preceded by Ox typeset i i 0100 echo i 64 i 0x80 echo i 128 1 2 1101001 echo i 105 i 16 a5 16 120 echo i 453 Unlike the Korn shell Bash doesn t keep track of the variable s base integer variables are displayed as decimal numbers You can always use printf to print integers in octal or hexadecimal format As you can see with Bash and the Korn shell it s easy to work with different bases This makes it possible to easily write functions to perform base conversion and arithmetic for example rs i os fa The alias Command An alias is a shorthand notation provided by the shell to allow customization of commands The shell keeps a list of aliases that is searched when a command is entered If the first word of a
43. 166d etc passwd gt tmp passwd Remove duplicate mv tmp passwd etc passwd mv etc passwd 444 modey mv cannot unlink etc passwd Naturally etc passwd is one of the most important files on a Unix system As such only the superuser is allowed to write to the file That s why the mv command failed Other Options The c option to unig behaves like unig with no options that is duplicate lines are removed except that each output line gets preceded by a count of the number of times the line occurred in the input sort names uniq c Count line occurrences 1 Charlie 1 Emanuel 1 Fred 1 Lucy 1 Ralph 2 Tony Two other options that won t be described enable you to tell unig to ignore leading characters fields on a line For more information consult your Unix User s Manual We would be remiss if we neglected to mention the programs awk and perl that can be useful when writing shell programs However to do justice to these programs requires more space than we can provide in this text We ll refer you to the document Awk A Pattern Scanning and Processing Language by Aho et al in the Unix Programmer s Manual Volume II for a description of awk Kernighan and Pike s The Unix Programming Environment Prentice Hall 1984 contains a detailed discussion of awk Learning Perl and Programming Perl both from O Reilly and Associates present a good tutorial and reference on the language respectively nS Gy Exercises
44. 2 Other Variables Used by the Shell Meaning The directories to be searched whenever cd is executed without a full path as argument The name of a file that the shell executes in the current environment when started interactively The editor used by fc If not set ed is used If set it specifies a file to be used to store the command history If not set or if the file isn t writable H0ME sh_history is used If set specifies the number of previously entered commands accessible for editing The default value is at least 128 The user s home directory the directory that cd changes to when no argument is supplied The Internal Field Separator characters used by the shell to delimit words when parsing the command line for the read and set commands when substituting the output from a back quoted command and when performing parameter substitution Normally it contains the three characters space horizontal tab and newline LINENO MAIL MAILCHECK MAILPATH PATH PPID PS1 PS2 PS4 PWD Set by the shell to the line number in the script it is executing This value is set before the line gets executed and starts at 1 The name of a file that the shell periodically checks for the arrival of mail If new mail arrives the shell displays a You have mail message See also MAILCHECK and MAILPATH The number of seconds specifying how often the shell is to check for the arrival of mail in the file in MAIL or in th
45. 5th downloading Korn shell duplicate lines deleting 2nd duplicating lines 2nd 3rd 4th 5th 6th 7th 8th dw command vi 4 rrevious SYMBOL A B C D E F G H 1 1 K L M N O P Q R S T U V W X Y Z e e operator e option fc command e option set command echo command 2nd 3rd 4th 5th 6th 7th escape characters 2nd edit mode commands vi 2nd 3rd edit modes emacs 2nd 3rd 4th 5th 6th 7th 8th commands 2nd cursor 2nd vi 2nd 3rd 4th 5th 6th command mode commands 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 1 th 12th 13th 14th 15th 16th 17th 18th 19th 20th input mode editing command history 2nd command line editing 2nd files sed command 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th phone book entries 2nd 3rd editors stream editor sed d option 2nd n option 2nd examples 2nd 3rd 4th 5th syntax elements of arrays assigning retrieving elif construct 2nd 3rd 4th 5th else construct 2nd 3rd 4th emacs line edit mode 2nd 3rd 4th 5th 6th 7th 8th commands 2nd cursor 2nd enabling trace mode 2nd end of line matching 2nd 3rd Enter command vi 2nd entering commands 2nd 3rd loops on one line 2nd 3rd entering multiple commands on one line entries phone book adding 2nd deleting 2nd 3rd 4th 5th 6th 7th 8th 9th displaying 2nd ed
46. 9th 10th 11th lines deleting 2nd duplicating 2nd 3rd 4th 5th 6th line numbering program 2nd pasting together 2nd 3rd 4th 5th sorting 2nd 3rd 4th 5th 6th 7th 8th 9th 10th pattern matching any character 2nd 3rd beginning of line 2nd end of line 2nd 3rd grep command 2nd 3rd 4th 5th 6th 7th 8th 9th parameter substitution constructs 2nd 3rd 4th precise number of characters 2nd 3rd 4th saving matched characters 2nd 3rd 4th specific characters 2nd 3rd summary of regular expressions 2nd zero or more characters 2nd 3rd 4th searching grep command 2nd 3rd 4th 5th 6th 7th 8th 9th 10th tilde tilde substitution 2nd 3rd 4th tilde substitution 2nd 3rd 4th tile tilde substitution time recording amount used time zones TZ variable 2nd time date displaying date command 2nd times command 2nd tools See commands 2nd See commands tr command d option s option examples 2nd 3rd 4th 5th 6th 7th 2nd 2nd syntax trace mode disabling enabling 2nd translating characters from standard input 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th 12th trap command 2nd 3rd 4th 5th 6th 7th 8th executing without arguments 2nd ignoring signals 2nd resetting traps 2nd signal numbers syntax traps creating 2nd 3rd displaying 2nd resetting 2nd signal numbers 2nd true command
47. As soon as one returns an exit status of zero the commands listed after the then that follows are executed up to another elif else or fi If none of the commands returns a zero exit status the commands listed after the optional else are executed You could rewrite the greetings program using this new format as shown cat greetings Program to print a greeting version 2 hour date cut c12 13 if hour ge O a hour le 11 then echo Good morning elif hour ge 12 a hour le 17 then echo Good afternoon else echo Good evening fi This version is easier to read and it doesn t have the tendency to disappear off the right margin due to excessive indentation Incidentally you should note that date provides a wide assortment of options One of these H can be used to get the hour directly from date date H 10 As an exercise you should change greetings to make use of this fact Yet Another Version of rem Another way to add some robustness to the rem program would be to check the number of entries that matched before doing the removal If there s more than one match you could issue a message to the effect and then terminate execution of the program But how do you determine the number of matching entries One approach is to do a normal grep on the phonebook file and then count the number of matches that come out with wc If the number of matches is greater than one the
48. Hasbrouck Heights NJ 07604 l 201 555 6000 Teri Zak Hayden Book Company see Maureen Connelly for address 201 555 6060 Would you like to Please select one of the above 1 6 4 Enter name to change Maureen Maureen Connelly Hayden Book Companu 10 Mulholland Drive Hasbrouck Heights NJ 07604 201 555 6000 Change this person y n y Enter changes with bin ed 101 1 p Maureen Connelly Hayden Book Companu 10 Mulholland Drive Hasbrouck Heights NJ 07604 201 555 6000 2s anu any Change the misspelling Hayden Book Company W 101 Would you like to Please select one of the above 1 6 6 The only function not tested here is removal of an entry Hopefully this example has given you some insight on how to develop larger shell programs and how to use the many different programming tools provided by the system Other than the shell built ins rolo relies on tr grep an editor sort and the standard file system commands such as mv and rm to get the job done The simplicity and elegance that enable you to easily tie all these tools together account for the deserved popularity of the Unix system See Appendix B for more information on downloading the rolo programs Chapter 15 Interactive and Nonstandard Shell Features introduces you to interactive features of the shell and two shells that have some nice features not found in the POSIX standard shell fe erevious Next
49. It It operator lu program 2nd 3rd 4th 5th 4 rrevious SYMBOL A B C D E F G H 1 J K L M N O P Q R S T U V W X Y Z m option set command MAIL variable MAILCHECK variable MAILPATH variable man command man pages matching patterns 2nd 3rd 4th amp amp construct 2nd 3rd 4th asterisk 2nd question mark L construct construct 2nd 3rd 4th any character 2nd 3rd beginning of line 2nd end of line 2nd 3rd grep command l option 2nd Qa n option 2n io v option 2n examples 2nd 3rd 4th 5th 6th 7th syntax parameter substitution constructs 2nd 3rd 4th precise number of characters 2nd 3rd 4th saving matched characters 2nd 3rd 4th specific characters 2nd 3rd summary of regular expressions 2nd zero or more characters 2nd 3rd 4th menu driven phone program See rolodex program messages job messages mkdir command 2nd modes access modes trace mode disabling enabling 2nd vi line edit mode 2nd 3rd 4th 5th commands 2nd 3rd 4th modes shell 2nd emacs 2nd 3rd 4th 5th 6th 7th 8th commands 2nd cursor 2nd vi 2nd 3rd 4th 5th 6th command mode commands 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th 12th 13th 14th 15th 16th 17th 18th 19th 20th input mode mon program 2nd 3rd 4th 5th 6th 7th 8th monitor s
50. Linux 8 and then get the most out of your system Sams Teach Yourself Red Hat Linux 8 0 in 24 Hours Aron Hsiao Sams Publishing 2003 ISBN 067232475xX This title offers a broad range of FreeBSD related topics It is detailed in its approach and offers information not found anywhere else FreeBSD Unleashed Michael Urban and Brian Tiemann Sams Publishing 2001 ISBN 0672322064 Learn FreeBSD from the ground up This book is the only beginning level tutorial that offers all the ins and outs of the FreeBSD operating system Sams Teach Yourself FreeBSD in 24 Hours Michael Urban and Brian Tiemann 2002 ISBN 0672324245 Other Publishers This is the set of standard documentation for System V Release 4 produced by AT amp T Although thorough this two book set dates from the early 1990s User s Reference Manual System Administrator s Reference Manual Prentice Hall 1992 ISBN 0139513108 The following book contains complete coverage of the Korn shell and is coauthored by its creator The New KornShell Command and Programming Language 2nd Edition D Korn and M Bolsky Prentice Hall 1995 ISBN 0131827006 An in depth reference to the C shell The Unix C Shell Field Guide G Anderson and P Anderson Prentice Hall 1986 ISBN 013937468xX A complete description of the awk language authored by its creators The AWK Programming Language A V Aho B W Kernighan and P J Weinberger Addison Wesley 1988 ISBN 020107981X
51. Reader Services Chapter 1 Introduction Chapter 2 A Quick Review of the Basics Some Basic Commands Working with Files Working with Directories Filename Substitution Standard Input Output and I O Redirection Pipes Standard Error More on Commands Command Summary Exercises Chapter 3 What Is the Shell The Kernel and the Utilities The Login Shell Typing Commands to the Shell The Shell s Responsibilities Chapter 4 Tools of the Trade Regular Expressions cut paste n E Exercises Chapter 5 And Away We Go Command Files Variables Built in Integer Arithmetic Exercises Chapter 6 Can Quote You on That The Single Quote The Double Quote The Backslash Command Substitution Exercises Chapter 7 Passing Arguments The Variable The Variable A Program to Look Up Someone in the Phone Book A Program to Add Someone to the Phone Book A Program to Remove Someone from the Phone Book n The shift Command Exercises Chapter 8 Decisions Decisions Exit Status The test Command The else Construct The exit Command The elif Construct The case Command The Null Command The amp amp and Constructs Exercises Chapter 9 Round and Round She Goes The for Command The until Command More on Loops The getopts Comm
52. This can consist of a single number as in c5 to extract character 5 a comma separated list of numbers as in c1 13 50 to extract characters 1 13 and 50 or a dash separated range of numbers as in c20 50 to extract characters 20 through 50 inclusive To extract characters to the end of the line you can omit the second number of the range so cut c5 data extracts characters 5 through the end of the line from each line of data and writes the results to standard output If file is not specified cut reads its input from standard input meaning that you can use cut as a filter ina pipeline Let s take another look at the output from the who command who root console Feb 24 08 54 steve tty02 Feb 24 12 55 george tty08 Feb 24 09 15 dawn tty10 Feb 24 15 55 As shown currently four people are logged in Suppose that you just want to know the names of the logged in users and don t care about what terminals they are on or when they logged in You can use the cut command to cut out just the usernames from the who command s output who cut c1 8 Extract the first 8 characters root steve george dawn The c1 8 option to cut specifies that characters 1 through 8 are to be extracted from each line of input and written to standard output The following shows how you can tack a sort to the end of the preceding pipeline to get a sorted list of the logged in users who cut cl 8 sort dawn george root ste
53. When the wc command executes it recognizes that no filename was specified and counts the lines on standard input unaware that standard input is not coming from the terminal but from the output of the who command Environment Control The shell provides certain commands that let you customize your environment Your environment includes your home directory the characters that the shell displays to prompt you to type in a command and a list of the directories to be searched whenever you request that a program be executed You ll learn more about this in Chapter 11 Your Environment Interpreted Programming Language The shell has its own built in programming language This language is interpreted meaning that the shell analyzes each statement in the language one line at a time and then executes it This differs from programming languages such as C and FORTRAN in which the programming statements are typically compiled into a machine executable form before they are executed Programs developed in interpreted programming languages are typically easier to debug and modify than compiled ones However they usually take much longer to execute than their compiled equivalents The shell programming language provides features you d find in most other programming languages It has looping constructs decision making statements variables and functions and is procedure oriented Modern shells based on the IEEE POSIX standard have many other features i
54. a command to follow In this case the shell finds that the date command is to be executed So it executes date and replaces the date on the command line with the output from the date After that it divides the command line into arguments in the normal manner and then initiates execution of the echo command echo Your current working directory is pwd Your current working directory is users steve shell ch6 Here the shell executes pwd inserts its output on the command line and then executes the echo Note that in the following section back quotes can be used in all the places where the construct is used The Construct The POSIX standard shell supports the newer construct for command substitution The general format is command where as in the back quoting method command is the name of the command whose standard output is to be substituted on the command line For example echo The date and time is date The date and time is Wed Aug 28 14 28 43 EDT 2002 This construct is better than back quotes for a couple of reasons First complex commands that use combinations of forward and back quotes can be difficult to read particularly if the typeface you re using doesn t have visually different single quotes and back quotes second constructs can be easily nested allowing command substitution within command substitution Although nesting can also be performed with back quotes it s a
55. add program 2nd change program 2nd 3rd design considerations 2nd display program 2nd listall program 2nd 3rd lu program 2nd PHONEBOOK variable 2nd 3rd 4th 5th rem program 2nd 3rd 4th 5th 6th 7th 8th 9th rolo program 2nd 3rd 4th 5th 6th 7th 8th 9th case statement echo command sample runs 2nd sample output 2nd 3rd 4th 5th 6th temporary files 2nd 3rd root directories rsh restricted shell 2nd 3rd 4th run command file passing arguments to 2nd running See executing 4 rrevious SYMBOL A B C D E F G H 1 CL K L M N O P Q R S T U V W X Y Z s s operator S command vi s conversion specification character s option fc command paste command 2nd sh command s options tr command 2nd Sams Teach Yourself FreeBSD in 24 Hours Sams Teach Yourself Red Hat Linux 8 0 in 24 Hours Sams Teach Yourself Shell Programming in 24 Hours Sams Teach Yourself Unix in 24 Hours saving matched characters 2nd 3rd 4th search order 2nd searching See also pattern matching files grep command 2nd 3rd 4th 5th 6th 7th 8th 9th 10th phone book entries 2nd 3rd 4th 5th search order 2nd secondary prompts Sed amp Awk sed command 2nd examples syntax selecting Shells 2nd semicolon semicolon command vi sending
56. append respectively You also know that you can redirect standard error from any command simply by writing command 2 gt file Sometimes you may want to explicitly write to standard error in your program You can redirect the standard output for a command to standard error by writing command gt amp 2 The notation gt amp specifies output redirection to a file associated with the file descriptor that follows File descriptor 0 is standard input descriptor 1 is standard output and descriptor 2 is standard error Note that no space is permitted between the gt and the amp So to write an error message to standard error you write echo Invalid number of arguments gt amp 2 Frequently you may want to collect the standard output and the standard error output from a program into the same file If you know the name of the file this is straightforward enough command gt foo 2 gt gt foo Here both the standard output and the standard error output from command will be written to foo You can also write command gt foo 2 gt amp 1 to achieve the same effect standard output is redirected to foo and standard error is redirected to standard output which has already been redirected to foo Note that because the shell evaluates redirection from left to right on the command line the last example cannot be written command 2 gt amp 1 gt foo because this would first redirect standard error to standard output your terminal b
57. appropriate message can be issued cat rem Remove someone from the phone book version 3 if ne 1 then echo Incorrect number of arguments echo Usage rem name exit 1 fi name 1 Find number of matching entries matches grep name phonebook wc lL If more than one match issue message else remove it if matches gt 1 then echo More than one match please qualify further elif matches eq 1 then grep v name phonebook gt tmp phonebook mv tmp phonebook phonebook else echo I couldn t find name in the phone book fl The positional parameter 1 is assigned to the variable name after the number of arguments check is performed to add readability to the program Subsequently using name is a lot clearer than using 1 The if elif else command first checks to see whether the number of matches is greater than one If it is the More than one match message is printed If it s not a test is made to see whether the number of matches is equal to one If it is the entry is removed from the phone book If it s not the number of matches must be zero in which case a message is displayed to alert the user of this fact Note that the grep command is used twice in this program first to determine the number of matches and then with the v option to remove the single matching entry Here are some sample runs of the third version of rem rem Inco
58. be placed into the environment of a command by preceding the command name with the assignment to the parameter on the command line as in PHONEBOOK HOME misc phone rolo Here the variable PHONEBOOK will be assigned the indicated value and then placed in rolo s environment The environment of the current shell remains unchanged as if PHONEBOOK HOME misc phone export PHONE BOOK rolo had been executed instead ns ca Functions Functions take the following form name compound command where compound command is a set of commands enclosed in or can bea for case until or while command Most often the function definition takes this form name command command command where name is the name of the function defined to the current shell functions can t be exported The function definition can span as many lines as necessary A return command can be executed to cause execution of the function to be terminated without also terminating the shell see the return command description For example nf ls we l defines a function called nf to count the number of files in your current directory ne Ga Previous next gt Job Control Shell Jobs Every command sequence run in the background is assigned a job number starting at one The lowest available number not in use is assigned A job may be referred to by a job_id which is a followed by the job number followed by t
59. book uses is to teach by example Properly chosen examples do a far superior job at illustrating how a particular feature is used than ten times as many words The old A picture is worth adage seems to apply just as well to examples You are encouraged to type in each example and test it on your system for only by doing can you become adept at shell programming You also should not be afraid to experiment Try changing commands in the program examples to see the effect or add different options or features to make the programs more useful or robust At the end of most chapters you will find exercises These can be used as assignments in a classroom environment or by yourself to test your progress This book teaches the IEEE POSIX standard shell Incompatibilities with earlier Bourne shell versions are noted in the text and these tend to be minor Acknowledgments from the first edition of this book We d like to thank Tony Iannino and Dick Fritz for editing the manuscript We d also like to thank Juliann Colvin for performing her usual wonders copy editing this book Finally we d like to thank Teri Zak our acquisitions editor and posthumously Maureen Connelly our production editor These two were not only the best at what they did but they also made working with them a real pleasure For the first revised edition of this book we d like to acknowledge the contributions made by Steven Levy and Ann Baker and we d like to also thank the fol
60. changed the values of x and y and that y is still exported vartest4 echo x y 100 10 So the subshell couldn t even change the value of the exported variable y it merely changed the copy of y that was passed to its environment when it was executed see Figure 11 5 Just as with local variables when a subshell goes away so do the values of the exported variables There is no way to change the value of a variable in a parent shell from within a subshell Figure 11 5 Execution of vartest4 exported local variables variables lt gt J gt exported local variables vartest4 _ variables gt subshell s In the case of a subshell executing another subshell for example the rolo program executing the Lu program the process is repeated The exported variables from the subshell are copied to the new subshell These exported variables may have been exported from above or newly exported from within the subshell After a variable is exported it remains exported to all subshells subsequently executed Consider a modified version of vartest4 cat vartest4 x 50 y 5 z 1 export z vartest5 and also consider vartest5 cat vartest5 echo x x echo y y echo z z When vartest4 gets executed the exported variable y will be copied into the subshell s environment vartest4 sets the value of x to 50 changes the value of y to 5 and sets the value of z to 1 Th
61. character and the end of line character known more formally as the newline character Multiple occurrences of whitespace characters are simply ignored by the shell When you type the command mv tmp mazewars games the shell scans the command line and takes everything from the start of the line to the first whitespace character as the name of the program to execute mv The set of characters up to the next whitespace character is the first argument to mv tmp mazewars The set of characters up to the next whitespace character Known as a word to the shell in this case the newline is the second argument to mv games After analyzing the command line the shell then proceeds to execute the mv command giving it the two arguments tmp mazewars and games see Figure 3 9 Figure 3 9 Execution of mv with two arguments mv arguments ee games As mentioned multiple occurrences of whitespace characters are ignored by the shell This means that when the shell processes this command line echo when do we eat it passes four arguments to the echo program when do we and eat see Figure 3 10 Figure 3 10 Execution of echo with four arguments Because echo takes its arguments and simply displays them at the terminal separating each by a space character the output from the following becomes easy to understand echo when do we eat when do we eat The fact is that the echo command never sees those blank spa
62. characters 0 through 9 The only restriction in specifying a range of characters is that the first character must be alphabetically less than the last character so that z f is not a valid range specification By mixing and matching ranges and characters in the list you can perform some complicated substitutions For example a np z matches all files that start with the letters a through n or p through z or more simply stated any lowercase letter but o If the first character following the is a the sense of the match is inverted That is any character is matched except those enclosed in the brackets So a z matches any character except a lowercase letter and o matches any file that doesn t end with the lowercase letter o Table 2 1 gives a few more examples of filename substitution Table 2 1 Filename Substitution Examples Command Description echo a Print the names of the files beginning with a Cat 2 Print all files ending in c rm Remove all files containing a period ls x List the names of all files beginning with x rm Remove all files in the current directory Note Be careful when you use this echo a b Print the names of all files beginning with a and ending with b cp programs Copy all files from programs into the current directory ls a z 0 9 List files that begin with a lowercase letter and don t end with a digit rs a Team LiB Previous g NEXT gt St
63. console Jul 25 07 55 2 pat tty03 Jul 25 09 26 3 steve tty04 Jul 25 10 58 4 george ttyl3 Jul 25 08 05 Note that number won t work too well for lines that contain backslashes or leading whitespace characters The following example illustrates this point number Here are some backslashes 1 Here are some backslashes Leading whitespace characters are removed from any line that s read The backslash characters are also interpreted by the shell when it reads the line You can use the r option to read to prevent it from interpreting the backslash character If we change the while read line in number to while read r line the output will look better number Here are some backslashes 1 Here are some backslashes In Chapter 12 More on Parameters you ll learn how to preserve the leading whitespace characters and also how to have some control over the parsing of the input data ne Ga rs ca The printf Command Although echo is adequate for displaying simple messages sometimes you ll want to print formatted output for example lining up columns of data Unix systems provide the printf command Those of you familiar with the C programming language will notice many similarities The general format of the printf command is printf format argl arg2 where format is a string that describes how the remaining arguments are to be displayed Note that the format string is a single argumen
64. current directory like cd but uses an array to keep a history of previous directories It allows the user to list the directory history and change back to any directory in it cat cdh CDHIST 0 PWD cdh typeset i cdlen i initialize CDHIST Q0 if eq 0 default to HOME with no arguments then set HOME fi cdlen CDHIST number of elements in CDHIST case in l print directory list i 0 while i lt cdlen do printf 3d s n i CDHIST i i i 1 done return 0 9 0 9 0 9 cd to dir in list i 1 remove leading cd CDHIST i cd to new dir cd 30 43 esac CDHIST cdlen PWwD The CDHIST array stores each directory visited by cdh and the first element CDHIST 0 is initialized with the current directory when the cdh file is run pwd users pat cdh Define cdh function cdh tmp cdh l 0 users pat 1 tmp When the cdh file was run CDHIST 0 was assigned users pat and the cdh function was defined when cdh tmp was executed cdlen was assigned the number of elements in CDHIST one and CDHIST 1 was assigned tmp The cdh l caused printf to display each element of CDHIST on this invocation cdlen was set to 2 because elements 0 and 1 of CDHIST contained data Note that the if statement at the beginning of the function sets 1 to HOME if no arguments are passed Let s try that out cdh pwd users pat
65. dact sys A new hire no JSK AMG reply wb collect mon The file directory documents contains the files plan dact sys A new hire no JSK and AMG reply The directory programs contains the files wb collect and mon At some point you may decide to further categorize the files in a directory This can be done by creating subdirectories and then placing each file into the appropriate subdirectory For example you might want to create subdirectories called memos proposals and letters inside your documents directory as shown in Figure 2 2 Figure 2 2 Directories containing subdirectories documents programs memos proposals letters wb collect mon a plan dact sys A newhire no JSK AMG reply documents contains the subdirectories memos proposals and letters Each of these directories in turn contains two files memos contains plan and dact proposals contains sys A and new hire and letters contains no JSK and AMG reply Although each file in a given directory must have a unique name files contained in different directories do not So for example you could have a file in your programs directory called dact even though a file by that name also exists in the memos subdirectory The Home Directory and Pathnames The Unix system always associates each user of the system with a particular directory When you log in to the system you are placed automatically into a directory called your home directory Although the location of users
66. development gt gt 1 s Delete the last two characters from each line 1 p The Unix operating system was pioneered by Ken Thompson and Dennis Ritchie at Bell Laboratories in the late 1960s One of the primary goals in the design of the Unix system was to create an environment that promoted efficient program development It s worth noting that the regular expression matches any line that contains no characters such a line can be created in ed by simply pressing Enter while in insert mode This regular expression is to be distinguished from one such as which matches any line that consists of a single space character Matching a Choice of Characters The Construct Suppose that you are editing a file and want to search for the first occurrence of the characters the In ed this is easy You simply type the command the This causes ed to search forward in its buffer until it finds a line containing the indicated string of characters The first line that matches will be displayed by ed ed intro 248 the Find line containing the in the late 1960s One of the primary goals in Notice that the first line of the file also contains the word the except it starts a sentence and so begins with a capital T You can tell ed to search for the first occurrence of the or The by using a regular expression J ust as in filename substitution the characters and can be used in a regular expression to spec
67. duplicate lines 2nd reversing sort order 2nd skipped fields 2nd writing sorted data to standard output 2nd linking files dangling symbolic links In command 2nd 3rd 4th 5th 6th symbolic links 2nd Linux in a Nutshell listall program 2nd 3rd listing files Is command 2nd 3rd 4th 5th 6th phone book entries 2nd 3rd variables 2nd In In option sort command 2nd In command 2nd 3rd 4th 5th 6th local variables 2nd logged in users displaying who command 2nd logical AND operator a 2nd logical negation operator 2nd logical OR operator 0 2nd login program 2nd 3rd login shell 2nd 3rd 4th getty program 2nd init program 2nd logins profile file 2nd 3rd login cycle login shell 2nd 3rd 4th looking up phone book entries 2nd looking up phone book entries 2nd 3rd loops 2nd breaking 2nd 3rd 4th 5th executing in background 2nd for command variable variable 2nd 3rd examples 2nd 3rd 4th 5th for without the list 2nd syntax 2nd I O redirection 2nd piping data into and out of 2nd shorthand notation 2nd 3rd skipping commands in 2nd 3rd 4th terminating 2nd typing on one line 2nd 3rd until command 2nd 3rd 4th 5th 6th 7th 8th examples 2nd 3rd 4th 5th 6th 7th 8th syntax 2nd while command 2nd 3rd Is command 2nd 3rd 4th 5th 6th
68. example the regular expression ACN matches the first character on the line whatever it is and stores it into register 1 To retrieve the characters stored in a particular register the construct n is used where n is from 1 9 So the regular expression NGAM matches the first character on the line and stores it in register 1 Then the expression matches whatever is stored in register 1 as specified by the 1 The net effect of this regular expression is to match the first two characters on a line if they are both the same character Go over this example if it doesn t seem clear The regular expression PN nO ae matches all lines in which the first character on the line is the same as the last character on the line 1 The matches all the characters in between Successive occurrences of the construct get assigned to successive registers So when the following regular expression is used to match some text NVC NING ND the first three characters on the line will be stored into register 1 and the next three characters into register 2 When using the substitute command in ed a register can also be referenced as part of the replacement string ed phonebook 114 1 p Alice Chebba 973 555 2015 Barbara Swingle 201 555 9257 Liz Stachiw 212 555 2298 Susan Goldberg 201 555 7776 Tony Iannino 973 555 1295 15 5 2 1 Switch the two fields 1 p 973 555 2015 Alice Chebba 201 555 9257 Barba
69. file to automatically start up the shell with one of the edit modes turned on fe revious next gt rs ca Command History As we Said before the shell keeps a history of previously entered commands Each time you press the Enter key to execute a command that command gets added to the end of this history list This command list is actually stored inside a file which means that you can access previously entered commands across login sessions By default the history list is kept in a file in your home directory under the name sh_history bash history for Bash unless it is started with the posix option You can change this filename to anything you want by setting the variable HISTFILE to the name of your history file This variable can be set and exported in your profile file Naturally there is a limit to the number of commands the shell records The default value of this limit varies by implementation but the POSIX standard requires it to be at least 128 the default value for the Korn shell is 128 the default value for Bash is 500 Each time you log in the shell automatically truncates your history file to this length You can control the size of your history file through the HISTFILE variable You may find that the default size isn t adequate for your needs in which case you may want to set the HISTFILE variable to a larger value such as 500 or 1000 The value you assign to HISTSIZE can be set and exported in your profile fil
70. first This book assumes that you are familiar with the fundamentals of the Unix system that is that you know how to log in how to create files edit them and remove them and how to work with directories But in case you haven t used the Unix system for a while we ll examine the basics in Chapter 2 A Quick Review of the Basics Besides the basic file commands filename substitution O redirection and pipes are also reviewed in Chapter 2 Chapter 3 What Is the Shell reveals what the shell really is You ll learn about what happens every time you log in to the system how the shell program gets started how it parses the command line and how it executes other programs for you A key point made in Chapter 3 is that the shell is just a program nothing more nothing less Chapter 4 Tools of the Trade provides tutorials on tools useful in writing shell programs Covered in this chapter are cut paste sed grep sort tr and unig Admittedly the selection is subjective but it does set the stage for programs that we ll develop throughout the remainder of the book Also in Chapter 4 is a detailed discussion of regular expressions which are used by many Unix commands such as sed grep and ed Chapters 5 through 10 teach you how to put the shell to work for writing programs You ll learn how to write your own commands use variables write programs that accept arguments make decisions use the Shell s for while and until looping
71. first before sending the remainder to the background mon fred Prompt comes back so you can continue working fred has logged on Note that a process id number is not printed by the shell when a command is sent to the background within a shell program Another Way to Pass Variables to a Subshell If you want to send the value of a variable to a subshell there s another way to do it besides setting the variable and then exporting it On the command line you can precede the name of the command with the assignment of as many variables as you want For example DBHOME uxn2 data DBID 452 dbrun places the variables DBHOME and DBID and their indicated values into the environment of dbrun and then dbrun gets executed These variables will not be known to the current shell they re created only for the execution of dbrun In fact execution of the preceding command behaves identically to typing DBHOME uxn2 data DBID 452 export DBHOME DBID dbrun Here s a short example cat fool echo x foo2 cat foo2 echo x fool x not known to fool or foo2 x 100 fool Try it this way 7 100 x is known to fool 100 and to its subshells echo x Still not known to current shell So variables defined this way otherwise behave as normal exported variables to the subshell ns Ga os fa Your profile File In Chapter 3 What Is the Shell you learned about the login sequence This sequence is completed whe
72. in the program at the point that it left off when the signal was received Signal number 1 is generated for hangup Either someone intentionally hangs up the line or the line gets accidentally disconnected You can modify the preceding trap to also remove the two specified files in this case by adding signal number 1 to the list of signals trap rm WORKDIR work1 WORKDIR dataout exit 1 2 Now these files will be removed if the line gets hung up or if the Delete key gets pressed The commands specified to trap must be enclosed in quotes if they contain more than one command Also note that the shell scans the command line at the time that the trap command gets executed and also again when one of the listed signals is received So in the preceding example the value of WORKDIR and will be substituted at the time that the trap command is executed If you wanted this substitution to occur at the time that either signal 1 or 2 was received for example WORKDIR may not have been defined yet you can put the commands inside single quotes trap rm WORKDIR work1 WORKDIR dataout exit 1 2 The trap command can be used to make your programs more user friendly In the next chapter when we revisit the rolo program the signal generated by the Delete key is caught by the program and brings the user back to the main menu In this way this key can be used to abort the current operation without exiting from the program trap with No Argume
73. in the regular expression which may itself be another regular expression So for example the regular expression xX matches zero one two three capital X s The expression XX matches one or more capital X s because the expression specifies a single X followed by zero or more X s A similar type of pattern is frequently used to match the occurrence of one or more blank spaces ed Lotsaspaces 85 1 p This is an example of a file that contains a lot of blank spaces Change multiple blanks to single blanks 1 s 9 1 p This is an example of a file that contains a lot of blank spaces The ed command 1 8 9 told ed to substitute all occurrences of a space followed by zero or more spaces with a single space The regular expression is often used to specify zero or more occurrences of any characters Bear in mind that a regular expression matches the longest string of characters that match the pattern Therefore used by itself this regular expression always matches the entire line of text As another example of the combination of and the regular expression e e matches all the characters from the first e on a line to the last one ed intro 248 1 s e e 1 p Th n Thompson and D S in th primary goals in th an nt program d nt Here s an interesting regular expression What do you think it matches A Za z A Za z That s right this matches
74. include your own bin sets your CDPATH changes your primary and secondary command prompts changes your erase character to a backspace Ctri h with the stty command and prints a friendly message using the greetings program from Chapter 8 Decisions Decisions cat HOME profile PATH bin usr bin usr lbin HOME bin CDPATH HOME HOME misc HOME documents export PATH CDPATH PS1 PS2 stty echoe erase CTRL h echo greetings Here s what a login sequence would look like with this profile login steve Password Good morning Output from greetings gt New PS1 rs ca The term Variable If you tend to use more than one type of terminal the profile is a good place to put some code to prompt for the terminal type and then set the TERM variable accordingly This variable is used by screen editors such as vi and other screen based programs A sample section of code from a profile file to prompt for the terminal type might look like this echo What terminal are you using xterm is the default c read TERM if z TERM then TERM xterm Fi export TERM Based on the terminal type entered you may also want to do things such as set up the function keys or the tabs on the terminal Even if you always use the same terminal type you should set the TERM variable in your profile file u Ga rs ca The 1z Variable The TZ variable is used by the date command and some Standard C library f
75. information provided is on an as is basis The authors and the publisher shall have neither liability nor responsibility to any person or entity with respect to any loss or damages arising from the information contained in this book Credits Acquisitions Editor Katie Purdum Development Editor Scott Meyers Managing Editor Charlotte Clapp Copy Editor Geneil Breeze Indexer Erika Millen Proofreader Jessica McCarty Technical Editor Michael Watson Interior Designer Gary Adair Cover Designer Gary Adair Page Layout Susan Geiselman Dedication To my father Harry Wood Patrick Wood To Gregory Linda and Julia for giving meaning to my life Stephen G Kochan 4 PREviows Next About the Authors Stephen G Kochan is the owner of TechFitness a technology based fitness company Prior to that he was president and CEO of Pipeline Associates a company specializing in color printing software Mr Kochan is the author of several best selling books on Unix and C programming including the best selling Programming in C He also acted as Series Editor for the Hayden Unix System Library Patrick Wood is the CTO of the New Jersey location of Electronics for Imaging He was a member of the technical staff at Bell Laboratories when he met Mr Kochan in 1985 Together they founded Pipeline Associates Inc a Unix consulting firm where he was the Vice President They coauthored Exploring the Unix System Unix System Securi
76. line mon m t 600 ann amp is executed the following occurs inside the while loop in mon getopts is executed and it stores the character m inside the variable option sets OPTIND to two and returns a zero exit status The case command is then executed to determine what was stored inside option A match on the character m indicates that the send mail option was selected so mailopt is set to TRUE Note that the inside the case is quoted This is to remove its special meaning as a pattern matching character from the shell The second time getopts is executed getopts stores the character t inside option stores the next command line argument 600 inside OPTARG sets OPTIND to three and returns a zero exit status The case command then matches the character t stored inside option The code associated with that case copies the value of 600 that was stored in OPTARG into the variable interval The third time getopts is executed getopts returns a nonzero exit status indicating the end of options The program then checks the value of OPTIND against to make sure that the username was typed on the command line If OPTIND is greater than then no more arguments remain on the command line and the user forgot the username argument Otherwise the shift command is executed to move the username argument into 1 The actual number of places to shift is one less than the value of OPTIND The rest of the mon program remains as before the only
77. line Also remember that the pattern matches are anchored In the case of the and constructs the variables must end with the specified pattern in the case of the and constructs the variable must begin with it Here are some simple examples to show how these constructs work var testcase echo var testcase echo var e testcas echo var testcase echo var s e testca echo var s e te echo var e stcase echo var s tcase echo var s e echo var test case echo var teas testcase Remove e from right Variable is unchanged Remove Remove Remove Remove Remove Remove smallest match from right longest match smallest match from left smallest match from left longest match from left test from left No match There are many practical uses for these constructs even though these examples don t seem to show it For example the following tests to see whether the filename stored inside the variable file ends in the two characters o if filex o file then file ends in o fi As another example here s a shell program that works just like the Unix system s basename command cat mybasename echo 1 The program displays its argument with all the characters up to the last removed mybasename usr spool uucppublic uucppublic mybasename HOME steve mybasename memos memos var
78. mark 2nd brackets 2nd examples 2nd multiple character matching 2nd 3rd 4th single character matching 2nd 3rd variables 2nd 3rd filenames linking dangling symbolic links In command 2nd 3rd 4th 5th 6th symbolic links 2nd listing Is command 2nd 3rd 4th 5th 6th moving between directories mv command 2nd ordinary files paths CDPATH variable 2nd 3rd 4th PATH variable 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th 12th 13th 14th phonebook file adding entries to 2nd 3rd deleting entries from 2nd 3rd 4th looking up entries 2nd 3rd reading data from 2nd renaming mv command 2nd 3rd searching grep command 2nd 3rd 4th 5th 6th 7th 8th 9th 10th sorting 2nd arithmetic sorts 2nd delimiter characters 2nd 3rd duplicate lines 2nd reversing sort order 2nd skipped fields 2nd writing sorted data to standard output 2nd source files special files temporary files 2nd 3rd fils reading command filters 2nd finding home directories 2nd phone book entries 2nd 3rd 4th 5th for command 2nd 3rd variable variable 2nd 3rd examples 2nd 3rd 4th 5th for without the list 2nd syntax 2nd foreground executing jobs in foreground running jobs in formatted output printing 2nd 3rd 4th 5th 6th 7th command syntax conversion specification characters 2nd conversion sp
79. mary had a little lammb mary had a little lammb Esc mary had a little lammb mary had a little lammb X mary had a little Lamb mary had a little lamb a mary had a little lamb_ mary had a little lamb_ da mary had a little lambda If you think that moving the cursor around by repeatedly pressing h and l is slow you re right The h and l commands may be preceded by a number that specifies the number of spaces to move the cursor mary had a little lambda Esc mary had a little Lambda mary had a little lambda 10h mary had a little Lambda mary had a little lambda 13h mary had a little Lambda mary had a little lambda 5x had a little lambda As you can see the x command can also be preceded by a number to tell it how many characters to delete You can easily move to the end of the line by typing the command had a little lambda had a little lambda To move to the beginning of the line you use the 0 that s a zero command had a little lambda 0 had a little lambda Two other commands useful in moving the cursor are the w and b commands The w command moves the cursor forward to the beginning of the next word where a word is a string of letters numbers and underscores delimited by blanks or punctuation The b command moves the cursor backward to the beginning of the previous word These commands may also be preceded by a number to specify the number of words to move forward or backward had a li
80. mexr e Table of Contents e Index Unix Shell Programming Third Edition By Stephen G Kochan Patrick Wood Publisher Sams Publishing Pub Date February 27 2003 ISBN 0 672 32490 3 Pages 456 Slots 1 Unix Shell Programming is a tutorial aimed at helping Unix and Linux users get optimal performance out of their operating out of their operating system It shows them how to take control of their systems and work efficiently by harnessing the power of the shell to solve common problems The reader learns everything he or she needs to know to customize the way a Unix system responds The vast majority of Unix users utilize the Korn shell or some variant of the Bourne shell such as bash Three are covered in the third edition of Unix Shell Programming It begins with a generalized tutorial of Unix and tools and then moves into detailed coverage of shell programming Topics covered include regular expressions the kernel and the utilities command files parameters manipulating text filters understanding and debugging shell scripts creating and utilizing variables tools processes and customizing the shell mer e Fe rrevious NEXT gt Table of Contents Index Unix Shell Programming Third Edition By Stephen G Kochan Patrick Wood Publisher Sams Publishing Pub Date February 27 2003 ISBN 0 672 32490 3 Pages 456 Slots 1 Copyright About the Authors We Want to Hear from You
81. must be specified for a signal to be ignored and is not equivalent to writing the following which has a separate meaning of its own trap 2 If you ignore a signal all subshells also ignore that signal However if you specify an action to be taken on receipt of a signal all subshells will still take the default action on receipt of that signal For the signals we ve described this means that the subshells will be terminated Suppose that you execute the command trap 2 and then execute a subshell which in turn executes other shell programs as subshells If an interrupt signal is then generated it will have no effect on the shells or subshells that are executing because they will all ignore the signal If instead of executing the previous trap command you execute trap 2 and then execute your subshells then on receiving the interrupt signal the current shell will do nothing it will execute the null command but all active subshells will be terminated they will take the default action termination Resetting Traps After you ve changed the default action to be taken on receipt of a signal you can change it back again with trap if you simply omit the first argument so trap 12 resets the action to be taken on receipt of signals 1 or 2 back to the default ns Ga i fo More on I O You know about the standard constructs lt gt and gt gt for input redirection output redirection and output redirection with
82. ns ca The read Command The general format of the read command is read variables When this command is executed the shell reads a line from standard input and assigns the first word read to the first variable listed in variables the second word read to the second variable and so on If there are more words on the line than there are variables listed the excess words get assigned to the last variable So for example the command read x y reads a line from standard input storing the first word read in the variable x and the remainder of the line in the variable y It follows from this that the command read text reads and stores an entire line into the shell variable text A Program to Copy Files Let s put the read command to work We ll write a simplified version of the cp command that will be a bit more user friendly than the standard Unix one We ll call it mycp and we ll have it take two arguments the source file and the destination file If the destination file already exists we ll tell the user and then ask him or her if he wants to proceed with the copy If the answer is yes we ll go ahead with it otherwise we won t cat mycp Copy a file if ne 2 then echo Usage mycp from to exit 1 fi from 1 to 2 See if the destination file already exists if e to then echo to already exists overwrite yes no read answer if answer yes then
83. on Instead of printing nothing if the requested user is not logged on we ll have the program print a message to that effect Here is version 3 of the program cat on determine if someone is logged on version 3 user 1 if who grep user gt dev null then echo user is logged on else echo user is not logged on fi If the user specified as the first argument to on is logged on the grep will succeed and the message user is logged on will be displayed otherwise the message user is not logged on will be displayed who Who s on root console Jul 8 10 37 barney tty03 Jul 8 12 38 fred tty04 Jul 8 13 40 joanne tty07 Jul 8 09 35 tony tty19 Jul 8 08 30 lulu tty23 Jul 8 09 55 on pat pat is not logged on on tony tony is logged on Another nice touch when writing shell programs is to make sure that the correct number of arguments is passed to the program If an incorrect number is supplied an error message to that effect can be displayed together with information on the proper usage of the program cat on determine if someone is logged on version 4 see if the correct number of arguments were supplied if ne 1 then echo Incorrect number of arguments echo Usage on user else user 1 if who grep user gt dev null then echo user is logged on else echo user is not logged on fi fi Compare this program with the prev
84. os i rs ca The test Command A built in shell command called test is most often used for testing one or more conditions in an if command Its general format is test expression where expression represents the condition you re testing test evaluates expression and if the result is true it returns an exit status of zero otherwise the result is false and it returns a nonzero exit status String Operators As an example of the use of test the following command returns a zero exit status if the shell variable name contains the characters julio test name julio The operator is used to test whether two values are identical In this case we re testing to see whether the contents of the shell variable name are identical to the characters julio If it is test returns an exit status of zero nonzero otherwise Note that test must see all operands name and julio and operators as separate arguments meaning that they must be delimited by one or more whitespace characters Getting back to the if command to echo the message Would you like to play a game if name contains the characters julio you would write your if command like this if test name julio then echo Would you like to play a game fL Why is it better to play it safe and enclose the message that is displayed by echo inside quotes When the if command gets executed the command that follows the if is executed and its exit status is tested The
85. output before getting into the explanation ls bin lu names progl prog2 ls bin lu nu progl mycp Usage mycp filel file2 mycp file s dir mycp names progl prog2 Usage mycp filel file2 mycp file s dir mycp names progl prog2 lu bin See what s around And what s in bin No arguments Last arg isn t a directory Legitimate use bin progl already exists overwrite yes no yes bin lu already exists overwrite yes no no ls l bin total 5 rWw r r 1 steve rW r r 1 steve rWw r r 1 steve rWw r r 1 steve rW r r 1 steve In the last case progl was overwritten and lu wasn t as per the user s request When the program starts execution it saves the number of arguments in the variable numargs This is done because it s changed later in the program by the shift command Next a loop is entered that is executed as long as the number of arguments is greater than one The purpose of this loop is to get the last argument on the line While doing this the loop stashes away the first argument into the shell variable filelist which contains a list of all the files to be copied The statement filelist filelist 1 says to take the previous value of filelist add on a space followed by the value of 1 and then store the result back into filelist Then the shift command is executed to move all the arguments over by one Eventually will be equal to one and the loop w
86. packed into a single line This is done by replacing the terminating newline characters in an entry with a special character We arbitrarily chose the caret The only restriction here is that this character not be used as part of the entry itself Using this technique the first entry shown would be stored in the phone book file as Steve s Ice Cream 444 6th Avenue New York City 10003 212 555 3021 and the second entry shown as YMCA 201 555 2344 You ll shortly see how convenient it becomes to process the entries when they re stored in this format Now we ll describe each program written for the rolodex program a next gt rolo rolo rolodex program to look up add remove and change entries from the phone book Set PHONEBOOK to point to the phone book file and export it so other progs know about it if it s set on entry then leave it alone PHONEBOOK HOME phonebook export PHONEBOOK if e PHONEBOOK then echo PHONEBOOK does not exist echo Should I create it for you y n c read answer if answer y then exit 1 fi gt PHONEBOOK exit 1 exit if the creation fails fi If arguments are supplied then do a lookup if ne O then lu exit fa Set trap on interrupt DELETE key to continue the loop trap continue 2 Loop until user selects exit while true do Display menu echo
87. processes are automatically terminated when you log off the system If you want a program to continue executing after you ve logged off you can run it with the nohup command or schedule it to run with at or from the cron Consult your Unix User s Manual for more details Because mon only checks once per minute for the user s logging on it won t hog the system s resources while it s running an important consideration when submitting programs to the background for execution Unfortunately after the specified user logs on there s a chance you might miss that one line message you may be cating a file and might not even notice it come and go right off your screen Also if you re editing a file with a screen editor such as vi when the message comes it may turn your screen into a mess and you still might miss the message A better alternative to writing the message to the terminal might be to mail it instead Actually you can let the user select his or her preference by adding an option to the program that if selected indicates that the message is to be mailed If the option is not selected the message can be displayed at the terminal In the version of mon that follows a m option has been added for this purpose cat mon Wait until a specified user logs on version 2 if 1 m then mailopt TRUE shift else mailopt FALSE fi if eq 0 o gt 1 then echo Usage mon m user echo m mean
88. profile file discussed later in this chapter u rs ca HOME James Your home directory is where you re placed whenever you log on to the system A special shell variable called HOME is also automatically set to this directory when you log on echo HOME users steve This variable can be used by your programs to identify your home directory It s also used by the cd command whenever you type just cd with no arguments pwd Where am I usr src lib libc port stdio cd pwd users steve There s no place like home You can change your HOME variable to anything you want but be warned that doing so may affect the operation of any programs that rely on it HOME users steve book Change it pwd users steve cd pwd See what happened users steve book es Gy Your PATH Return for a moment to the rolo program from Chapter 10 rolo Liz Liz Stachiw 212 555 2298 Let s see what directory this program was created in pwd users steve bin Okay now change directory to anywhere you want cd Go home And now try to look up Liz in the phone book rolo Liz sh rolo not found Reading and Printing Data Unless you already know where this discussion is leading you are likely to get the preceding results Whenever you type in the name of a program to be executed the shell searches a list of directories until it 1 finds the requested program
89. prog1 in your current directory and you want to copy it into your bin directory directly below Take another look at the mycp program and determine what happens if you type in mycp progl bin The e test on bin will succeed because e tests for existence of a file and mycp will display the already exists message and wait for a yes no answer If the second argument is a directory mycp should check to see whether the from file exists inside this directory The next version of mycp performs this check It also has the modified echo command that includes the c to suppress the terminating newline cat mycp Copy a file version 2 if ne 2 then echo Usage mycp from to exit 1 fi from 1 to 2 See if destination file is a directory if d to then to to basename from fL See if the destination file already exists if e to then echo to already exists overwrite yes no c read answer if answer yes then echo Copy not performed exit 0 fi Ti Either destination doesn t exist or yes was typed cp from to proceed with the copy If the destination file is a directory the program changes the variable to to more precisely identify the file inside the directory as to basename from This ensures that the following test on the existence of the ordinary file to will be done on the file in the director
90. quotes indicate to the shell that variable name substitution is still to be performed inside the quotes So the shell substitutes for x Because filename substitution is not done inside double quotes is then passed to echo as the value to be displayed So if you want to have the value of a variable substituted but don t want the shell to treat the substituted characters specially you must enclose the variable inside double quotes Here s another example illustrating the difference between double quotes and no quotes address 39 East 12th Street gt New York N Y 10003 echo address 39 East 12th Street New York N Y 10003 echo address 39 East 12th Street New York N Y 10003 It makes no difference whether the value assigned to address is enclosed in single quotes or double quotes The shell displays the secondary command prompt in either case to tell you it s waiting for the corresponding closed quote After assigning the two line address to address the value of the variable is displayed by echo Notice that the address is displayed on a single line The reason is the same as what caused echo one two three four to be displayed as one two three four Recalling that the shell removes spaces tabs and newlines that is whitespace characters from the command line and then cuts it up into arguments in the case of echo address the shell simply removes the embedded newline character treating it as
91. r command allows you to substitute the first occurrence of one string with the next To re execute the last cat command on the file planB instead of planA you could type r cat planA planB cat docs planB or even more simply you could have typed r cat A B cat docs planB Bash has the built in command re executes the previous command and string re executes the most recent command line from your history that begins with string l cat docs planB d date Thu Oct 24 14 39 40 EST 2002 Note that no spaces can exist between and string The fc command can be used with the s option to do the same thing with any POSIX compliant shell the r command is actually an alias to the fc command in the Korn shell more on that later in this chapter fc s cat cat docs planB fc s B C cat docs planC rs ca Functions Bash and the Korn shell both have function features not available in the POSIX standard shell Local Variables Bash and Korn shell functions can have local variables making recursive functions possible They are defined with the typeset command as in typeset i j If a variable of the same name as a local function variable exists it is saved when the typeset is executed and restored when the function exits Note that the typeset command is not part of the POSIX standard Shell After using the shell for a while you may develop a set of functions that you like to use during
92. red Find blue objects Stop it again Back to the background Keep working until it s ready rrevious Next gt The Restricted Shell rsh Although the restricted shell is not part of the POSIX standard it is supported by every Bourne shell variant we know of The restricted shell is almost the same as the regular shell but it s designed to restrict a user s capabilities by disallowing certain actions that the standard shell allows This allows an administrator to let users who should not have complete access to the system use the shell It is usually found in usr lib rsh and is started as the login shell for a user who should not have full capabilities on a system for example a game user or data entry clerk The list of actions disallowed is very short Cannot change directory cd Cannot change PATH ENV or SHELL variables Cannot specify a path to a command Cannot redirect output gt and gt gt Cannot exec programs These restrictions are enforced after the profile is executed when logging in and the user is logged off if he presses Break or Delete while the profile is being interpreted These simple restrictions allow the writer of a restricted user s profile to have control over what commands that user can use The following example shows a simple setup for a restricted environment cat profile User restrict s profile PATH usr rbin users restrict bin export PATH SHELL usr lib rsh Some commands use SHE
93. reference the same directory mv file s dir ps pwd rm file s rmdir dir s sort file s wc file s who Move file s into directory dir List information about active processes Display current working directory path Remove files s Remove empty directory dir s Sort lines of file s or standard input if not supplied Count the number of lines words and characters in file s or standard input if not supplied Display who s logged in Exercises 1 Given the following files in your current directory ls feb96 jan12 02 jan19 02 jan26 02 jan5 02 jan95 jan96 jan97 jan98 mar98 memol memo10 memo2 memo2 sv What would be the output from the following commands echo echo m a df z echo jan echo jan feb mar echo echo echo echo echo 10 9 A Z a i fjm ae bnr 2 What is the effect of the following command sequences ls we l rm who wc l mv progs users steve backup ls c we l rm 0 who sort cd pwd cp memol plotdata 2 gt errors amp Chapter 3 What Is the Shell IN THIS CHAPTER The Kernel and the Utilities The Login Shell Typing Commands to the Shell The Shell s Responsibilities In this chapter you ll learn what the shell is and what it does Team LiB u Ig next 5 The Kernel and the Utilities The Unix system is itself logically divided into two pieces the kernel and the utilities see Figure 3
94. reply You know that two directories are directly below steve s home directory documents and programs In fact this can be verified at the terminal by issuing the ls command ls documents programs The Ls command lists the two directories documents and programs the same way it listed other ordinary files in previous examples To change your current working directory issue the cd command followed by the name of the directory to change to cd documents After executing this command you will be placed inside the documents directory as depicted in Figure 2 5 Figure 2 5 cd documents users pat steve ruth gt documents programs r J memos proposals letters wb collect mon plan dact sysA newhire no JSK AMG reply You can verify at the terminal that the working directory has been changed by issuing the pwd command pwd users steve documents The easiest way to get one level up in a directory is to issue the command CO are because by convention always refers to the directory one level up Known as the parent directory see Figure 2 6 cd pwd users steve Figure 2 6 cd users pat gt steve ruth documents programs memos proposals letters wb collect mon plan dact sysA newhire no JSK AMG reply If you wanted to change to the letters directory you could get there with a single cd command by specifying the relative path documents letters
95. runs a null command in the background Also make sure that the program correctly handles the case where the process number loops back around to 1 after the maximum is reached So collect t 900 gt stats amp should start up collect to gather the desired statistics every 15 minutes and write them into the file stats Write a shell program called wgrep that searches a file for a given pattern just as grep does For each line in the file that matches print a window around the matching line That is print the line preceding the match the matching line and the line following the match Be sure to properly handle the special cases where the pattern matches the first line of the file and where the pattern matches the last line of the file 7 Modify wgrep to take an optional w option that specifies the window size so wgrep w 3 UNIX text should print three lines before and after each line from text that contains the pattern UNIX 8 Modify wgrep to take a variable number of filenames as arguments Precede each output line with the name of the file in which the match occurs as grep does Fe erevious next 4 Previous NEXT Chapter 10 Reading and Printing Data IN THIS CHAPTER e The read Command e The printf Command e Exercises In this chapter you ll learn how to read data from the terminal or from a file using the read command and how to print formatted data to standard output using the printf command ne Ga
96. s a highly manufactured example of the output from a for command piped into wc foriinl234 gt do gt echo i gt done wc l 4 Typing a Loop on One Line If you find yourself frequently executing loops directly at the terminal you ll want to use the following shorthand notation to type the entire loop on a single line Put a semicolon after the last item in the list and one after each command in the loop Don t put a semicolon after the do Following these rules the loop for iin1il234 do echo i done becomes for i in 1 2 3 4 do echo i done And you can type it in directly this way for i in 1 2 3 4 do echo i done 1 2 The same rules apply to while and until loops if commands can also be typed on the same line using a similar format if 1 1 then echo yes fi yes if 1 2 then echo yes else echo no fi no Note that no semicolons appear after the then and the else rs ca The getopts Command Let s extend our mon program further We ll add a t option to it that specifies the time interval in seconds to perform the check Now our mon program takes both m and t options We ll allow it to take these options in any order on the command line provided that if they are used they appear before the name of the user that we re monitoring So valid mon command lines look like this mon ann mon m ann mon t 600 ann mon m t 600 ann mon t 600 m ann and invali
97. s output use space as delimiter addresses intro lotsaspaces names numbers phonebook In the preceding example the output from Ls is piped to paste which merges the lines s option from standard input separating each field with a space d option Of course you ll recall from Chapter 2 A Quick Review of the Basics that the command echo would have worked just as well and is certainly more straightforward ns Gy rs i sed sed is a program used for editing data It stands for stream editor Unlike ed sed cannot be used interactively However its commands are similar The general form of the sed command is sed command file where command is an ed style command applied to each line of the specified file If no file is specified standard input is assumed As sed applies the indicated command to each line of the input it writes the results to standard output Recall the file intro from previous examples cat intro The Unix operating system was pioneered by Ken Thompson and Dennis Ritchie at Bell Laboratories in the late 1960s One of the primary goals in the design of the Unix system was to create an environment that promoted efficient program development Suppose that you want to change all occurrences of Unix in the text to UNIX This can be easily done in sed as follows sed s Unix UNIX intro Substitute Unix with UNIX The UNIX operating system was pioneered by Ken Thompson and D
98. same time there s a chance that the phone book file can get messed up because the same temporary file will be used by all rolo users H Naturally the chances of this happening that is the preceding two commands being executed at the same time by more than one user are rather small but nevertheless there still is that chance Anyway it brings up an important point when dealing with temporary files in general 1 Actually it depends on the users default file creation mask known as umask If one person has created tmp phonebook and it s not writable by anyone else the next person who comes along and tries to create it will get an error message from the shell The net result is that the first user s file will get properly updated and the second user s won t neither file will get corrupted When writing shell programs to be run by more than one person make your temporary files unique One way is to create the temporary file in the user s home directory for example Another way is to choose a temporary filename that will be unique for that particular process To do this you can use the special shell variable which contains the process id number PID of the current process echo 4668 ps PID TTY TIME COMMAND 4668 co 0 09 sh 6470 co 0 03 ps As you can see is equal to the process id number of your login shell Because each process on the Unix system is given a unique process id number using the value of in
99. see Figure 2 7 cd documents letters pwd users steve documents letters Figure 2 7 cd documents letters users pat steve ruth docu programs letters lt memos proposals wb collect mon 4 fo plan dact sys A newhire no JSK AMG reply You can get back up to the home directory by using a single cd command to go up two directories as shown cd ifo pwd users steve Or you can get back to the home directory using a full pathname rather than a relative one cd users steve pwd users steve Finally there is a third way to get back to the home directory that is also the easiest Typing the command cd without an argument always places you back into your home directory no matter where you are in your directory path cd pwd users steve More on the 1s Command When you type the ls command the files contained in the current working directory are listed But you can also use Ls to obtain a list of files in other directories by supplying an argument to the command First let s get back to your home directory cd pwd users steve Now let s take a look at the files in the current working directory ls documents programs If you supply the name of one of these directories to the ls command you can get a list of the contents of that directory So you can find out what s contained in the documents directory simply by typing the command ls d
100. shell from replacing the first range with all the files in your directory named a through z and the second range with all the files in your directory named A through Z What do you think happens if no such files exist By reversing the two arguments to tr you can use it to translate all uppercase letters to lowercase tr A Z a z lt intro the unix operating system was pioneered by ken thompson and dennis ritchie at bell laboratories in the late 1960s one of the primary goals in the design of the unix system was to create an environment that promoted efficient program development The s Option You can use the s option to tr to squeeze out multiple occurrences of characters in to chars In other words if more than one consecutive occurrence of a character specified in to chars occurs after the translation is made the characters will be replaced by a single character For example the following command translates all colons into tab characters replacing multiple tabs with single tabs tr s 2 IT So one colon or several consecutive colons on the input will be replaced by a single tab character on the output Suppose that you have a file called Lotsaspaces that has the contents as shown cat lotsaspaces This is an example ofa file that contains a lot of blank spaces You can use tr to squeeze out the multiple spaces by using the s option and by specifying a single space character as the fi
101. term pattern is used here because the shell allows you to use the same pattern matching characters that it accepts in filename substitution and case values to match zero or more characters to match any single character to match any single character from the specified set and to match any single character not in the specified set When you write the construct variablespattern the shell looks inside variable to see whether it ends with the specified pattern If it does the contents of variable are substituted on the command line with the shortest matching pattern removed from the right If you use the construct variable pattern the shell once again looks inside variable to see whether it ends with pattern This time however it removes the longest matching pattern from the right This is relevant only if the is used in pattern Otherwise the and behave the same way The is used in a similar way to force the pattern matching to occur on the left rather than the right So the construct variable pattern tells the shell to substitute the value of variable on the command line with pattern removed from the left Finally the shell construct variable pattern works like the form only the longest occurrence of pattern is removed from the left Remember that in all four cases no permanent changes are made to the variable itself you are affecting only what gets substituted on the command
102. the arguments aren t correctly specified for example wrong number of arguments or more than two arguments and the last one isn t a directory In the case of grep an exit status of zero Success is returned if it finds the specified pattern in at least one of the files a nonzero value is returned if it can t find the pattern or if an error occurs the arguments aren t correctly specified or it can t open one of the files In a pipeline the exit status is that of the last command in the pipe So in who grep fred the exit status of the grep is used by the shell as the exit status for the pipeline In this case an exit status of zero means that fred was found in who s output that is fred was logged on at the time that this command was executed The Variable The shell variable is automatically set by the shell to the exit status of the last command executed Naturally you can use echo to display its value at the terminal cp phonebook phone2 echo 0 Copy succeeded cp nosuch backup cp cannot access nosuch echo 2 Copy failed who See who s logged on root console Jul 8 10 06 wilma tty03 Jul 8 12 36 barney tty04 Jul 8 14 57 betty tty15 Jul 8 15 03 who grep barney barney tty04 Jul 8 14 57 echo Print exit status of last command grep 0 grep succeeded who grep fred echo 1 grep failed echo 0 Exit status of last echo Note that the numeric result of
103. the single argument l see Figure 3 13 Figure 3 13 Execution of wc l lt users arguments When wc begins execution this time it sees that it was passed the single argument 1l Because no filename was specified wc takes this as an indication that the number of lines appearing on standard input is to be counted So wc counts the number of lines on standard input unaware that it s actually counting the number of lines in the file users The final tally is displayed at the terminal without the name of a file because wc wasn t given one The difference in execution of the two commands is important for you to understand If you re still unclear on this point review the preceding section Pipeline Hookup Just as the shell scans the command line looking for redirection characters it also looks for the pipe character For each such character that it finds it connects the standard output from the command preceding the to the standard input of the one following the It then initiates execution of both programs So when you type who wc l the shell finds the pipe symbol separating the commands who and wc It connects the standard output of the former command to the standard input of the latter and then initiates execution of both commands When the who command executes it makes a list of who s logged in and writes the results to standard output unaware that this is not going to the terminal but to another command instead
104. to the specified process where job is a process id number or job_id and signal is a number or one of the signal names specified in lt signal h gt see the description of trap later in the chapter kill l lists these names A signal number supplied with the l option lists the corresponding signal name A process id used with the l option lists the name of the signal that terminated the specified process if it was terminated by a signal The s option can also be used when a signal name is supplied in which case the dash before the name is not used see the following example If signal isn t specified TERM is used Examples kill 9 1234 Kill HUP 2 Kill s TERM 2 kill 1 Note that more than one process id can be supplied to the kill command on the command line The newgrp Command General Format newgrp group This command changes your real group id GID to group If no argument is specified it changes you back to your default group Examples newgrp shbook Change to group shbook newgrp Change back to default group If a password is associated with the new group and you are not listed as a member of the group you will be prompted to enter it newgrp l changes you back to your login group The pwd Command General Format pwd This command tells the shell to print your working directory which is written to standard output Examples pwd users steve documents memos cd pwd users steve
105. to the string new in the command s to be re executed vi Line Edit Mode The shell has a built in implementation of the vi screen editor scaled down to work on single lines When vi mode is turned on you are by default placed in a state similar to vi s input mode Commands can be typed just the same as when vi mode is off At any time however you can press the Esc key to be placed in edit mode At this point most vi commands will be interpreted by the shell The current command line can be edited as can any of the lines in the command history Pressing Enter at any point in either command or input mode causes the command being edited to be executed Table A 4 lists all the editing commands in vi mode Note count is any integer and may be omitted Table A 4 vi Editing Commands Input Mode Commands Command Meaning erase Ctrl w kill eof Ctril v Enter Esc Edit Mode Commands Command count k count count j count count G string Erase character usually Ctri h or delete previous character Delete the previous blank separated word Line kill character normally Ctri u or delete the entire current line End of file character normally Ctrl d terminate the shell if the current line is empty Quote next character editing characters and the erase and kill characters may be entered in a command line or in a search string if preceded by a Ctrl v Execute the current l
106. up editor on the confirmed entry echo line c tr Q12 gt tmp ed echo Enter changes with EDITOR bin ed trap 2 don t abort if DELETE hit while editing EDITOR tmp ed Remove old entry now and insert new one grep v line PHONEBOOK gt tmp phonebook tr 012 lt tmp ed echo gt gt tmp phonebook last echo was to put back trailing newline translated by tr sort tmp phonebook o PHONEBOOK rm tmp ed tmp phonebook The change program allows the user to edit an entry in the phone book The initial code is virtually identical to rem it finds the matching entries and then prompts the user to select the one to be changed The selected entry is then written into the temporary file tmp ed with the characters translated to newlines This unfolds the entry into separate lines for convenient editing The program then displays the message echo Enter changes with EDITOR bin ed which serves a dual purpose It tells the user what editor will be used to make the change while at the same time setting the variable EDITOR to bin ed if it s not already set This technique allows the user to use his or her preferred editor by simply assigning its name to the variable EDITOR and exporting it before executing rolo EDITOR vi export EDITOR rolo The signal generated by the Delete key 2 is ignored so that if the user presses this key while in the edi
107. use shell variables ne Ga a Command Files A shell program can be typed directly at the terminal as in who wc l or it can be first typed into a file and then the file can be executed by the shell For example suppose that you need to find out the number of logged in users several times throughout the day It s not unreasonable to type in the preceding pipeline each time you want the information but for the sake of example let s type this pipeline into a file We ll call the file nu for number of users and its contents will be just the pipeline shown previously cat nu who wc l To execute the commands contained inside the file nu all you now have to do is type nu as the command 1 name to the shell 1 Note that the error produced here varies between different shells nu sh nu cannot execute Oops We forgot to mention one thing Before you can execute a program this way you must change the file s permissions and make it executable This is done with the change mode command chmod To add execute permission to the file nu you simply type chmod x file s The x says make the file s that follow executable The shell requires that a file be both readable and executable by you before you can execute it ls l nu rw rw r 1 steve steve 12 Jul 10 11 42 nu chmod x nu Make it executable ls l nu FWXrWwWXr x 1 steve steve 12 Jul 10 11 42 nu Now that you ve made it exec
108. users steve The cd executed in cdtest had no effect on your current directory Because the current directory is part of the environment when a cd is executed from a subshell the current directory of that subshell is altered There is no way to change the current directory of a parent shell from a subshell When cd is invoked it sets the PWD shell variable to the full pathname of the new current directory so the command echo PWD produces the same output as the pwd command pwd users steve echo PWD users steve cd bin echo PWD users steve bin cd also sets OLDPWD to the full pathname of the previous current directory Incidentally cd is a shell built in command CDPATH The CDPATH variable works like the PATH variable It specifies a list of directories to be searched by the shell whenever you execute a cd command This search is done only if the specified directory is not given by a full pathname and if CDPATH is not null obviously So if you type in cd users steve the shell changes your directory directly to users steve but if you type cd memos the shell looks at your CDPATH variable to find the memos directory And if your CDPATH looks like this echo CDPATH users steve users steve docs the shell first looks in your current directory for a memos directory and if not found then looks in users steve for a memos directory and if not found there tries users steve docs in a l
109. values to variables directly at your terminal count 1 Assign character 1 to count my bin users steve bin Assign users steve bin to my bin So now that you know how to assign values to variables what good is it Glad you asked Displaying the Values of Variables The echo command is used to display the value stored inside a shell variable To do this you simply write echo variable The character is a special character to the shell If a valid variable name follows the the shell takes this as an indication that the value stored inside that variable is to be substituted at that point So when you type echo count the shell replaces count with the value stored there then it executes the echo command echo count 1 Remember the shell performs variable substitution before it executes the command see Figure 5 1 Figure 5 1 echo count echo i ae eee You can have the value of more than one variable substituted at a time echo my bin users steve bin echo my bin count users steve bin 1 In the second example the shell substitutes the value of my bin and count and then executes the echo command see Figure 5 2 Figure 5 2 echo my_ bin count users steve bin The values of variables can be used anywhere on the command line as the next examples illustrate ls my bin mon nu testx pwd Where are we users steve documents memos cd my bin Change to my bin
110. with usr bin perl forces the shell to execute usr bin perl on it You have to use this feature with caution however because many programs such as Perl don t reside in a standard place on every Unix system Also this is not a feature specified by the POSIX standard even though it s found in every modern shell we ve seen and is even implemented at the operating system level on many Unix versions Fe previous Ne lt T rs ca The env File When you start the shell one of the first things it does is look in your environment for a variable called ENV If it finds it the file specified by ENV will be executed much like the profile is executed when logging in The ENV file usually contains commands to set up the shell s environment Throughout this chapter we ll mention various things that you may want to put into this file If you do decide to have an ENV file you should set and export the ENV variable inside your profile file cat profile ENV HOME alias export ENV For Bash users the ENV file is read only when Bash is invoked with the name sh or with the posix command line option or after set o posix is executed all of which force POSIX standard compliance By default when a noninteractive Bash shell is started for example when you run a shell program it reads commands from the file specified by the BASH ENV environment variable and when an interactive Bash shell is started for example by typing bash a
111. you never know the difference between standard output and standard error ls n List all files beginning with n n not found Here the not found message is actually being written to standard error and not standard output by the ls command You can verify that this message is not being written to standard output by redirecting the ls command s output ls n gt foo n not found So you still get the message printed out at the terminal even though you redirected standard output to the file foo The preceding example shows the raison d tre for standard error so that error messages will still get displayed at the terminal even if standard output is redirected to a file or piped to another command You can also redirect standard error to a file by using the notation command 2 gt file No space is permitted between the 2 and the gt Any error messages normally intended for standard error will be diverted into the specified file similar to the way standard output gets redirected ls n 2 gt errors cat errors n not found rs ca More on Commands Typing More Than One Command on a Line You can type more than one command on a line provided that you separate each command with a semicolon For example you can find out the current time and also your current working directory by typing in the date and pwd commands on the same line date pwd Sat Jul 20 14 43 25 EDT 2002 users pat bin You can s
112. your interactive work sessions A good place to define such functions is inside your ENV file so that they will be defined whenever you start up a new shell Automatically Loaded Functions The Korn shell allows you to set up a special variable called FPATH that is similar to your PATH variable If you try to execute a function that is not yet defined the Korn shell searches the colon delimited list of directories in your FPATH variable for a file that matches the function name If it finds such a file it executes it in the current shell Presumably somewhere inside the file will be a definition for the specified function ns Ga os fa Integer Arithmetic Both Bash and the Korn shell support evaluating arithmetic expressions without arithmetic expansion The syntax is similar to but without the dollar sign Because expansion is not performed the construct can be used without variable assignment or the colon operator x 10 x x 12 echo x 120 The real value of this construct is that it allows arithmetic expressions to be used rather than test in if while and until commands The comparison operators set the exit status to a nonzero value if the result of the comparison is false and to a zero value if the result is true So writing i 100 has the effect of testing i to see whether it is equal to 100 and setting the exit status appropriately This knowledge makes integer arithmetic ideal for incl
113. yourself a lot of programming headaches in the future There is another way to test whether a string is null and that s with either of the last two operators listed previously in Table 8 1 The n operator returns an exit status of zero if the argument that follows is not null Think of this operator as testing for nonzero length The z operator tests the argument that follows to see whether it is null and returns an exit status of zero if it is Think of this operator as testing to see whether the following argument has zero length So the command test n day returns an exit status of 0 if day contains at least one character The command test z dataflag returns an exit status of 0 if dataflag doesn t contain any characters Be forewarned that both of the preceding operators expect an argument to follow therefore get into the habit of enclosing that argument inside double quotes nullvar nonnullvar abc test n nullvar echo test echo test echo test echo nonnullvar Snullvar nonnullvar Does nullvar have nonzero length No And what about nonnullvar Yes Does nullvar have zero length Yes And nonnullvar No Note that test can be picky about its arguments For example if the shell variable symbol contains an equals sign look at what happens if you try to test it for zero length echo symbol test z symboLl sh
114. zero 1 echo one 2 echo two 3 echo three 4 echo four 5 echo five 6 echo six 7 echo seven 8 echo eight 9 echo nine esac Now to test it number 0 zero number 3 three number Try no arguments Usage number digit number 17 Try a two digit number The last case shows what happens when you type in more than one digit 1 doesn t match any of the values listed in the case so none of the echo commands is executed Special Pattern Matching Characters The shell lets you use the same special characters for specifying the patterns in a case as you can with filename substitution That is can be used to specify any single character can be used to specify zero or more occurrences of any character and can be used to specify any single character enclosed between the brackets Because the pattern matches anything just as when it s used for filename substitution it matches all the files in your directory it s frequently used at the end of the case as the catchall value That is if none of the previous values in the case match this one is guaranteed to match Here s a second version of the number program that has such a catchall case cat number Translate a digit to English version 2 if ne 1 then echo Usage number digit exit 1 fi case 1 in 0 1 2 3 4 5 6 7 8 9 E esac number 9 nine number 99 e
115. 1 Given the following variable assignments EDITOR bin vi DB EDITFLAG yes PHONEBOOK What will be the results of the following commands echo EDITOR echo DB users pat db echo EDITOR bin ed echo PHONEBOOK echo DB users pat db ed EDITFLAG EDITOR bin ed Rewrite the home program from Exercise 5 in Chapter 7 to use the set command and the IFS to extract the home directory from etc passwd What happens to the program if one of the fields in the file is null as in steve 203 100 users steve usr bin ksh Here the fifth field is null Using the fact that the shell construct var gives the number of characters stored in var rewrite wc in the shell Be sure to use integer arithmetic Notes Change your IFS variable to just a newline character so that leading whitespace characters on input are preserved and also use the r option to the shell s read command so that terminating backslash characters on the input are ignored Write a function called rightmatch that takes two arguments as shown rightmatch value pattern where value is a sequence of one or more characters and pattern is a shell pattern that is to be removed from the right side of value The shortest matching pattern should be removed from value and the result written to standard output Here is some sample output rightmatch test c c test rightmatch usr spool uucppublic usr spool right
116. 15 Look what happens when you look up Susan grep Susan phonebook Susan Goldberg 201 555 7776 Susan Topple 212 555 4932 There are two lines that contain Susan thus explaining the two lines of output One way to overcome this problem would be to further qualify the name For example you could specify the last name as well grep Susan Goldberg phonebook grep can t open Goldberg Susan Goldberg 201 555 7776 Susan Topple 212 555 4932 Recalling that the shell uses one or more whitespace characters to separate the arguments on the line the preceding command line results in grep being passed three arguments Susan Goldberg and phonebook see Figure 6 1 Figure 6 1 grep Susan Goldberg phonebook arguments Se ee MM Goldberg phonebook When grep is executed it takes the first argument as the pattern and the remaining arguments as the names of the files to search for the pattern In this case grep thinks it s supposed to look for Susan in the files Goldberg and phonebook So it tries to open the file Goldberg can t find it and issues the error message grep can t open Goldberg Then it goes to the next file phonebook opens it searches for the pattern Susan and prints the two matching lines The problem boils down to trying to pass whitespace characters as arguments to programs This can be done by enclosing the entire argument inside a pair of single quotes as in grep Susan Goldberg phonebook
117. 2 fi done done both the while and the for loops will be exited if error is nonnull Skipping the Remaining Commands in a Loop The continue command is similar to break only it doesn t cause the loop to be exited merely the remaining commands in the loop to be skipped Execution of the loop then continues as normal Like the break an optional number can follow the continue so continue n causes the commands in the innermost n loops to be skipped but execution of the loops then continues as normal for file do if e file then echo file not found continue fi Process the file done Each value of file is checked to make sure that the file exists If it doesn t a message is printed and further processing of the file is skipped Execution of the loop then continues with the next value in the list Note that the preceding example is equivalent to writing for file do if e file then echo file not found else Process the file fi done Executing a Loop in the Background An entire loop can be sent to the background for execution simply by placing an ampersand after the done for file in memo 1 4 gt do gt run file gt done amp Send it to the background 1 9932 request id is laser1 85 standard input request id is laser1 87 standard input request id is laser1 88 standard input request id is laser1 92 standard input I O Redirec
118. 2nd sh command 2nd 3rd 4th x option set command xtrace shell mode fe revious Fe rrevious SYMBOL A B C D E F G H 1 J K L M N O P Q R S T U V W X Y Z Y command vi Fe revious 4 rrevious SYMBOL A B C D E F G H 1 CL K L M N O P Q R S T U V W X Y Z zero 0 exit status zero exit status Fe rrevious
119. 2nd 3rd cd command 2nd 3rd 4th 5th 6th 7th 8th 9th Option 2nd cdh function 2nd 3rd 4th CDPATH variable 2nd 3rd 4th cdtest program change program 2nd 3rd changing command prompt 2nd directories cd command 2nd 3rd 4th 5th groups 2nd characters pound sign equal sign underscore extracting from files cut command 2nd 3rd 4th 5th 6th 7th 8th 9th 10th matching multiple characters 2nd 3rd 4th single characters 2nd 3rd newline characters pattern matching any character 2nd 3rd beginning of line 2nd end of line 2nd 3rd grep command 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th 12th 13th 14th parameter substitution constructs 2nd 3rd 4th precise number of characters 2nd 3rd 4th saving matched characters 2nd 3rd 4th specific characters 2nd 3rd summary of regular expressions 2nd zero or more characters 2nd 3rd 4th repeating echo command 2nd translating from standard input 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th 12th whitespace characters child processes closing standard input standard output comamnds bg 2nd 3rd 4th comma command vi command files 2nd comments 2nd 3rd 4th pound sign creating 2nd examples 2nd 3rd 4th execute permissions 2nd passing arguments to 2nd 3rd 4th variable 2nd variable 2nd n variable 2nd phonebook file
120. 5EDT cmd wc x 0 To protect against both of these problems occurring you can use the option to set This tells set not to interpret any subsequent arguments on the command line as options It also prevents set from displaying all your variables if no other arguments follow as was the case when you typed a null line So the set command in words should be changed to read set line With the addition of a while loop and some integer arithmetic the words program can be easily modified to count the total number of words on standard input giving you your own version of wc w cat words Count all of the words on standard input count 0 while read line do set line count count done echo count After each line is read the set command is executed to take advantage of the fact that will be assigned the number of words on the line The option is supplied to set just in case any of the lines read begins with a or consists entirely of whitespace characters The value of is then added into the variable count and the next line is read When the loop is exited the value of count is displayed This represents the total number of words read words lt etc passwd 567 wc w lt etc passwd Check against wc 567 Our version is a lot slower than wc because the latter is written in C 1 Here s a quick way to count the number of files in your directory 1 Th
121. Apr 21 18 30 xtr ln users steve wb link wb to pat s bin ls l total 5 rWXr Xr x 1 pat DP3822 1358 Jan 15 11 01 lcat rWXr Xr x 2 steve DP3725 89 Jun 25 13 30 wb rwXr xr x 1 pat DP3822 504 Apr 21 18 30 xtr Note that steve is still listed as the owner of wb even though the listing came from pat s directory This makes sense because really only one copy of the file exists and it s owned by steve The only stipulation on linking files is that for ordinary links the files to be linked together must reside on the same file system If they don t you ll get an error from ln when you try to link them To determine the different file systems on your system execute the df command The first field on each line of output is the name of a file system To create links to files on different file systems or perhaps on different networked systems you can use the s option to the Ln command This creates a symbolic link Symbolic links behave a lot like regular links except that the symbolic link points to the original file if the original file is removed the symbolic link no longer works Let s see how symbolic links work with the previous example rm wb ls l total 4 rWXr Xr x 1 pat DP3822 1358 Jan 15 11 01 lcat rwXr xr x 1 pat DP3822 504 Apr 21 18 30 xtr ln s users steve wb symwb Symbolic link to wb ls l total 5 rWXr Xr x 1 pat DP3822 1358 Jan 15 11 01 lcat lrwxr xr x 1 pat DP3822 15 Jul 20 15 22 symwb
122. Backspace c The line without a terminating newline Ar Formfeed n Newline r Carriage return XE Tab character v Vertical tab character AX Backslash character Onnn The character whose ASCII value is nnn where nnn is a one to three digit octal number that starts with a zero Remember to quote these characters so that the echo command interprets them and not the shell Examples echo bin docs mail mise src echo echo X tY X Y echo n nSales Report Sales Report echo Wake up a Wake up The eval Command General Format eval args List all files in the current directory Skip a line Print X and Y separated by a tab Skip two lines before displaying Sales Report Print message and beep terminal Execution of this command causes the shell to evaluate args and then execute the results This is useful for causing the shell to effectively double scan a command line Example x abc def y x Assign x to y echo y x eval echo y abc def The exec Command General Format exec command args When the shell executes the exec command it initiates execution of the specified command with the indicated arguments Unlike other commands executed as a new process command replaces the current process that is no new process is created After command starts execution there is no return to the program that initiated the exec If just I O redirection is spe
123. Before the shell starts execution of the desired program it redirects the standard output of the program to the indicated file As far as the program is concerned it never knows that its output is being redirected It just goes about its merry way writing to standard output which is normally your terminal you ll recall unaware that the shell has redirected it to a file Let s take another look at two nearly identical commands wc l users 5 users wc l lt users 5 In the first case the shell analyzes the command line and determines that the name of the program to execute is wc and it is to be passed two arguments l and users see Figure 3 12 Figure 3 12 Execution of wc l users arguments JA When wc begins execution it sees that it was passed two arguments The first argument l tells it to count the number of lines The second argument specifies the name of the file whose lines are to be counted So wc opens the file users counts its lines and then prints the count together with the filename at the terminal Operation of wc in the second case is slightly different The shell spots the input redirection character lt when it scans the command line The word that follows on the command line is the name of the file input is to be redirected from Having gobbled up the lt users from the command line the shell then starts execution of the wc program redirecting its standard input from the file users and passing it
124. Bell Labs Since then the IEEE created standards based on the Bourne shell and the other more recent shells The current version of this standard as of this revision is the Shell and Utilities volume of IEEE Std 1003 1 2001 also known as the POSIX standard This shell is what we propose to teach you about in this book The examples in this book were tested on both SunOS 5 7 running on a Sparcstation Ultra 30 and on Silicon Graphics IRIX 6 5 running on an Octane some examples were also run on Red Hat Linux 7 1 and Cygwin All examples except some Bash examples in Chapter 15 were run using the Korn shell although many were also run with Bash Many Unix systems are still around that have Bourne shell derivatives and utilities not compliant with the POSIX standard We ll try to note this throughout the text wherever possible however there are so many different versions of Unix from so many different vendors that it s simply not possible to mention every difference If you do have an older Unix system that doesn t supply a POSIX compliant shell there s still hope We ll list resources at the end of this book where you can obtain free copies of three different POSI X compliant shells Because the shell offers an interpreted programming language programs can be written modified and debugged quickly and easily We turn to the shell as our first choice of programming language After you become adept at programming in the shell you too may turn to it
125. City 10003 to 3 and 212 555 3021 to 4 After executing the set the program enters a for loop that will be executed exactly six times no matter how many lines are contained in the entry this ensures uniformity of our rolodex cards the program can be easily modified to draw larger sized cards if needed If the set command was executed on Steve s Ice Cream as shown previously 5 and 6 would be null thus resulting in two blank lines to fill out the bottom of the card The printf command displays a line exactly 38 characters wide the leading followed by a space followed by the first 34 characters of line followed by a space and a Fe previous NEXT rem Remove someone from the phone book name 1 Get matching entries and save in temp file grep name PHONEBOOK gt tmp matches if s tmp matches then echo I can t find name in the phone book exit 1 fi Display matching entries one at a time and confirm removal while read line do display line echo Remove this entry y n c read answer lt dev tty if answer y then break fi done lt tmp matches rm tmp matches if answer y then if grep v line PHONEBOOK gt tmp phonebook then mv tmp phonebook PHONEBOOK echo Selected entry has been removed else echo Entry not removed fi fi The rem program collects all matching entries into a tempora
126. EBOOK and set it to HOME phonebook If you then export this variable Lu rem and add which are executed as subshells by rolo can use the value of PHONEBOOK to reference the file One advantage of this is if in the future you change the location of the phonebook file all you ll have to do is change this one variable in rolo the other three programs can remain untouched Here is the new rolo program followed by modified Lu add and rem programs cd users steve bin cat rolo rolo rolodex program to look up add and remove people from the phone book Set PHONEBOOK to point to the phone book file and export it so other progs know about it PHONEBOOK HOME phonebook export PHONEBOOK if f PHONEBOOK then echo No phone book file in HOME exit 1 Ti If arguments are supplied then do a lookup if ne O then lu exit fai validchoice set it null Loop until a valid selection is made until n validchoice do Display menu echo Would you like to 1 Look someone up 2 Add someone to the phone book 3 Remove someone from the phone book Please select one of the above 1 3 c Read and process selection read choice echo case choice in 1 echo Enter name to look up c read name lu name validchoice TRUE 2 echo Enter name to be added c read name echo Enter number c rea
127. Harry Dick Instead of first entering the names into a file we ll enter them directly from the terminal sort Tony Barbara Harry Dick Ctrl d Barbara Dick Harry Tony Because no filename was specified to the sort command the input was taken from standard input the terminal After the fourth name was typed in the Ctrl and d keys were pressed to signal the end of the data At that point the sort command sorted the four names and displayed the results on the standard output device which is also the terminal This is depicted in Figure 2 11 Figure 2 11 sort command Tony Barbara Barbara Dick Harry Harry Dick Tony The wc command is another example of a command that takes its input from standard input if no filename is specified on the command line So the following shows an example of this command used to count the number of lines of text entered from the terminal we l This is text that is typed on the standard input device Ctril d Note that the Ctrl d that is used to terminate the input is not counted as a separate line by the wc command Furthermore because no filename was specified to the wc command only the count of the number of lines 3 is listed as the output of the command Recall that this command normally prints the name of the file directly after the count Output Redirection The output from a command normally intended for standard output can be easily diverted to a file instead This c
128. Here s the phonebook file from previous examples cat phonebook Alice Chebba 973 555 2015 Barbara Swingle 201 555 9257 Liz Stachiw 212 555 2298 Susan Goldberg 201 555 7776 Susan Topple 212 555 4932 Tony Iannino 973 555 1295 You know how to look up someone in the file by using grep grep Cheb phonebook Alice Chebba 973 555 2015 And you know that if you want to look up someone by the full name you d better put quotes around it to keep the argument together grep Susan T phonebook Susan Topple 212 555 4932 It would be nice to write a shell program that you could use to look up someone Let s call the program Lu and have it take as its argument the name of the person to look up cat lu Look someone up in the phone book grep 1 phonebook Here s a sample use of Lu lu Alice Alice Chebba 973 555 2015 lu Susan Susan Goldberg 201 555 7776 Susan Topple 212 555 4932 lu Susan T grep can t open T phonebook Susan Goldberg 201 555 7776 phonebook Susan Topple 212 555 4932 In the preceding example you were careful to enclose Susan T in double quotes so what happened Look again at the grep executed in the lu program grep 1 phonebook Even though enclosing Susan T inside double quotes results in its getting passed to lu as a single argument when the shell substitutes this value for 1 on grep s command line it then passes it as two arguments to grep Remember we h
129. LL variable export SHELL cd users restrict restdir Don t leave user in HOME directory ls l profile Restricted user shouldn t own his profile rw r r 1 pat groupl 179 Sep 14 17 50 profile ls usr rbin Directory of restricted commands cat Harmless commands echo ls mail Let them send us mail red Restricted editor write ls users restrict bin restrict s command directory adventure Lots of games backgammon chess hearts poker rogue Here we have a restricted environment for a user When this user logs in his PATH is changed to search just the directories usr rbin and users restrict bin He can run only commands found in these two directories Any other command will get a command not found response The user is effectively bottled up in the directory users restrict restdir and cannot cd out of it The profile is owned by a user other than the restricted one and the permissions are such that only the owner can change the file Don t let a restricted user alter his or her profile because the profile is executed before any restrictions are applied One quick note about the commands in usr rbin They were simply copied from the bin and usr bin directories You can put almost any command from bin and usr bin in usr rbin just use common sense in choosing the commands you allow restricted users to use For example don t give them access to the shell a compiler or chmod because these may be used to
130. Note that because file is not executed by a subshell variables set and or changed within file remain in effect after execution of the commands in file is complete The alias Command General Format alias name string name string The alias command assigns string to the alias name Whenever name is used as a command the shell substitutes string performing command line substitution after string is in place Examples alias ll ls l alias dir basename pwd If an alias ends with a blank the word following the alias is also checked to see whether it s an alias The format alias name causes the alias for name to be printed out alias with no arguments lists all aliases alias returns an exit status of zero unless a name is given as in alias name for which no alias has been defined The bg Command General Format bg job_id If job control is enabled the job identified by job_id is put into the background If no argument is given the most recently suspended job is put into the background Example bg 2 The break Command General Format break Execution of this command causes execution of the innermost for while or until loop to be immediately terminated Execution continues with the commands that immediately follow the loop If the format break n is used where n is an integer greater than or equal to 1 execution of the n innermost loops is automatically terminated The case Command General Fo
131. Placing a at the beginning of the pipeline causes the exit status of the pipeline to be the logical negation of the last command in the pipeline If the command sequence is terminated by an ampersand character amp it is run asynchronously in the background The shell displays the process id number and job id of the command at the terminal Typing of a command can continue to the next line if the last character on the line is a backslash character i The characters amp amp cause the command that follows to be executed only if the preceding command returns a zero exit status The characters cause the command that follows to be executed only if the preceding command returns a nonzero exit status As an example in who grep fred gt dev null amp amp echo fred s logged on the echo is executed only if the grep returns a zero exit status rs fay ns mi Comments If a word begins with the character the shell treats the remainder of the line as a comment and simply ignores it fe revious next gt Fe previous next Parameters and Variables There are three different types of parameters shell variables special parameters and positional parameters Shell Variables A shell variable name must start with an alphabetic or underscore _ character and can be followed by any number of alphanumeric or underscore characters Shell variables can be assigned values on the command line by writing variable v
132. Print the exit status 1 Even if name is null the shell still passes three arguments to test the first one null see Figure 8 2 Figure 8 2 test name julio with name null arguments Other operators can be used to test character strings These operators are summarized in Table 8 1 Table 8 1 test String Operators Operator Returns TRUE exit status of 0 if string strings string is identical to string string strings string is not identical to string gt string string is not null n string string is not null and string must be seen by test Zz string string is null and string must be seen by test You ve seen how the operator is used The operator is similar only it tests two strings for inequality That is the exit status from test is zero if the two strings are not equal and nonzero if they are Let s look at three similar examples day monday test day monday echo 0 True The test command returns an exit status of 0 because the value of day is equal to the characters monday Now look at the following day monday test day monday echo 1 False Here we assigned the characters monday including the space character that immediately followed to day Therefore when the previous test was made test returned false because the characters monday were not identical to the characters monday If you wanted these two values to be considered
133. SERT Xx w S Matches a followed by any two characters wood only if it appears at the beginning of the line x only if it is the last character on the line a line containing just the characters INSERT a line that contains no characters zero or more consecutive x s one or more consecutive x s zero or more characters w followed by zero or more characters followed by ans chars any character in chars tT lower or uppercase t a z lowercase letter a zA Z lower or uppercase letter chars any character 0 9 any nonnumeric character not in chars a zA Z any nonalphabetic character min max at least min and at most max x 1 5 at least 1 and at and at most 5 x s occurrences of previous regular SAPrESSIQOS 0 9 3 9 anywhere from 3 to 9 successive digits 0 9 3 exactly 3 digits 0 9 3 at least 3 digits Wesa store characters matched NEA first character on line and stores it in between parentheses in next register 1 register 1 9 A 1 first and second characters on the line if they re the same fe revious next gt rs i cut This section teaches you about a useful command known as cut This command comes in handy when you need to extract that is cut out various fields of data from a data file or the output of a command The general format of the cut command is cut cchars file where chars specifies what characters you want to extract from each line of file
134. TZ EST5EDT export y 10 As you can see there are actually more exported variables here than you were initially led to believe Note that y shows up on the list together with other variables that were exported when you logged on Note that the variables listed include those that have been inherited from a parent shell ns Ga rs ca Psi and ps2 The characters that the shell displays as your command prompt are stored in the variable PS1 You can change this variable to be anything you want As soon as you change it it ll be used by the shell from that point on echo PS1 E o PSl gt gt pwd users steve gt PS1 I await your next command master I await your next command master date Wed Sep 18 14 46 28 EDT 2002 I await your next command master PS1 Back to normal Your secondary command prompt normally gt is kept in the variable PS2 where you can change it to your heart s content echo PS2 a 3 PS2 gt for x in 12 3 gt do gt Cho x gt done 1 2 Like any other shell variables after you log off the system the values of those variables go with it So if you change PS1 the shell will use the new value for the remainder of your login session Next time you log in however you ll get the old value again You can make the change yourself every time you log in or you can have the change made automatically by adding it to your
135. When found it initiates its execution This list of directories is contained in a special shell variable called PATH This variable is automatically set for you when you log on to the system See what it s set to now 1 Actually the shell is a bit more intelligent because it keeps track of where it finds each command you execute When you re execute one of these commands the shell remembers where it was found and doesn t go searching for it again This feature is known as hashing echo PATH bin usr bin Chances are that your PATH has a slightly different value As noted the PATH specifies the directories that the shell searches to execute a command These directories are separated from one another by colons In the preceding example three directories are listed bin usr bin and which you ll recall stands for the current directory So whenever you type in the name of a program say for example rolo the shell searches the directories listed in PATH from left to right until it finds an executable file called rolo First it looks in bin then in usr bin and finally in the current directory for an executable file called rolo As soon as it finds rolo the shell executes it if the shell doesn t find rolo the shell issues a not found message The path bin usr bin specifies to search bin followed by the current directory followed by usr bin To have the current directory searched first you put the
136. When the shell sees the first single quote it ignores any otherwise special characters that follow until it sees the closing quote grep Susan Goldberg phonebook Susan Goldberg 201 555 7776 In this case the shell encountered the first and ignored any special characters until it found the closing So the space between Susan and Goldberg which would have normally delimited the two arguments was ignored by the shell The shell therefore divided the command line into two arguments the first Susan Goldberg which includes the space character and the second phonebook It then executed grep passing it these two arguments see Figure 6 2 Figure 6 2 grep Susan Goldberg phonebook arguments Susan Goldberg phonebook grep then took the first argument Susan Goldberg and looked for it in the file specified by the second argument phonebook Note that the shell removes the quotes from the command line and does not pass them to the program No matter how many space characters are enclosed between quotes they are preserved by the shell echo one two three four one two three four echo one two three four one two three four In the first case the shell removes the extra whitespace characters from the line and passes echo the four arguments one two three and four see Figure 6 3 Figure 6 3 echo one two three four In the second case the space characters are preserved and the shell treats the
137. a failure for some commands can vary from one Unix version to the next but success is always signified by a zero exit status Let s now write a shell program called on that tells us whether a specified user is logged on to the system The name of the user to check will be passed to the program on the command line If the user is logged on we ll print a message to that effect otherwise we ll say nothing Here is the program cat on determine if someone is logged on user 1 if who grep user then echo user is Logged on fi This first argument typed on the command line is stored in the shell variable user Then the if command executes the pipeline who grep user and tests the exit status returned by grep If the exit status is zero grep found user in who s output In that case the echo command that follows is executed If the exit status is nonzero the specified user is not logged on and the echo command is skipped The echo command is indented from the left margin for aesthetic reasons only tab characters are usually used for such purposes because it s easier to type a tab character than an equivalent number of spaces In this case just a single command is enclosed between the then and fi When more commands are included and when the nesting gets deeper indentation can have a dramatic effect on the program s readability Later examples will help illustrate this point Here are some sample uses of on
138. able 2nd 3rd 4th 5th 6th ignoreeof shell mode ignoring signals 2nd in line input redirection 2nd 3rd 4th 5th 6th 7th 8th 9th creating archives with 2nd 3rd 4th 5th example 2nd 3rd 4th info command init program 2nd Input Field Separator IFS variable 2nd 3rd 4th 5th input mode commands vi 2nd input redirection 2nd 3rd 4th 5th 6th 7th 8th input See I O input output input output See I O inserting phone book entries 2nd INT signal integer arithmetic arithmetic bases 2nd 3rd arithmetic expressions 2nd integer types 2nd integer expansion 2nd 3rd integer operators 2nd 3rd integers integer operators 2nd 3rd interpreted programming languages 2nd interrupt signals ignoring 2nd sending signal numbers 2nd trapping 2nd 3rd 4th 5th ison program passing arguments to 2nd 4 rrevious SYMBOL A B C D E F G H 1 J K L M N O P Q R S T U V W X Y Z j command vi 2nd 3rd job numbers job_ids 2nd jobs See also processes active jobs printing list of 2nd defined job_ids 2nd killing 2nd 3rd 4th messages running in background 2nd running in foreground 2nd status of stopping 2nd 3rd 4th 5th 6th suspending 2nd 3rd 4th terminating 2nd jobs command 2nd 3rd Joy Bill Fe eevious Fe rrevious SYMBOL A
139. ad this same sort of discussion when we talked about variable substitution first the shell substitutes the value of the variable then it divides the line into arguments You can alleviate this problem by enclosing 1 inside double quotes why not single in the Lu program cat lu Look someone up in the phone book version 2 grep 1 phonebook Now let s try it again lu Tony Tony Iannino 973 555 1295 lu Susan T Susan Topple 212 555 4932 This still works Now try this again rs ca A Program to Add Someone to the Phone Book Let s continue with the development of programs that work with the phonebook file You ll probably want to add someone to the file particularly because our phonebook file is so small You can write a program called add that takes two arguments the name of the person to be added and the number Then you can simply write the name and number separated from each other by a tab character onto the end of the phonebook file cat add Add someone to the phone book echo 1 2 gt gt phonebook Although you can t tell there s a tab character that separates the 1 from the 2 in the preceding echo command This tab must be quoted to make it to echo without getting gobbled up by the shell Let s try out the program add Stromboli Pizza 973 555 9478 lu Pizza See if we can find the new entry Stromboli Pizza 973 555 9478 So far so good cat
140. alue in answer and test would issue an error message if the quotes were omitted Special echo Escape Characters A slight annoyance with mycp is that after the echo command is executed to alert the user that the file already exists the response that is typed by the user appears on the next line This happens because the echo command always automatically displays a terminating newline character after the last argument This can be suppressed if the last two characters given to echo are the special escape characters c This tells echo to leave the cursor right where it is after displaying the last argument and not to go to the next line So if you changed the echo command in mycp to read like this echo to already exists overwrite yes no c the user s input would be typed right after the message on the same line Bear in mind that the c is interpreted by echo and not by the shell meaning that it must be quoted so that the backslash makes it to echo echo interprets other special characters These must each be preceded by a backslash They re summarized in Table 10 1 Table 10 1 echo Escape Characters Character Prints b Backspace c The line without a terminating newline f Formfeed n Newline r Carriage return Bas Tab character W Backslash character Onnn The character whose ASCII value is nnn where nnn is a one to three digit octal number An Improved Version of mycp Suppose that you have a program called
141. alue variable value Filename substitution is not performed on value Positional Parameters Whenever a shell program is executed the name of the program is assigned to the variable 0 and the arguments typed on the command line to the variables 1 2 and respectively Positional parameters can also be assigned values with the set command Parameters 1 through 9 can be explicitly referenced Parameters greater than nine must be enclosed inside braces as in 10 Special Parameters Table A 1 summarizes the special shell parameters Table A 1 Special Parameter Variables Parameter Meaning The number of arguments passed to the program or the number of parameters set by executing the set statement 7 Collectively references all the positional parameters as 1 2 Same as except when double quoted collectively references all the positional parameters as 1 2 0 The name of the program being executed The process id number of the program being executed The process id number of the last program sent to the background for execution The exit status of the last command not executed in the background The current option flags in effect see the set statement In addition to these parameters the shell has some other variables that it uses Table A 2 summarizes the more important of these variables Variable CDPATH ENV FCEDIT HISTFILE HISTSIZE HOME IFS Table A
142. ame has been inadvertently entered more than once You can easily find such duplicate entries by first sorting etc passwd and piping the results into uniq d as done previously sort etc passwd uniq d Find duplicate entries in etc passwd So there are no duplicate entries But we think that you really want to find duplicate entries for the same username This means that you want to just look at the first field from each line of etc passwd recall that the leading characters of each line of etc passwd up to the colon are the username This can t be done directly through an option to unig but can be accomplished indirectly by using cut to extract the username from each line of the password file before sending it to uniq sort etc passwd cut f1 d uniq d Find duplicates cem harry So there are multiple entries in etc passwd for cem and harry If you wanted more information on the particular entries you could grep them from etc passwd grep n cem etc passwd 20 cem 91 91 users cem 166 cem 91 91 users cem grep n harry etc passwd 29 harry 103 103 Harry Johnson users harry 79 harry 90 90 Harry Johnson users harry The n option was used to find out where the duplicate entries occur In the case of cem there are two entries on lines 20 and 166 in harry s case the two entries are on lines 29 and 79 If you now want to remove the second cem entry you could use sed sed
143. ames and no other information Type in the program and test it out Fe erevious next a Chapter 6 Can I Quote You on That IN THIS CHAPTER The Single Quote The Double Quote The Backslash Command Substitution Exercises This chapter teaches you about a unique feature of the shell programming language the way it interprets quote characters Basically the shell recognizes four different types of quote characters The single quote character The double quote character The backslash character The back quote character The first two and the last characters in the preceding list must occur in pairs whereas the backslash character is unary in nature Each of these quotes has a distinct meaning to the shell We ll cover them in separate sections of this chapter rs fi lt Previous next gt The Single Quote There are several reasons that you might need to use quotes in the shell One of these is to keep characters otherwise separated by whitespace characters together Let s look at an example Here s a file called phonebook that contains names and phone numbers cat phonebook Alice Chebba 973 555 2015 Barbara Swingle 201 555 9257 Liz Stachiw 212 555 2298 Susan Goldberg 201 555 7776 Susan Topple 212 555 4932 Tony Iannino 973 555 1295 To look up someone in our phonebook file which has been kept small here for the sake of example you use grep grep Alice phonebook Alice Chebba 973 555 20
144. amiliarity with a line based editor such as ex or ed See Appendix B For More Information for more information on these editors Matching Any Character The Period A period in a regular expression matches any single character no matter what it is So the regular expression specifies a pattern that matches an r followed by any single character The regular expression matches an x that is surrounded by any two characters not necessarily the same The ed command searches forward in the file you are editing for the first line that contains any three characters surrounded by blanks ed intro 248 1 p Print all the lines The Unix operating system was pioneered by Ken Thompson and Dennis Ritchie at Bell Laboratories in the late 1960s One of the primary goals in the design of the Unix system was to create an environment that promoted efficient program development Look for three chars surrounded by blanks The Unix operating system was pioneered by Ken Repeat last search Thompson and Dennis Ritchie at Bell Laboratories 1 S p 0 XXX g Change all p os to XXX 1 p Let s see what happened The Unix operating system was XXXneered by Ken ThomxXXn and Dennis Ritchie at Bell Laboratories in the late 1960s One of the primary goals in the design of the Unix system was to create an environment that XXXmoted efficient XxXXgram development In the first search ed started searching from the beginning of the file and
145. and Exercises Chapter 10 Reading and Printing Data The read Command The printf Command Exercises Chapter 11 Your Environment Local Variables Exported Variables PS1 and PS2 HOME ames Your PATH Your Current Directory More on Subshells Your profile File The TERM Variable The TZ Variable Exercises Chapter 12 More on Parameters Parameter Substitution The 0 Variable The set Command The IFS Variable The readonly Command The unset Command Exercises Chapter 13 Loose Ends The eval Command The wait Command The trap Command More on I O Functions The type Command Exercises Chapter 14 Rolo Revisited Design Considerations rolo add il display rem change listall Sample Output Exercises Chapter 15 Interactive and Nonstandard Shell Features Getting the Right Shell The ENV File Command Line Editing Command History The vi Line Edit Mode The Line Edit Mode Other Ways to Access Your History Functions Integer Arithmetic The alias Command Arrays Job Control The Restricted Shell rsh Miscellaneous Features Compatibility Summary Exercises Appendix A Shell Summary Startup Commands Comments Parameters and Variables Command Re entry Quoting Filename Substitution I O Redirection Exported Variable
146. andard Input Output and I O Redirection Standard Input and Standard Output Most Unix system commands take input from your terminal and send the resulting output back to your terminal A command normally reads its input from a place called standard input which happens to be your terminal by default Similarly a command normally writes its output to standard output which is also your terminal by default This concept is depicted in Figure 2 9 Figure 2 9 Typical Unix command standard input standard output a ms ees gt Recall that executing the who command results in the display of the currently logged in users More formally the who command writes a list of the logged in users to standard output This is depicted in Figure 2 10 Figure 2 10 who command Fa ra Fa ai tty01 oko tty36 pat tty21 ruth tty24 steve tty25 h hs W hs hs If a sort command is executed without a filename argument the command takes its input from standard input As with standard output this is your terminal by default Sep 12 Sep 12 Sep 12 Sep 12 Sep 12 07 30 13 32 10 10 13 07 13 03 When entering data to a command from the terminal the Ctrl and d keys denoted Ctrl d in this text must be simultaneously pressed after the last data item has been entered This tells the command that you have finished entering data As an example let s use the sort command to sort the following four names Tony Barbara
147. any alphabetic character followed by zero or more alphabetic characters This is pretty close to a regular expression that matches words ed intro 248 1 s A Za z A Za z X g 1 p X XX X X X X X X X X X X X X X X X 1960X XXX XX xX XXXXXXXK XK XX XXX X X X The only thing it didn t match in this example was 1960 You can change the regular expression to also consider a sequence of digits as a word ed intro 248 1 s A Za z0 9 A Za z0 9 X g 1 p X XX X X X X X X X X X X X X X XXX XX XX X X X XX X XX X X X X X X X X X X We could expand on this somewhat to consider hyphenated words and contracted words for example don t but we ll leave that as an exercise for you As a point of note if you want to match a dash character inside a bracketed choice of characters you must put the dash immediately after the left bracket and after the inversion character if present or immediately before the right bracket So the expression 0 9 matches a single dash or digit character If you want to match a right bracket character it must appear after the opening left bracket and after the if present So a z matches a right bracket or a lowercase letter Matching a Precise Number of Characters In the preceding examples you saw how to use the asterisk to specify that one or more occurrences of the preceding regular expression are to be matched For instance the regular expressi
148. apability is known as output redirection If the notation gt file is appended to any command that normally writes its output to standard output the output of that command will be written to file instead of your terminal who gt users This command line causes the who command to be executed and its output to be written into the file users Notice that no output appears at the terminal This is because the output has been redirected from the default standard output device the terminal into the specified file cat users oko tty 1 Sep 12 07 30 ai tty15 Sep 12 13 32 ruth tty21 Sep 12 10 10 pat tty24 Sep 12 13 07 steve tty25 Sep 12 13 03 If a command has its output redirected to a file and the file already contains some data that data will be lost Consider this example echo line 1 gt users cat users line 1 echo line 2 gt gt users cat users line 1 line 2 The second echo command uses a different type of output redirection indicated by the characters gt gt This character pair causes the standard output from the command to be appended to the specified file Therefore the previous contents of the file are not lost and the new output simply gets added onto the end By using the redirection append characters gt gt you can use cat to append the contents of one file onto the end of another cat Tilel This is in filel cat file2 This is in file2 cat filel gt gt file2 Append
149. ar rationale for their order of presentation fe rrevious next rs ca The evat Command This section describes another of the more unusual commands in the shell eval Its format is as follows eval command line where command line is a normal command line that you would type at the terminal When you put eval in 1 front of it however the net effect is that the shell scans the command line twice before executing it For the simple case this really has no effect 1 Actually what happens is that eval simply executes the command passed to it as arguments so the shell processes the command line when passing the arguments to eval and then once again when eval executes the command The net result is that the command line is scanned twice by the shell eval echo hello hello But consider the following example without the use of eval pipe ls pipe wc l No such file or directory wc No such file or directory l No such file or directory Those errors come from ls The shell takes care of pipes and I O redirection before variable substitution so it never recognizes the pipe symbol inside pipe The result is that the three arguments wc and l are passed to ls as arguments Putting eval in front of the command sequence gives the desired results eval ls pipe wc l 16 The first time the shell scans the command line it substitutes as the value of pipe Then eval causes it to rescan t
150. aracter on line newline is removed command or command Executes command and inserts standard output at that point Tilde Substitution Each word and shell variable on a command line is checked to see whether it begins with an unquoted If it does the rest of the word or variable up to a is considered a login name and is looked up in a system file typically etc passwd If that user exists his home directory replaces the and his login name If that user doesn t exist the text is unchanged A by itself or followed by a is replaced by the HOME variable Arithmetic Expressions General Format expression The shell evaluates the integer arithmetic expression expression can contain constants shell variables which don t have to be preceded by dollar signs and operators The operators in order of decreasing precedence are z unary minus bitwise NOT x 9 T lt lt gt gt amp amp expr expr expr3 logical negation multiplication division remainder addition subtraction left shift right shift comparison equal not equal bitwise AND bitwise exclusive OR bitwise OR logical AND logical OR conditional operator assignment Parentheses may be used to override operator precedence The exit status is zero true if the last expression is nonzero and one false if the last expression is zero The C operators sizeof and may be available in your shell implemen
151. arameter matching pattern is removed Shell filename substitution characters and may be used in pattern Same as pattern except the largest matching pattern is removed parameterxpattern Same as pattern except pattern is removed from the right side parameter pattern Same as pattern except the largest matching pattern is removed from the right side ns Ga rs ca Command Re entry The shell keeps a list or history of recently entered commands The number of commands available is determined by the HISTSIZE variable default at least 128 and the file in which the history is kept is determined by the HISTFILE variable default HOME sh_ history Because the command history is stored in a file these commands are available after you log off and back on There are three ways you can access the command history The fc Command The built in command fc allows you to run an editor on one or more commands in the command history When the edited command s is written and you leave the editor the edited version of the command s is executed The editor is determined by the FCEDIT variable default ed The e option may be used with fc to specify the editor rather than FCEDIT The s option causes commands to be executed without first invoking an editor A simple editing capability is built in to the fc s command an argument of the form old new may be used to change the first occurrence of the string old
152. arameter or parameter parameter value parameter value parameter value parameter value parameter value parameter value parameter value parameter value parameter parameter pattern parameter pattern Table A 3 Parameter Substitution Meaning Substitute the value of parameter Substitute the value of parameter if it s set and non null otherwise substitute value Substitute the value of parameter if it s set otherwise substitute value Substitute the value of parameter if it s set and non null otherwise substitute value and also assign it to parameter Substitute the value of parameter if it s set otherwise substitute value and also assign it to parameter Substitute the value of parameter if it s set and non null otherwise write value to standard error and exit If value is omitted write parameter parameter null or not set instead Substitute the value of parameter if it s set otherwise write value to standard error and exit If value is omitted write parameter parameter null or not set instead Substitute value if parameter is set and non null otherwise substitute null Substitute value if parameter is set otherwise substitute null Substitute the length of parameter If parameter is or the result is not specified Substitute the value of parameter with pattern removed from the left side The smallest portion of the contents of p
153. art up when that user logs in tl The last bit of information the program to start up is stored after the last colon of each line If nothing follows the last colon the standard shell usr bin sh is assumed by default The following three lines show typical lines from etc passwd for three users of the system sue pat and bob 1 The file s name passwd derives from a time when encrypted versions of the users passwords were Stored in this file along with other user information The encrypted passwords are no longer stored in etc passwd but for security reasons are now kept in the etc shadow file which is not readable by normal users sue 15 47 users sue pat 99 7 users pat usr bin ksh bob 13 100 users data users data bin data_entry After Login checks the password you typed in against the one stored in etc shadow it then checks for the name of a program to execute In most cases this will be usr bin sh usr bin ksh or bin bash In other cases it may be a special custom designed program The main point here is that you can set up a login account to automatically run any program whatsoever whenever someone logs in to it The shell just happens to be the program most often selected So login initiates execution of the standard shell on sue s terminal after validating her password see Figure 3 4 Figure 3 4 login executes usr bin sh usr bin sh login sue Password Welcome I I
154. as a standard operating system For programmers who have been using Unix for many years now this came as no surprise The Unix system provides an elegant and efficient environment for program development After all this is what Dennis Ritchie and Ken Thompson strived for when they developed Unix at Bell Laboratories in the late 1960s One of the strongest features of the Unix system is its wide collection of programs More than 200 basic commands are distributed with the standard operating system These commands also known as tools do everything from counting the number of lines in a file to sending electronic mail to displaying a calendar for any desired year But the real strength of the Unix system comes not entirely from this large collection of commands but also from the elegance and ease with which these commands can be combined to perform far more sophisticated functions To further this end and also to provide a consistent buffer between the user and the guts of the Unix system the kernel the shell was developed The shell is simply a program that reads in the commands you type and converts them into a form more readily understood by the Unix system It also includes some fundamental programming constructs that let you make decisions loop and store values in variables The standard shell distributed with Unix and Linux systems derives from AT amp T s distribution which evolved from a version originally written by Stephen Bourne at
155. ast ditch effort to find the directory If the directory that it finds is not relative to your current one the cd command prints the full path to the directory to let you know where it s taking you cd users steve cd memos users steve docs memos cd bin users steve bin Like the PATH variable use of the period for specifying the current directory is optional so users steve users steve docs is equivalent to users steve users steve docs J udicious use of the CDPATH variable can save you a lot of typing especially if your directory hierarchy is fairly deep and you find yourself frequently moving around in it or if you re frequently moving around into other directory hierarchies as well Unlike the PATH you ll probably want to put your current directory first in the CDPATH list This gives you the most natural use of CDPATH because you re used to doing a cd x to switch to the subdirectory x If the current directory isn t listed first you may end up in an unexpected directory or oo os fa More on Subshells It s important for you to understand the way subshells work and how they interact with your environment You know now that a subshell can t change the value of a variable in a parent shell nor can it change its current directory Suppose that you want to write a program to set values for some variables that you like to use whenever you log on For example assume that you have the following file called var
156. at means all files in the directory The quotes are needed in the assignments made to the variables message and text because of the embedded spaces In the preceding example you are reminded that the shell still does filename substitution after variable name substitution meaning that the is replaced by the names of all the files in the current directory before the echo is executed There is a way to overcome this annoyance and it s through the use of double quotes fe erevious Next lt Previous Jinext The Double Quote Double quotes work similarly to single quotes except that they re not as restrictive Whereas the single quotes tell the shell to ignore all enclosed characters double quotes say to ignore most In particular the following three characters are not ignored inside double quotes e Dollar signs e Back quotes e Backslashes The fact that dollar signs are not ignored means that variable name substitution is done by the shell inside double quotes x echo x addresses intro lotsaspaces names nu numbers phonebook stat echo Sx x echo x Here you see the major differences between no quotes single quotes and double quotes In the first case the shell sees the asterisk and substitutes all the filenames from the current directory In the second case the shell leaves the characters enclosed within the single quotes alone which results in the display of x In the final case the double
157. at string anywhere on the command line The command will then be displayed If no line in the history contains the string the shell beeps the terminal When the is entered the current line is replaced by a tmp Esc test test_ The search is begun when the Enter key is pressed test_ Enter echo this is a test To execute the command that results from the search Enter must be pressed again echo this is a test Hit Enter again this is a test If the command that s displayed isn t the one you re interested in you can continue the search through the command history by simply typing and pressing Enter The shell uses the string that you entered the last time you executed the search command When you ve found the command in the history either by k j or you can edit the command using the other vi commands we ve already discussed Note that you don t actually change the command in the history That command cannot be changed after it is entered Instead you are editing a copy of the command in the history which will itself be entered in the history when you press Enter Table 15 1 summarizes the basic vi line edit commands Table 15 1 Basic vi Line Edit Commands Command Meaning h Move left one character ik Move right one character b Move left one word w Move right one word 0 Move to Start of line Move to end of line x Delete character at cursor dw Delete word at cursor rc Change characte
158. atching characters 2nd 3rd 4th pipe symbol 2nd 3rd displaying null values 2nd 3rd reassigning to positional parameters 2nd 3rd variables 2nd 3rd 4th 5th variable 2nd variable 2nd 3rd variable variable 2nd 3rd variable 2nd 3rd 4th 5th 6th variable 2nd 3rd n variable 2nd variable construct italicize variable 2nd 0 0 variable 2nd arithmetic expansion 2nd 3rd assigning to other variables assigning values to 2nd assigning variables to CDPATH 2nd 3rd 4th displaying values of ENV 2nd examples 2nd exporting 2nd 3rd 4th 5th 6th 7th 8th 9th 10th FCEDIT 2nd filename substitution 2nd 3rd FPATH HISTFILE HISTSIZE HOME 2nd 3rd IFS 2nd 2nd IFS variable 2nd 3rd 4th 5th LINENO listing 2nd local variables 2nd MAIL MAILCHECK MAILPATH naming conventions null values 2nd 3rd passing to subshells 2nd 3rd 4th PATH 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th 12th PHONEBOOK 2nd 3rd 4th 5th pointers creating positional parameters defined 2nd left shifting 2nd 3rd PPID 2n 2 3rd 2n 2 WwW iQ r PS1 PS2 PS4 W ge D read only variables 2nd creating 2nd removing from environment 2nd 4 ERM 2nd 2nd IN N unsetting 2nd 3rd variable substitution 2nd vartest program 2nd vartest2 pro
159. ay of integers to the shell simply by giving the array name to typeset i typeset i data Integer calculations can be performed on array elements using the construct typeset i array array 0 100 array 1 50 array 2 array array 1 echo array 2 150 i 1 echo array i 50 array 3 array 0 array 2 echo array 3 250 Note that not only can you omit the dollar signs and the curly braces when referencing array elements inside double parentheses you also can omit them outside when the array is declared to be of integer type Also note that dollar signs are not needed before variables used in subscript expressions The following program called reverse reads in up to 4096 lines from standard input and then writes them back out to standard output in reverse order cat reverse read lines to array buf typeset i line 0 while line lt 4096 amp amp read buf line do line line 1 done now print the lines in reverse order while line gt 0 do line line 1 echo buf line done reverse line one line two line three Ctrl d line three line two line one The first while loop executes as long as 4096 or fewer lines have been read and there is more data to be read from standard input recall the amp amp described at the end of Chapter 8 Decisions Decisions The following function cdh changes the
160. be true if either the first expression is true or the second expression is true n mailopt o r HOME mailfile This command will be true if the variable mailopt is not null or if the file HOME mailfile is readable by you The o operator has lower precedence than the a operator meaning that the expression a eq 0 0 b eq 2 A ogr eq 10 gets evaluated by test as a eq 0 0 b eq 2 s eg eq 10 Naturally you can use parentheses to change this order if necessary a eq 0 0 Sb eq 2 a ngg eq 10 You will see many uses of the test command throughout the book Table A 11 in Appendix A Shell Summary summarizes all available test operators ns Ga os fa The eise Construct A construct known as the else can be added to the if command with the general format as shown if command then command command else command command fi Execution of this form of the command starts as before command is executed and its exit status tested If it s zero the commands that follow between the then and the else are executed and the commands between the else and fi are skipped Otherwise the exit status is nonzero and the commands between the then and else are skipped and the commands between the else and fi are executed In either case only one set of commands gets executed the first set if the exit status is zero and the second set if it s nonzero Let s now write a modified version of
161. be used at the beginning or in the middle as well echo t1 chapt1 echo t chapt1 chapt2 chapt3 chapt4 echo x X In the first echo the t1 specifies all filenames that end in the characters t1 In the second echo the first matches everything up to a t and the second everything after thus all filenames containing a t are printed Because there are no files ending with x no substitution occurs in the last case Therefore the echo command simply displays x Matching Single Characters The asterisk matches zero or more characters meaning that x matches the file x as well as x1 x2 xabc and so on The question mark matches exactly one character So cat prints all files with one character names just as cat x prints all files with two character names beginning with x ls a aa aax alice bb CC reportl report2 report3 echo abc echo a aa echo aa bb cc echo aa aax alice bb cc reportl report2 report3 In the preceding example the matches two characters and the matches zero or more up to the end The net effect is to match all filenames of two or more characters Another way to match a single character is to give a list of the characters to use in the match inside square brackets For example abc matches one letter a b or c It s similar to the but it allows you to choose the characters that will be matched The specification 0 9 matches the
162. by rolo cat rolosubs Archived programs used by rolo echo Extracting lu cat gt lu lt lt THE END OF DATA Look someone up in the phone book if ne 1 then echo Incorrect number of arguments echo Usage lu name exit 1 Fa name 1 grep name PHONEBOOK if ne 0 then echo I couldn t find name in the phone book fi THE END OF DATA echo Extracting add cat gt add lt lt THE END OF DATA Program to add someone to the phonebook file if ne 2 then echo Incorrect number of arguments echo Usage add name number exit 1 fi echo 1 2 gt gt PHONEBOOK sort o PHONEBOOK PHONEBOOK THE END OF DATA echo Extracting rem cat gt rem lt lt THE END OF DATA Remove someone from the phone book if ne 1 then echo Incorrect number of arguments echo Usage rem name exit 1 fi name 1 Find number of matching entries matches grep name PHONEBOOK wc L If more than one match issue message else remove it if matches gt 1 then echo More than one match please qualify further elif matches eq 1 then grep v name PHONEBOOK gt tmp phonebook mv tmp phonebook PHONEBOOK else echo I couldn t find name in the phone book fi THE END OF DATA To be complete this archive should probably include rolo as well but we didn t here to conserve
163. bypass the restricted shell The mail and write commands are safe even though they have shell escapes because the shell looks at the SHELL variable and runs restricted if the first character of its name is r The restricted editor red is the same as ed except it doesn t allow shell escapes and it only allows editing of files in the current directory Note that most restricted shells are not really very secure They should not be used to contain hostile users Even though some restricted shells are more secure than others if you give a restricted user certain commands such as env he will be able to break out into a nonrestricted shell If you can t find the restricted shell on your system you can copy or link your Bash Korn shell or even old Bourne shell to any filename that begins with r and make that file the login shell of the restricted user When the shell starts up it checks the first letter of the command name that was used to invoke it if that letter is r it will be a restricted shell rs i a Miscellaneous Features Other Features of the ca Command The argument to cd always means the previous directory pwd usr src cmd cd usr spool uucp pwd usr spool uucp cd cd to previous directory usr src cmd cd prints out name of new directory cd usr spool uucp As you can see cd can be used to toggle between two directories with no effort at all The Korn shell s cd command has the ca
164. cally Fields are delimited by space or tab characters by default If a different delimiter is to be used the t option must be used sort 1n data Skip the first field in the sort 14 9 23 2 15 6 5 11 2 12 5 27 3 33 The t Option As mentioned if you skip over fields sort assumes that the fields being skipped are delimited by space or tab characters The t option says otherwise In this case the character that follows the t is taken as the delimiter character Look at our sample password file again cat etc passwd root 0 0 The super User usr bin ksh steve 203 100 users steve usr bin ksh bin 3 3 The owner of system files cron 1 1 Cron Daemon for periodic tasks george 75 75 users george usr lib rsh pat 300 300 users pat usr bin ksh uucp 5 5 usr spool uucppublic usr lib uucp uucico asg 6 6 The Owner of Assignable Devices sysinfo 10 10 Access to System Information usr bin sh mail 301 301 usr mail If you wanted to sort this file by username the first field on each line you could just issue the command sort etc passwd To sort the file instead by the third colon delimited field which contains what is known as your user id you would want an arithmetic sort skipping the first two fields 2n specifying the colon character as the field delimiter t sort 2n t etc passwd Sort by user id root 0 0 The Super User usr bin ksh cron 1
165. ce limitations However almost all this information is also available on the Web and in printed form rs i rs ca Documentation on the Web The authors of this book maintain the Web site at www kochan wood com You can download the rolo program get answers to the exercises and find out more about Unix the C programming language and shell programming there The best place on the Web for information on the POSIX standard is at www unix org This site is maintained by The Open Group an international consortium that worked with the IEEE to create the current POSIX specification The complete specification is available on its Web site You must register first to read it but registration is free The URL for accessing the documentation is www unix org online html You can also purchase for download a printable copy of the Open Group Unix documentation at www opengroup org pubs catalog un htm The standards publications are only available for download or on CD ROM although you may still be able to find some older out of print hard copy versions at Amazon com The Free Software Foundation maintains online documentation for a variety of Unix utilities including Bash at www fsf org manual David Korn the developer of the Korn shell maintains www kornshell com It contains documentation downloads information on books on the Korn shell and links to information on other shells If you want to experiment with the Korn s
166. ces they have been gobbled up by the shell When we discuss quotes in Chapter 6 Can Quote You on That you ll see how you can include blank spaces in arguments to programs We mentioned earlier that the shell searches the disk until it finds the program you want to execute and then asks the Unix kernel to initiate its execution This is true most of the time However there are some commands that the shell knows how to execute itself These built in commands include cd pwd and echo So before the shell goes searching the disk for a command the shell first determines whether it s a built in command and if it is the shell executes the command directly Variable and Filename Substitution Like any other programming language the shell lets you assign values to variables Whenever you specify one of these variables on the command line preceded by a dollar sign the shell substitutes the value assigned to the variable at that point This topic is covered in complete detail in Chapter 5 And Away We Go The shell also performs filename substitution on the command line In fact the shell scans the command line looking for filename substitution characters or before determining the name of the program to execute and its arguments Suppose that your current directory contains the files as shown ls mrs todd progl shortcut sweeney Now let s use filename substitution for the echo command echo List all files m
167. change is the use of the interval variable to specify the number of seconds to sleep os fa Exercises 1 Modify the prargs program to precede each argument by its number So typing prargs a b c d should give the following output 1 a 2 DE 3 d 2 Modify the mon program to also print the tty number that the user logs on to That is the output should say sandy logged onto tty13 if sandy logs on to tty13 3 Adda f option to mon to have it periodically check for the existence of a file ordinary file or directory instead of for a user logging on So typing mon f usr spool uucppublic steve newmemo amp should cause mon to periodically check for the existence of the indicated file and inform you when it does by displaying a message or by mail if the m option is also selected Add a n option to mon that inverts the monitoring function So mon n sandy checks for sandy logging off the system and mon n f tmp dataout amp periodically checks for the removal of the specified file Write a program called collect that runs in the background and counts the number of users logged in at the end of each interval and also the number of processes run during that interval Allow the interval to be specified with a t option see the previous exercise with the default 10 minutes Use the fact that the special shell variable is set to the process number of the last command executed in the background and that
168. chapt1 chapt2 chapt3 chapt4 Suppose that you want to print their contents at the terminal Well you could take advantage of the fact that the cat command allows you to specify more than one filename at a time When this is done the contents of the files are displayed one after the other cat chaptl chapt2 chapt3 chapt4 But you can also type in cat and get the same results The shell automatically substitutes the names of all the files in the current directory for the The same substitution occurs if you use with the echo command echo chapt1 chapt2 chapt3 chapt4 Here the is again replaced with the names of all the files contained in the current directory and the echo command simply displays them at the terminal Any place that appears on the command line the shell performs its substitution echo chapt1 chapt2 chapt3 chapt4 chaptl chapt2 chapt3 chapt4 The can also be used in combination with other characters to limit the filenames that are substituted For example let s say that in your current directory you have not only chapt1 through chapt4 but also files a b and c ls chapt1l chapt2 chapt3 chapt4 To display the contents of just the files beginning with chapt you can type in cat chapt The chapt matches any filename that begins with chapt All such filenames matched are substituted on the command line The is not limited to the end of a filename it can
169. cho echo echo echo echo echo echo echo echo echo echo zero one two three four five S1X seven eight nine Bad argument please specify a single digit Bad argument please specify a single digit Here s another program called ctype that prints the type of the single character given as an argument Character types recognized are digits uppercase letters lowercase letters and special characters anything not in the first three categories As an added check the program makes sure that just a single character is given as the argument cat ctype Classify character given as argument if ne 1 then echo Usage ctype char exit 1 fi Ensure that only one character was typed char 1 numchars echo char wc c if numchars ne 1 then echo Please type a single character exit 1 fa Now classify it case char in 0 9 echo digit a z echo lowercase letter A Z echo uppercase letter X echo special character esac Some sample runs ctype a Please type a single character ctype 7 Please type a single character The x Option for Debugging Programs Something seems to be amiss The counting portion of our program doesn t seem to be working properly This seems like a good point to introduce the shell s x option You can trace the execution of any program by typ
170. cified the input and or output for the shell is accordingly redirected Examples exec bin sh Replace current process with sh exec lt datafile Reassign standard input to datafile The exit Command General Format exit n Execution of exit causes the current shell program to be immediately terminated The exit status of the program is the value of the integer n if supplied If n is not supplied the exit status is that of the last command executed prior to the exit An exit status of zero is used by convention to indicate success and nonzero to indicate failure Such as an error condition This convention is used by the shell in evaluation of conditions for if while and until commands and with the amp amp and constructs Examples who grep user gt dev null exit Exit with status of last grep exit 1 Exit with status of 1 if finduser If finduser returns an exit status of zero then then fi Note that executing exit from a login shell has the effect of logging you off The export Command General Format export variables The export command tells the shell that the indicated variables are to be marked as exported that is their values are to be passed down to subshells Examples export PATH PS1 export dbhome x1 y1 date Variables may be set when exported using the form export variable value So lines such as PATH PATH HOME bin export PATH CDPATH HOME usr spool uucppublic e
171. cked for a user logging on to the system One of the comments we made back then is that it would be nice if the program could somehow automatically send itself to the background for execution because that s how it s really meant to be run Now you know how to do it You simply enclose the until loop and the commands that follow inside parentheses and send it into the background cat mon Wait until a specified user logs on version 4 Set up default values mai lopt FALSE interval 60 process command options while getopts mt option do case option in m mailopt TRUE t interval 0PTARG 47 echo Usage mon m t n user echo m means to be informed by mail echo t means check every n secs exit 1 esac done Make sure a user name was specified if OPTIND gt then echo Missing user name exit 2 fi shiftcount OPTIND 1 shift shiftcount user 1 Send everything that follows into the background Check for user logging on until who grep user gt dev null do sleep interval done When we reach this point the user has logged on if mailopt FALSE then echo user has logged on else runner who am i cut c1 8 echo user has logged on mail runner fi amp The entire program could have been enclosed in parentheses but we arbitrarily decided to do the argument checking and parsing
172. cksum can be generated on the sending end for each file in the archive and then commands included in the shell archive can verify the sum on the receiving end If they don t match an error message can be displayed Fe previous Ne lt T rs ca Functions The POSIX standard shell supports the concept of functions note that older shells may not support this feature To define a function you use the general format name command command where name is the name of the function the parentheses denote to the shell that a function is being defined and the commands enclosed between the curly braces define the body of the function These commands will be executed whenever the function is executed Note that at least one whitespace character must separate the from the first command and that a semicolon must separate the last command from the closing brace if they occur on the same line The following defines a function called nu that displays the number of logged in users nu who wc l You execute a function the same way you execute an ordinary command simply by typing its name to the shell nu 22 Arguments listed after the function on the command line are assigned to the positional parameters 1 2 just as with any other command Here s a function called nrrun that runs tbl nroff and lp on the file given as its argument nrrun tbl 1 nroff mm Tlp lp nrrun memol Run it on memol r
173. command can be used to terminate a job running in the background The argument to it can be a process number or a percent sign followed by a job number a current job a previous job or another also current job pic chaptl troff gt aps out amp 1 886 jobs 1 Running pic chaptl troff gt aps out amp kill 1 1 Done pic chaptl troff gt aps out amp The preceding kill could have used or to refer to the same job The first few characters of the command sequence can also be used to refer to a job for example kill pic would have worked in the preceding example Stopped Jobs and the fg and bg Commands If you are running a job in the foreground without an amp and you want to suspend it you can press the Ctrl z key The job stops executing and the shell prints the message n Stopped SIGTSTP sequence The stopped job is made the current job To have it continue executing you must use the fg or bg command The fg command with no arguments causes the current job to resume execution in the foreground and bg causes the current job to resume execution in the background You can also use a job number the first few characters of the pipeline a a or a preceded by a to specify any job to the fg and bg commands These commands print out the command sequence to remind you what is being brought to the foreground or sent to the background troff memo photo Ctrl z 1
174. command line is an alias it is replaced by the text of the alias An alias is defined by using the alias command The format is alias name string where name is the name of the alias and string is any string of characters For example alias ll ls l assigns ls l to the alias Ll Now when the alias ll is typed in the shell replaces it with ls l You can type arguments after the alias name on the command line as in LL Fee which looks like this after alias substitution has been performed tls l c The shell performs its normal command line processing both when the alias is set and when it is used so quoting can be tricky For example recall that the shell keeps track of your current working directory inside a variable called PWD cd users steve letters echo PWD users steve letters You can create an alias called dir that gives you the base directory of your current working directory by using the PWD variable and one of the parameter substitution constructs described in an earlier section of this chapter alias dir echo PWD Let s see how this alias works alias dir echo PWD Define alias pwd Where are we users steve dir Execute alias steve cd letters Change directory dir Execute the alias again steve cd usr spool One more try dir steve It seems that no matter what the current directory is the dir alias prints out steve That s because we weren t careful about o
175. commands and use the read command to read data from the terminal or from a file Chapter 6 Can Quote You on That is devoted entirely to a discussion on one of the most intriguing and often confusing aspects of the shell the way it interprets quotes By this point in the book all the basic programming constructs in the shell will have been covered and you will be able to write shell programs to solve your particular problems Chapter 11 Your Environment covers a topic of great importance for a real understanding of the way the shell operates the environment You ll learn about local and exported variables subshells special shell variables such as HOME PATH and CDPATH and how to set up your profile file Chapter 12 More on Parameters and Chapter 13 Loose Ends tie up some loose ends and Chapter 14 Rolo Revisited presents a final version of a phone directory program called rolo that is developed throughout the book Chapter 15 Interactive and Nonstandard Shell Features discusses features of the shell that either are not formally part of the IEEE POSIX standard shell but are available in most Unix and Linux shells or are mainly used interactively instead of in programs Appendix A Shell Summary summarizes the features of the IEEE POSIX standard shell Appendix B For More Information lists references and resources including the Web sites where different shells can be downloaded The philosophy this
176. cute this in the current shell echo x 100 pwd Where am I users steve cd bin ls Change to bin and do an ls add greetings lu number phonebook rem rolo pwd users steve No change cd bin This should change me pwd users steve bin If the commands enclosed in the braces are all to be typed on the same line a space must follow the left brace and a semicolon must appear after the last command As the example cd bin ls shows the parentheses are useful for doing some commands without affecting your current environment You can also use them for other purposes sort 2002data o 2002data plotdata 2002data amp 1 3421 The parentheses group the sort and plotdata commands together so that they can both be sent to the background for execution with their order of execution preserved Input and output can be piped to and from these constructs and I O can be redirected In the next example a ls 2 nroff command for double spaced output is effectively tacked to the beginning of the file memo before being sent to nroff echo ls 2 cat memo nroff Tlp lp In the command sequence progl prog2 prog3 2 gt errors all messages written to standard error by the three programs are collected into the file errors As a final example let s return to the mon program from Chapter 9 Round and Round She Goes As you ll recall this program periodically che
177. cuted This process continues with successive words in the list being assigned to var and the commands in the loop body being executed until the last word in the list word is assigned to var and the body of the loop executed At that point no words are left in the list and execution of the for command is then finished Execution then continues with the command that immediately follows the done So if there are n words listed after the in the body of the loop will have been executed a total of n times after the loop has finished Here s a loop that will be executed a total of three times for iinil2 3 do echo i done To try it out you can type this in directly at the terminal just like any other shell command fori in 12 3 gt do gt echo i gt done While the shell is waiting for the done to be typed to close off the for command it displays your secondary command prompt When it gets the done the shell then proceeds to execute the loop Because three words are listed after the in 1 2 and 3 the body of the loop in this case a single echo command will be executed a total of three times The first time through the loop the first word in the list 1 is assigned to the variable i Then the body of the loop is executed This displays the value of i at the terminal Then the next word in the list 2 is assigned to i and the echo command re executed resulting in the display of 2 at the terminal The third word i
178. d 3rd looking up 2nd rolo program listing 2nd 3rd 4th sample program output 2nd 3rd 4th 5th 6th phonebook file adding entries to 2nd 3rd deleting entries from 2nd 3rd 4th looking up entries 2nd 3rd PHONEBOOK variable 2nd 3rd 4th 5th PIDs process ids 2nd pipe character pipe symbol 2nd 3rd construct 2nd 3rd 4th pattern matching pipes 2nd 3rd 4th pipeline hookup 2nd piping data into loops 2nd plus sign pointers to variables creating positional parameters 2nd 3rd 4th defined 2nd left shifting 2nd 3rd 4th reassigning values to 2nd 3rd 4th 5th 6th POSIX standard Web site POSIX standard shell pound sign 2nd 3rd 4th 5th 6th PPID variable PPIDs parent process ids precise number of characters matching 2nd 3rd 4th printf command 2nd 3rd 4th 5th 6th 7th 8th conversion specification characters 2nd conversion specification modifiers 2nd examples 2nd 3rd 4th 5th 6th syntax printing command information formatted output 2nd 3rd 4th 5th 6th 7th command syntax conversion specification characters 2nd conversion specification modifiers 2nd examples 2nd 3rd 4th 5th 6th job status list of active jobs 2nd working directory 2nd process ids PIDs 2nd processes See also jobs background processes waiting for 2nd 3rd 4th 5th child processes
179. d 4th 5th 6th 7th 8th syntax 2nd s option 2nd examples 2nd 3rd 4th 5th 6th 7th syntax trap 2nd 3rd 4th 5th 6th 7th 8th executing without arguments 2nd ignoring signals 2nd resetting traps 2nd signal numbers syntax true type 2nd 3rd 4th umask 2nd unalias 2nd uniq c option d option 2nd 3rd examples 2nd 3rd syntax unset 2nd 3rd 4th until 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th examples 2nd 3rd 4th 5th 6th 7th 8th syntax 2nd line edit mode 2nd 3rd 4th 5th 6th 7th 2nd L 2nd 3rd V IO N en n Q n I N n In N oy commands 2nd 3rd 4th dw h N oa 3rd n N Qa In N es OC WW UJ y 2 3rd N gt a w Q 3 l N 3 2 x N gt a w a wait 2nd variable 2nd example syntax 2nd while 2nd 3rd 4th 5th who 2nd wv 2nd commaneds read exit status 2nd 3rd 4th 5th commans executing 2nd 3rd 4th comments 2nd 3rd 4th 5th comments pound sign comparing values case command command syntax 2nd 3rd pattern matching characters 2nd 3rd 4th pipe symbol 2nd 3rd compatibility of shells 2nd contents of files displaying cat command 2nd continuation character continue command 2nd 3rd 4th conversion specification characters 2nd conversion specification modifiers 2nd Coorinated Univ
180. d by the nested command substitution otherwise the two examples are the same The expr Command Although the POSIX standard shell supports built in integer arithmetic operations older shells don t It s likely that you may see command substitution with a Unix program called expr which evaluates an expression given to it on the command line expr 1 2 3 Each operator and operand given to expr must be a separate argument thus explaining the output from the following expr 1 2 1 2 The usual arithmetic operators are recognized by expr for addition for subtraction for division for multiplication and for modulus remainder expr 10 20 2 20 Multiplication division and modulus have higher precedence than addition and subtraction Thus in the preceding example the division was performed before the addition expr 17 6 expr syntax error What happened here The answer The shell saw the and substituted the names of all the files in your directory It has to be quoted to keep it from the shell expr 17 6 17 6 That s not the way to do it Remember that expr must see each operator and operand as a separate argument the preceding example sends the whole expression in as a single argument expr 17 6 102 Naturally one or more of the arguments to expr can be the value stored inside a shell variable because the shell takes care of the substitution first an
181. d names Frequently you want to sort the lines in a file and have the sorted data replace the original Typing sort names gt names won t work it ends up wiping out the names file However with the o option it is okay to specify the same name for the output file as the input file sort names o names cat names Charlie Emanuel Fred Lucy Ralph Tony Tony The n Option Suppose that you have a file containing pairs of x y data points as shown cat data 5 27 2 12 3 33 23 2 15 6 14 9 Suppose that you want to feed this data into a plotting program called plotdata but that the program requires that the incoming data pairs be sorted in increasing value of x the first value on each line The n option to sort specifies that the first field on the line is to be considered a number and the data is to be sorted arithmetically Compare the output of sort used first without the n option and then with it sort data 5 11 14 9 15 6 2 12 23 2 3 33 5 27 sort n data Sort arithmetically 5 11 2 12 3 33 5 27 14 9 15 6 23 2 Skipping Fields If you had to sort your data file by the y value that is the second number in each line you could tell sort to skip past the first number on the line by using the option 1n instead of n The 1 says to skip the first field Similarly 5n would mean to skip the first five fields on each line and then sort the data numeri
182. d number add name number validchoice TRUE 3 echo Enter name to be removed c read name rem name validchoice TRUE echo Bad choice esac done cat add Program to add someone to the phone book file if ne 2 then echo Incorrect number of arguments echo Usage add name number exit 1 fi echo 1 2 gt gt PHONEBOOK sort o PHONEBOOK PHONEBOOK cat lu Look someone up in the phone book if ne 1 then echo Incorrect number of arguments echo Usage lu name exit 1 fi name 1 grep name PHONEBOOK if ne 0 then echo I couldn t find name in the phone book Fa cat rem Remove someone from the phone book if ne 1 then echo Incorrect number of arguments echo Usage rem name exit 1 fi name 1 Find number of matching entries matches grep name PHONEBOOK wc lL If more than one match issue message else remove it if matches gt 1 then echo More than one match please qualify further elif matches eq 1 then grep v name PHONEBOOK gt tmp phonebook mv tmp phonebook PHONEBOOK else echo I couldn t find name in the phone book fi In an effort to be more user friendly a test was added to the end of lu to see whether the grep succeeds if it doesn t a message is displayed to the user Now to test it cd Ret
183. d ones look like this mon Missing user name mon t600 ann Need a space after t mon ann m Options must appear first mon t ann Missing argument after t If you start writing the code to allow this sort of flexibility on the command line you will soon discover that it can start to get a bit complex Luckily the shell provides a built in command called getopts that exists for the express purpose of processing command line arguments The general format of the command is getopts options variable The getopts command is designed to be executed inside a loop Each time through the loop getopts examines the next command line argument and determines whether it is a valid option This determination is made by checking to see whether the argument begins with a minus sign and is followed by any single letter contained inside options If it does getopts stores the matching option letter inside the specified variable and returns a zero exit status If the letter that follows the minus sign is not listed in options getopts stores a question mark inside variable before returning with a zero exit status It also writes an error message to standard error If no more arguments are left on the command line or if the next argument doesn t begin with a minus sign getopts returns a nonzero exit status Suppose that you want getopts to recognize the options a i and r for a command called foo Your getopts call might look like this getopts air optio
184. d processes waiting for 2nd 3rd 4th 5th background executing jobs in background running jobs in backquote 2nd 3rd 4th backslash 2nd 3rd 4th 5th backslash inside double quotes 2nd 3rd 4th line continuation character 2nd bases arithmetic 2nd 3rd Bash shell beginning of line matching 2nd bg command 2nd 3rd 4th 5th 6th books AWK Programming Language The Learning Pearl Programming Pearl Unix C Shell Field Guide The Unix in a Nutshell colon System V Edition Unix Programmer s Manual Volume II Unix Programming Environment The Bourne shell Bourne Stephen 2nd braces brackets filename substitution 2nd 3rd break command 2nd 3rd 4th 5th 6th Break key breaking loops 2nd breaking loops 2nd 3rd 4 rrevious SYMBOL A B C D E F G H L J K L M N O P Q R S T U V W X Y Z c escape character c escape characters C command vi c conversion specification character c option sh command uniq command wc command C option set command caret 2nd 3rd case command 2nd 3rd pattern matching characters 2nd 3rd 4th amp amp construct 2nd 3rd 4th asterisk 2nd question mark L construct construct 2nd 3rd 4th pipe symbol 2nd 3rd syntax 2nd 3rd case statement cat command
185. definitions 2nd lines of text sed command 2nd phone book entries 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th 12th 13th variable definitions 2nd delimiter characters sort command 2nd 3rd design rolodex program 2nd destination files checking 2nd 3rd directories 2nd 3rd 4th See also files changing cd command 2nd 3rd 4th 5th copying files between 2nd creating mkdir command 2nd current directory 2nd current working directories changing 2nd 3rd 4th 5th defined displaying 2nd deleting rmdir command 2nd directory structure 2nd home directories identifying 2nd listing files in Is command 2nd 3rd 4th moving files between 2nd parent directories pathnames 2nd pathname pathname full pathnames relative pathnames root directories working directory printing 2nd disabling trace mode display program 2nd displaying current working directory pwd command 2nd date time date command 2nd file contents cat command 2nd list of files Is command 2nd 3rd 4th 5th 6th list of variables 2nd logged in users who command 2nd phone book entries 2nd 3rd 4th traps 2nd variable values documentation man pages tutorials Web sites 2nd Cygwin Free Software Foundation Korn shell 2nd POSIX standard dollar sign 2nd 3rd 4th 5th double quotes 2nd 3rd 4th
186. directory pwd users steve bin number 99 echo There are number bottles of beer on the wall There are 99 bottles of beer on the wall Here are some more examples command sort command names Charlie Emanuel Fred Lucy Ralph Tony Tony Command wc option l file names command option file 7 names So you see even the name of a command can be stored inside a variable Because the shell performs its substitution before determining the name of the program to execute and its arguments it scans the line command option file and turns it into wc l names Then it executes wc passing the two arguments l and names Variables can even be assigned to other variables as shown in the next example valuel 10 value2 valuel echo value2 valuel Didn t do that right value2 valuel echo value2 10 That s better Remember that a dollar sign must always be placed before the variable name whenever you want to use the value stored in that variable The Null Value What do you think happens when you try to display the value of a variable that has no value assigned to it Try it and see echo nosuch Never assigned it a value You don t get an error message Did the echo command display anything at all Let s see whether we can more precisely determine that echo nosuch Surround its value with colons So you see no characters were substituted by the shell fo
187. dth modifier is a positive number that specifies the minimum field width for printing an argument The argument is right justified within this field unless the flag is used printf 20s 20s n stringl string2 stringl string2 printf 20s 20s n stringl string2 stringl string2 printf 5d 5d 5d n 1 10 100 1 10 100 printf 5d 5d 5d n 1 10 100 1 10 100 printf 5d 5d 5d n 1 10 100 1 10 100 The width modifier can be useful for lining up columns of text or numbers note that signs for numbers and leading 0 0x and 0X characters are counted as part of the argument s width The width specifies a minimum size for the field if the width of an argument exceeds width it is not truncated The precision modifier is a positive number that specifies a minimum number of digits to be displayed for d u 0 Sx and X This results in zero padding on the left of the value printf 5d 4X n 10 27 00010 001B For strings the precision modifier specifies the maximum number of characters to be printed from the string if the string is longer than precision characters it is truncated on the right printf 5s n abcdefg abcde A width can be combined with precision to specify both a field width and zero padding for numbers or truncation for strings printf 10 5x 5 4x 5 4d n 1 10 100 0x00001 000a 0100 printf 9 5s n abcdefg abcde printf 9 5s n abcdefg abcde
188. e cat profile HISTSIZE 500 export HISTSIZE Be reasonable about the values that you assign to HISTSIZE The larger the value the more disk space you will need to store the history file and the longer it will take the shell to search through the entire history file ns fy ua NeXt gt The vi Line Edit Mode After turning on the vi line editor you will be placed in input mode You probably won t even notice anything different about input mode because you can type in and execute commands almost the same as before you started the vi line editor set o vi echo hello hello pwd users pat To make use of the line editor you must enter command mode by pressing the ESCAPE or Esc key usually in the upper left corner of the keyboard When you enter command mode the cursor moves to the left one space to the last character typed in The current character is whatever character the cursor is on we ll say more about the current character in a moment When in command mode you can enter vi commands Note that vi commands are not followed by an Enter One problem often encountered when typing in long commands is that you may notice an error in a command line after you finish typing it in Invariably the error is at the beginning of the line In command mode you can move the cursor around without disturbing the command line After you ve moved the cursor to the place where the error is you can change the letter or letters
189. e a program called myrm that takes as arguments the names of files to be removed If the global variable MAXFILES is set take it as the maximum number of files to remove without question If the variable is not set use 10 as the maximum If the number of files to be removed exceeds this count ask the user for confirmation before removing the files ls we l 25 myrm Remove them all Remove 25 files y n n files not removed MAXFILES 100 myrm ls All files removed If MAXFILES is set to zero the check should be suppressed 2 Here are two programs called progl and prog2 cat progl e1 100 export el e2 200 e3 300 prog2 cat prog2 echo el1 e2 e3 e4 What output would you expect after typing the following e2 20 export e2 e4 40 progl Modify rolo from this chapter so that a person running the program can keep his or her phone book file in any directory and not just in the home directory This can be done by requiring that the user set an exported variable called PHONEBOOK to the name of the phone book file before executing rolo Check to make sure that this variable is set to a valid file If the variable is not set have the program assume that the phone book file is in the user s home directory as before Here are some examples PHONEBOOK users steve personal lu Gregory Gregory 973 555 0370 PHONEBOOK users pat phonebook lu Toritos El Toritos 973 555 2236 In the precedi
190. e and commands to compare the count of each extracted file against the count of the original file If a discrepancy occurs an error should be noted as in add expected 345 characters extracted 343 Fe previous next gt 4 Previous NEXT Chapter 14 Rolo Revisited IN THIS CHAPTER Design Considerations rolo add lu display rem change listall Sample Output Exercises This chapter presents a final version of the rolo program This version is enhanced with additional options and also allows for more general types of entries other than just names and numbers The sections in this chapter discuss the individual programs in rolo starting with rolo itself At the end of this chapter sample output is shown u Ga a Design Considerations A more practical type of rolodex program would permit more than just the names and numbers to be stored in the phone book You d probably want to keep addresses maybe even electronic mail addresses there as well The new rolo program allows entries in the phone book to consist of multiple lines For example a typical entry might be Steve s Ice Cream 444 6th Avenue New York City 10003 212 555 3021 To increase the flexibility of the program we re allowing an individual entry to contain as many lines as desired So another entry in the phone book might read YMCA 201 555 2344 To logically separate one entry from the next inside the phone book file each entry is
191. e and could not be written as mv file filex because the shell would substitute the value of filex for the second argument As mentioned in Chapter 7 Passing Arguments to access positional parameters 10 and above you must enclose the number inside the curly braces as in 11 parameter value This construct says to substitute the value of parameter if it is not null and to substitute value otherwise For example in the command line echo Using editor EDITOR bin vi the shell substitutes the value of EDITOR if it s not null and the value bin vi otherwise It has the same effect as writing if n EDITOR then echo Using editor EDITOR else echo Using editor bin vi Ti The command line EDITOR bin ed tmp edfile starts up the program stored in the variable EDITOR presumably a text editor or bin ed if EDITOR is null Here s a simple test of this construct from the terminal EDITOR bin ed echo EDITOR bin vi bin ed EDITOR Set it null echo EDITOR bin vi bin vi parameter value This version is similar to the last only if parameter is null not only is value used but it is also assigned to parameter as well note the in the construct You can t assign values to positional parameters this way that means that parameter can t be a number A typical use of this construct would be in testing to see whether an exported variable has been set and if
192. e files listed in MAILPATH The default is 600 A value of 0 causes the shell to check before displaying each command prompt A list of files to be checked for the arrival of mail Each file is delimited by a colon and can be followed by a percent sign and a message to be displayed when mail arrives in the indicated file You have mail is often the default A colon delimited list of directories to be searched when the shell needs to find a command to be executed The current directory is specified as or if it heads or ends the list suffices The process id number of the program that invoked this shell that is the parent process The primary command prompt normally The secondary command prompt normally gt Prompt used during execution trace x option to shell or set x Default is Pathname of the current working directory Parameter Substitution In the simplest case the value of a parameter can be accessed by preceding the parameter with a dollar sign Table A 3 summarizes the different types of parameter substitution that can be performed Parameter substitution is performed by the shell before filename substitution and before the command line is divided into arguments The presence of the colon after parameter in Table A 3 indicates that parameter is to be tested to see whether it s set and not null Without the colon a test is made to check whether parameter is set only Parameter p
193. e format shift n is used instead the shift is to the left n places Examples set abcd echo n abcd shift echo n bac shift 2 echo n The test Command General Format test condition or condition The shell evaluates condition and if the result of the evaluation is TRUE returns a zero exit status If the result of the evaluation is FALSE a nonzero exit status is returned If the format condition is used a space must appear immediately after the and before the condition is composed of one or more operators as shown in Table A 11 The a operator has higher precedence than the o operator In any case parentheses can be used to group subexpressions J ust remember that the parentheses are significant to the shell and so must be quoted Operators and operands including parentheses must be delimited by one or more spaces so that test sees them as separate arguments test is often used to test conditions in an if while or until command Examples see if perms is executable if test x etc perms then fi see if it s a directory or a normal file that s readable if d file o f file a r file then fi Table A 11 test Operators Operator File Operators b file C file d file e file f file g file h file k file L file p file r file S file s file t fd Returns TRUE zero exit sta
194. e late 1960s One of the primary goals in the design of the UNIX system was to create an The i option to grep indicates that upper and lowercase letters are not to be distinguished in the matching process That is the command grep i the intro tells grep to ignore case when matching the pattern against the lines in intro Therefore lines containing the or The will be printed as will lines containing THE THe tHE and so on Table 4 5 shows other types of regular expressions that you can specify to grep and the types of patterns they ll match Table 4 5 Some grep Examples Command Prints grep A Z list Lines from list containing a capital letter grep 0 9 data Lines from data containing a number grep A Z 9 list Lines from List containing five character patterns that start with a capital letter and end with a digit grep pic filelist Lines from filelist that end in pic The v Option Sometimes you re interested not in finding the lines that contain a specified pattern but those that don t To do this with grep is simple You use the v option In the next example grep is used to find all the lines in intro that don t contain the characters UNIX grep v UNIX intro Print all lines that don t contain UNIX Thompson and Dennis Ritchie at Bell Laboratories in the late 1960s One of the primary goals in environment that promoted efficient program development The 1 Option At times y
195. e logged in Another approach to determine the number of logged in users bypasses the use of a file The Unix system allows you to effectively connect two commands together This connection is known as a pipe and it enables you to take the output from one command and feed it directly into the input of another command A pipe is effected by the character which is placed between the two commands So to make a pipe between the who and wc l commands you simply type who wc L who wc l 5 The pipe that is effected between these two commands is depicted in Figure 2 12 Figure 2 12 Pipeline process who wc l ai ttyOi Sep 12 oko tty36 Sep 12 ruth tty24 Sep 12 steve tty25 Sep 12 F w w When a pipe is set up between two commands the standard output from the first command is connected directly to the standard input of the second command You know that the who command writes its list of logged in users to standard output Furthermore you know that if no filename argument is specified to the wc command it takes its input from standard input Therefore the list of logged in users that is output from the who command automatically becomes the input to the wc command Note that you never see the output of the who command at the terminal because it is piped directly into the wc command This is depicted in Figure 2 13 Figure 2 13 Pipeline process A pipe can be made between any two programs provided t
196. e name of a command to execute it s frequently useful to know where that command is coming from In other words is the command actually defined as a function Is it a shell program Is ita shell built in Is it a standard Unix command This is where the type command comes in handy The type command takes one or more command names as its argument and tells you what it knows about it Here are some examples nu who we l type pwd pwd is a shell builtin type troff troff is usr bin troff type cat cat is bin cat type nu nu is a function Fe previous Next F os fa Exercises 1 Using eval write a program called recho that prints its arguments in reverse order So recho one two three should produce three two one Assume that more than nine arguments can be passed to the program 2 Modify the shar program presented in this chapter to handle directories shar should recognize input files from different directories and should make sure that the directories are created if necessary when the archive is unpacked Also allow shar to be used to archive an entire directory ls rolo lu add rem rolo shar rolo lu rolo add rolo rem gt rolosubs shar shar rolo gt rolo shar In the first case shar was used to archive three files from the rolo directory In the last case shar was used to archive the entire rolo directory 3 Modify shar to include in the archive the character count for each fil
197. e new for old in current directory s path mycd if le 1 then normal case 0 or 1 argument cd 1 elif eq 2 then special case substitute 2 for 1 cd echo PWD sed s 1 2 else cd can t have more than two arguments echo mycd bad argument count exit 1 fi myfuncs Read in definition pwd users steve mycd users pat Change directory pwd Did it work users pat mycd pat tony Substitute tony for pat pwd users tony After a function has been defined its execution will be faster than an equivalent shell program file That s because the shell won t have to search the disk for the program open the file and read its contents into memory Another advantage of functions is the capability to group all your related shell programs in a single file if desired For example the add Lu and rem programs from Chapter 11 Your Environment can be defined as functions inside rolo The template for such an approach is shown cat rolo rolo program written in function form Function to add someone to the phonebook file add put commands from add program here Function to look someone up in the phone book tu 4 put commands from lu program here Function to remove someone from the phone book rem put commands from rem program here rolo rolodex program to look up add and remove people from the pho
198. e shell to the shell just type ina q This output indicates that two lines in the file ed cmd contain the word shell If the pattern does not exist in the specified file s the grep command simply displays nothing grep cracker ed cmd You saw in the section on sed how you could print all lines containing the string UNIX from the file intro with the command sed n UNIX p intro But you could also use the following grep command to achieve the same result grep UNIX intro Recall the phonebook file from before cat phone book Alice Chebba 973 555 2015 Barbara Swingle 201 555 9257 Jeff Goldberg 201 555 3378 Liz Stachiw 212 555 2298 Susan Goldberg 201 555 7776 Tony Iannino 973 555 1295 When you need to look up a particular phone number the grep command comes in handy grep Susan phone book Susan Goldberg 201 555 7776 The grep command is useful when you have a lot of files and you want to find out which ones contain certain words or phrases The following example shows how the grep command can be used to search for the word shell in all files in the current directory grep shell cmdfiles shell that enables sophisticated ed cmd files and is independent of the shell ed cmd to the shell just type in a q grep cmd occurrence of the word shell grep cmd grep shell grep cmd every use of the word shell As noted when more than one file is specified to grep each output line is prec
199. e shell searches the disk until it finds the named program When found the shell asks the kernel to initiate the program s execution and then the shell goes to sleep until the program has finished Step 5 The kernel copies the specified program into memory and begins its execution This copied program is called a process in this way the distinction is made between a program that is kept in a file on the disk and a process that is in memory doing things Figure 3 6 Command cycle 7 If the program writes output to standard output it will appear at your terminal unless redirected or piped into another command Similarly if the program reads input from standard input it will wait for you to type in input unless redirected from a file or piped from another command Step 6 When the command finishes execution control once again returns to the shell which awaits your next command Steps 7 and 8 Note that this cycle continues as long as you re logged in When you log off the system execution of the shell then terminates and the Unix system starts up a new getty or rlogind and so on at the terminal and waits for someone else to log in This cycle is illustrated in Figure 3 7 Figure 3 7 Login cycle It s important for you to recognize that the shell is just a program It has no special privileges on the system meaning that anyone with the capability and devotion can create his own shell program This is in fact the reason w
200. e the with the text you type and the rest of the line is updated with the matching command reverse i search _ c reverse i search c echo this is a test reverse i search c echo this is a test d reverse i search cd cd tmp Note how Bash highlights the matching part of the command by placing the cursor on it As with the Korn shell the command is executed by pressing Enter When you ve found the command in the history either by Ctri p Ctril n or Ctri r you can edit the command using the other emacs commands we ve already discussed Note that you don t actually change the command in the history That command cannot be changed after it is entered Instead you are editing a copy of the command in the history which will itself be entered in the history when you press Enter Table 15 2 summarizes the basic line edit commands Table 15 2 Basic emacs Line Edit Commands Command Meaning Ctri b Move left one character Ctri f Move right one character Esc f Move forward one word Esct b Move back one word Ctrit a Move to start of line Ctri e Move to end of line Ctri d Delete current character Esc d Delete current word erase char User defined erase character usually or Ctri h delete previous character Ctri p Get previous command from history Ctrl n Get next command from history Ctril r string Search history for the most recent command line containing string rs ca Other Ways to Access Your History T
201. ease select one of the above 1 3 c Read and process selection read choice echo case choice in 1 echo Enter name to look up c read name lu name 2 echo Enter name to be added c read name echo Enter number c read number add name number 3 echo Enter name to be removed c read name rem name echo Bad choice esac A single echo command is used to display the menu at the terminal taking advantage of the fact that the quotes preserve the embedded newline characters Then the read command is executed to get the selection from the user and store it in the variable choice A case statement is next entered to determine what choice was made If choice 1 was selected the user wants to look up someone in the phone book In that case the user is asked to enter the name to be looked up and the lu program is called passing it the name typed in by the user as the argument Note that the double quotes around name in lu name are necessary to ensure that two or more words typed in by the user are handed over to Lu as a single argument A similar sequence occurs if the user selects menu items 2 or 3 The programs Lu rem and add are from earlier chapters Lu is from page 137 rem from page 167 and add from page 138 Here are some sample runs of rolo rolo Would you like to 1 Look someone up 2 Add someone to the phone book 3 Remove someone from the pho
202. echnique is used in a final version of the rolo program that s presented in Chapter 14 Rolo Revisited The following program called number2 is a final version of the line numbering program presented in Chapter 10 Reading and Printing Data This program faithfully prints the input lines to standard output preceded by a line number Notice the use of printf to right align the line numbers cat number2 Number lines from files given as argument or from standard input if none supplied final version Modify the IFS to preserve leading whitespace on input ise Just a newline appears between the quotes Lineno 1 cat while read r line do printf 5d s n lineno line Lineno Lineno 1 done Here s a sample execution of number number2 words 1 2 Count all of the words on standard input 3 5 count 0 6 while read line 7 do 8 set line 9 count count 10 done Li 12 echo count Because the IFS has an influence on the way things are interpreted by the shell if you re going to change it in your program it s usually wise to save the old value first in another variable such as OIFS and then restore it after you ve finished the operations that depend on the changed IFS ns Ga rs ca The readonly Command The readonly command is used to specify variables whose values cannot be subsequently changed For example readonly PATH HOME makes t
203. echo Copy not performed exit 0 fi ri Either destination doesn t exist or yes was typed cp from to proceed with the copy And now for the test Ls addresses intro lotsaspaces mycp names nu numbers phonebook stat mycp Usage mycp from to mycp names names2 ls l names rWw r r 1 steve rW r r 1 steve mycp names numbers What files are around No arguments Make a copy of names Did it work steve 43 Jul 20 11 12 names steve 43 Jul 21 14 16 names2 Try to overwrite an existing file numbers already exists overwrite yes no no Copy not performed To complete the test cases try answering yes and ensuring that the program proceeds with the copy There are a few things worthy of mention with the mycp program First if the file already exists the echo command that prompts for the yes no response is executed The read command that follows causes the shell to wait for you to type something in Note that the shell does not prompt you when it s waiting for you to enter data it s up to you to add your own prompt message to the program The data that is typed is stored in the variable answer and is then tested against the characters yes to determine whether the copy is to proceed The quotes around answer in the test answer yes are necessary in case the user just presses the Enter key without typing any data In that case the shell would store a null v
204. ecification modifiers 2nd examples 2nd 3rd 4th 5th 6th Fox Brian FPATH variable Free Software Foundation Web site FreeBSD Unleashed full pathnames functions 2nd 3rd advantages 2nd arguments automatically loaded functions 2nd cdh 2nd 3rd 4th defining 2nd deleting 2nd executing 2nd mycd function 2nd recursive functions 2nd stopping execution of terminating execution of 2nd 4 rrevious SYMBOL A B C D E F G H L J K L M N O P Q R S T U V W X Y Z ge ge operator getopts command 2nd 3rd 4th 5th 6th 7th 8th 9th 10th getty program 2nd Greenwich Mean Time greg command l option 2nd a n option 2n a v option 2n examples 2nd 3rd 4th 5th 6th 7th syntax grep command 2nd grouping commands 2nd 3rd 4th 5th 6th 7th 8th groups changing 2nd gt gt operator 4 Previous 4 rrevious SYMBOL A B C D E F G H 1 1 K L M N O P Q R S T U V W X Y Z h command vi 2nd 3rd h option set command hash command 2nd hierarchical directory structure 2nd HISTFILE variable 2nd history command 2nd history file See command history HISTSIZE variable 2nd home directories identifying 2nd HOME variable 2nd 3rd HUP signal hyphen fe revious 4 rrevious
205. ed wc c names 27 names Finally the w option can be used to count the number of words contained in the file wc w names 5 names Some commands require that the options be listed before the filename arguments For example sort names r is acceptable whereas wc names lis not Let s generalize by saying that command options should precede filenames on the command line Making a Copy of a File The cp Command To make a copy of a file the cp command is used The first argument to the command is the name of the file to be copied known as the source file and the second argument is the name of the file to place the copy into known as the destination file You can make a copy of the file names and call it saved names as follows Cp names saved names Execution of this command causes the file named names to be copied into a file named saved names As with many Unix commands the fact that a command prompt was displayed after the cp command was typed indicates that the command executed successfully Renaming a File The nv Command A file can be renamed with the mv command The arguments to the mv command follow the same format as the cp command The first argument is the name of the file to be renamed and the second argument is the new name So to change the name of the file saved names to hold it for example the following command would do the trick mv saved names hold it When executing an mv or cp com
206. ed arg that contained a digit for example and you wanted to display the positional parameter referenced by arg You could simply write eval echo arg The only problem is that just the first nine positional parameters can be accessed this way to access positional parameters 10 and greater you must use the n construct eval echo arg Here s how the eval command can be used to effectively create pointers to variables x 100 ptrx x eval echo ptrx Dereference ptrx 100 eval ptrx 50 Store 50 in var that ptrx points to echo x See what happened 50 fe revious next gt rs ca The wait Command If you submit a command line to the background for execution that command line runs in a subshell independent of your current shell the job is said to run asynchronously At times you may want to wait for the background process also Known as a child process because it s spawned from your current shell the parent to finish execution before proceeding For example you may have sent a large sort into the background and now want to wait for the sort to finish because you need to use the sorted data The wait command is for such a purpose Its general format is wait process id where process id is the process id number of the process you want to wait for If omitted the shell waits for all child processes to complete execution Execution of your current shell will be suspended until the process or processes
207. ed on the command line Then the program exits Otherwise the until loop is executed until the variable validchoice is nonnull The only way it can ever become nonnull is if the command validchoice TRUE is executed inside the case on selection of either 1 2 or 3 Otherwise the program continues to loop until one of these three choices is made rolo Bill Quick lookup Billy Bach 201 555 7618 rolo Let s have the menu this time Would you like to 1 Look someone up 2 Add someone to the phone book 3 Remove someone from the phone book Please select one of the above 1 3 4 Bad choice Would you like to 1 Look someone up 2 Add someone to the phone book 3 Remove someone from the phone book Please select one of the above 1 3 0 Bad choice Would you like to 1 Look someone up 2 Add someone to the phone book 3 Remove someone from the phone book Please select one of the above 1 3 1 Enter name to look up Tony Tony Iannino 973 555 1295 The Variable and Temporary Files If two or more people on your system use the rolo program at the same time a potential problem may occur Look at the rem program and see whether you can spot it The problem occurs with the temporary file tmp phonebook that is used to create a new version of the phone book file grep v name phonebook gt tmp phonebook mv tmp phonebook phonebook If more than one person uses rolo to remove an entry at the
208. eded by the name of the file containing that line It s generally a good idea to enclose your grep pattern inside a pair of single quotes to protect it from the shell For instance if you want to find all the lines containing asterisks inside the file stars typing grep stars does not work as expected because the shell sees the asterisk and automatically substitutes the names of all the files in your current directory ls circles polka dots squares stars stripes grep stars In this case the shell took the asterisk and substituted the list of files in your current directory Then it started execution of grep which took the first argument circles and tried to find it in the files specified by the remaining arguments as shown in Figure 4 1 Figure 4 1 grep stars circles polka dots squares grep Bye __wgumens stars stripes stars Enclosing the asterisk in quotes however removes its special meaning from the shell grep stars The asterisk is a special character that KKK 5 4 20 The quotes told the shell to leave the enclosed characters alone It then started execution of grep passing it the two arguments without the surrounding quotes the shell removes them in the process and stars see Figure 4 2 Figure 4 2 grep stars There are characters other than that otherwise have a special meaning and must be quoted when used in a pattern The whole topic o
209. en echo lt lt lt echo x gt gt gt displays the value of x which is x os fa Command Substitution Command substitution refers to the shell s capability to insert the standard output of a command at any point in a command line There are two ways in the shell to perform command substitution by enclosing a shell command with back quotes and with the construct The Back Quote The back quote is unlike any of the previously encountered types of quotes Its purpose is not to protect characters from the shell but to tell the shell to execute the enclosed command and to insert the standard output from the command at that point on the command line The general format for using back quotes is command where command is the name of the command to be executed and whose output is to be inserted at that 1 point 1 Note that using the back quote for command substitution is no longer the preferred method however we cover it here because of the large number of older canned shell programs that still use this construct Also you should know about back quotes in case you ever need to write shell programs that are portable to older Unix systems with shells that don t support the newer construct Here is an example echo The date and time is date The date and time is Wed Aug 28 14 28 43 EDT 2002 When the shell does its initial scan of the command line it notices the back quote and expects the name of
210. en it exports z This makes the value of z accessible to any subshell subsequently run by vartest4 vartest5 is such a subshell and when it is executed the shell copies into its environment the exported variables from vartest4 y and z This should explain the following output vartest4 X y 5 z 1 This entire operation is depicted in Figure 11 6 Figure 11 6 Subshell execution exported local variables login variables shell l i exported local variables vartest4 variables subshell F lt gt exported local variables vartests J variables subshell F lt gt To summarize the way local and exported variables work 1 Any variable that is not exported is a local variable whose existence will not be known to subshells 2 Exported variables and their values are copied into a subshell s environment where they may be accessed and changed However such changes have no effect on the variables in the parent shell 3 Exported variables retain this characteristic not only for directly spawned subshells but also for subshells spawned by those subshells and so on down the line 4 A variable can be exported any time before or after it is assigned a value export p If you simply type export p you ll get a list of the variables and their values exported by your shell export p export LOGNAME steve export PATH bin usr bin export TIMEOUT 600 export
211. ennis Ritchie at Bell Laboratories in the late 1960s One of the primary goals in the design of the UNIX system was to create an environment that promoted efficient program development For now get into the habit of enclosing your sed command in a pair of single quotes Later you ll know when the quotes are necessary and when to use double quotes instead The sed command s Unix UNIX is applied to every line of intro Whether or not the line gets changed by the command it gets written to standard output all the same Note that sed makes no changes to the original input file To make the changes permanent you must redirect the output from sed into a temporary file and then move the file back to the old one sed s Unix UNIX intro gt temp Make the changes mv temp intro And now make them permanent Always make sure that the correct changes were made to the file before you overwrite the original a cat of temp could have been included between the two commands shown previously to ensure that the sed succeeded as planned If your text included more than one occurrence of Unix on a line the preceding sed would have changed just the first occurrence on each line to UNIX By appending the global option g to the end of the s command you can ensure that multiple occurrences of the string on a line will be changed In this case the sed command would read sed s Unix UNIX g intro gt temp Suppose that you wanted to extrac
212. entical to substitution of other types of variables so when the shell sees tbl 1 it replaces the 1 with the first argument supplied to the program new hire As another example the following program called ison lets you know if a specified user is logged on cat ison who grep 1 who See who s on root console Jul 7 08 37 barney tty03 Jul 8 12 28 fred tty04 Jul 8 13 40 joanne tty07 Jul 8 09 35 tony tty19 Jul 8 08 30 lulu tty23 Jul 8 09 55 ison tony tony tty19 Jul 8 08 30 ison pat Not logged on Fe erevious Next a fo The s Variable Whenever you execute a shell program the special shell variable gets set to the number of arguments that were typed on the command line As you ll see in the next chapter this variable can be tested by the program to determine whether the correct number of arguments was typed by the user The next program called args was written just to get you more familiar with the way arguments are passed to shell programs Study the output from each example and make sure that you understand it cat args Look at the program echo arguments passed echo arg 1 1 arg 2 2 arg 3 3 args abc Execute it 3 arguments passed arg 1 a arg 2 b arg 3 c args ab Try it with two arguments 2 arguments passed arg 1 a arg 2 b arg 3 Unassigned args are null args Try it with no arguments 0 arguments passed arg 1 arg 2 arg3 args
213. entire string of characters enclosed between the quotes as a single argument when executing echo see Figure 6 4 Figure 6 4 echo one two three four arguments echo P e e one two three four As we mentioned all special characters are ignored by the shell if they appear inside single quotes That explains the output from the following file users steve bin progl echo file users steve bin progl echo file not interpreted file echo addresses intro lotsaspaces names nu numbers phonebook stat echo echo lt gt gt gt amp lt gt gt gt amp Even the Enter key will be ignored by the shell if it s enclosed in quotes echo How are you today gt John How are you today John After typing the first line the shell sees that the quote isn t matched so it waits for you to type in the closing quote As an indication that the shell is waiting for you to finish typing in a command it changes your prompt character from to gt This is known as your secondary prompt character and is displayed by the shell whenever it s waiting for you to finish typing a command Quotes are also needed when assigning values containing whitespace or special characters to shell variables message I must say this sure is fun echo message I must say this sure is fun text means all files in the directory echo text names nu numbers phonebook st
214. equal omitting the double quotes would have caused the Shell to eat up the trailing space character and test would have never seen it day monday test day monday echo 0 True Although this seems to violate our rule about always quoting shell variables that are arguments to test it s okay to omit the quotes if you re sure that the variable is not null and not composed entirely of whitespace characters You can test to see whether a shell variable has a null value with the third operator listed in Table 8 1 test day This returns true if day is not null and false if it is Quotes are not necessary here because test doesn t care whether it sees an argument in this case Nevertheless you are better off using them here as well because if the variable consists entirely of whitespace characters the shell will get rid of the argument if not enclosed in quotes blanks 7 test blanks Is it not null echo 1 False it s null test blanks And now echo 0 True it s not null In the first case test was not passed any arguments because the shell ate up the four spaces in blanks In the second case test got one argument consisting of four space characters obviously not null In case we seem to be belaboring the point about blanks and quotes realize that this is a sticky area that is a frequent source of shell programming errors It s good to really understand the principles here to save
215. equest id is laserl 33 standard input Functions exist only in the shell in which they re defined that is they can t be passed down to subshells Further because the function is executed in the current shell changes made to the current directory or to variables remain after the function has completed execution db gt PATH PATH uxn2 data gt PS1 DB gt cd uxn2 data gt db Execute it DB As you see a function definition can continue over as many lines as necessary The shell displays your secondary command prompt until you close the definition with the You can put definitions for commonly used functions inside your profile so that they ll be available whenever you log in Alternatively you can group the definitions in a file say myfuncs and then execute the file in the current shell by typing myfuncs This has the effect of causing any functions defined inside myfuncs to be read in and defined to the current shell The following function called mycd takes advantage of the fact that functions are run in the current environment It mimics the operation of the Korn shell s cd command which has the capability to substitute portions of the current directory s path with something else see the discussion of cd in Chapter 15 Interactive and Nonstandard Shell Features for more details cat myfuncs See what s inside new cd function mycd dir Switches dir mycd old new Substitut
216. equirement that a command appear particularly in if commands Suppose that you want to make sure that the value stored in the variable system exists in the file users steve mail systems and if it doesn t you want to issue an error message and exit from the program So you start by writing something like if grep system users steve mail systems gt dev null then but you don t know what to write after the then because you want to test for the nonexistence of the system in the file and don t want to do anything special if the grep succeeds Unfortunately the shell requires that you write a command after the then Here s where the null command comes to the rescue if grep system users steve mail systems gt dev null then else echo system is not a valid system exit 1 fi So if the system is valid nothing is done If it s not valid the error message is issued and the program exited Remember this simple command when these types of situations arise ns Ga os fa The ss and Constructs The shell has two special constructs that enable you to execute a command based on whether the preceding command succeeds or fails In case you think this sounds similar to the if command well it is It s sort of a shorthand form of the if If you write command amp amp command anywhere where the shell expects to see a command command will be executed and if it returns an exit status of zero command will be e
217. ermine which key sequence is used with the stty command The pressing of the Delete key at the terminal sends what s known as a signal to the executing program The program can specify the action that should be taken on receipt of the signal This is done with the trap command whose general format is trap commands signals where commands is one or more commands that will be executed whenever any of the signals specified by signals is received Numbers are assigned to the different types of signals and the more commonly used ones are summarized in Table 13 1 A more complete list is given under the trap command in Appendix A Shell Summary Table 13 1 Commonly Used Signal Numbers Signal Generated for 0 Exit from the shell 1 Hangup 2 Interrupt for example Delete Ctrl c key 15 Software termination signal sent by kill by default As an example of the trap command the following shows how you can remove some files and then exit if someone tries to abort the program from the terminal trap rm WORKDIR work1 WORKDIR dataout exit 2 From the point in the shell program that this trap is executed the two files work1 and dataout will be automatically removed if signal number 2 is received by the program So if the user interrupts execution of the program after this trap is executed you can be assured that these two files will be cleaned up The exit that follows the rm is necessary because without it execution would continue
218. ersal Time copying files checking destination files 2nd 3rd cp command 2nd 3rd 4th echo escape characters 2nd final code listing 2nd 3rd 4th 5th 6th initial code listing 2nd 3rd read command counting words in files wc command 2nd cp command 2nd 3rd 4th Ctrl j command vi Ctrl I command vi Ctrl m command vi Ctri v command vi Ctrl w command vi curly braces current directory 2nd current program replacing with new program 2nd 3rd current working directories changing cd command 2nd 3rd 4th 5th defined displaying pwd command 2nd cut command doption 2nd 3rd 4th 5th foption 2nd 3rd 4th 5th examples 2nd 3rd 4th syntax Cygwin Web site 4 rrevious SYMBOL A B C D E F G H 1 J K L M N O P Q R S T U V W X Y Z d d operator D command vi d conversion specification character d option cut command 2nd 3rd 4th 5th paste command 2nd sed command 2nd uniq command 2nd 3rd d options tr command 2nd dangling symbolic links data types integer 2nd date command 2nd 3rd date time displaying date command 2nd defining aliases 2nd 3rd 4th 5th functions 2nd Delete key 2nd deleting aliases directories rmdir command 2nd duplicate lines 2nd files rm command 2nd function
219. esses intro lotsaspaces names nu numbers phonebook stat There s a lot to be learned from this small example Was the list of files stored into the variable x when was executed or did the shell do the substitution when echo x was executed The answer is that the shell does not perform filename substitution when assigning values to variables Therefore assigns the single character to x This means that the shell did the filename substitution when executing the echo command In fact the precise sequence of steps that occurred when echo x was executed is as follows 1 The shell scanned the line substituting as the value of x 2 The shell rescanned the line encountered the and then substituted the names of all files in the current directory 3 The shell initiated execution of echo passing it the file list as arguments see Figure 5 3 Figure 5 3 echo x addresses arguments lotsaspaces echo p amp names o phonebook This order of evaluation is important Remember first the shell does variable substitution then does filename substitution and then parses the line into arguments The variable Construct Suppose that you have the name of a file stored in the variable filename If you wanted to rename that file so that the new name was the same as the old except with an X added to the end your first impulse would be to type mv filename filenamex When the shell scans th
220. et of programs that enable you to look up or add someone to the phone book would be complete without a program to remove someone from the phone book We ll call the program rem and have it take as its argument the name of the person to be removed What should the strategy be for developing the program Essentially you want to remove the line from the file that contains the specified name The v option to grep can be used here because it prints lines from a file that don t match a pattern cat rem Remove someone from the phone book grep v 1 phonebook gt tmp phonebook mv tmp phonebook phonebook The grep writes all lines that don t match into the file tmp phonebook m phonebook file is replaced by the new one from tmp After the grep is done the old 1 tmp is a directory on all Unix systems that anyone can write to It s used by programs to create temporary files Each time the system gets rebooted all the files in tmp are usually removed rem Stromboli Pizza Remove this entry cat phonebook Alice Chebba 973 555 2015 Barbara Swingle 201 555 9257 Billy Bach 201 555 7618 Liz Stachiw 212 555 2298 Susan Goldberg 201 555 7776 Susan Topple 212 555 4932 Tony Iannino 973 555 1295 rem Susan cat phonebook Alice Chebba 973 555 2015 Barbara Swingle 201 555 9257 Billy Bach 201 555 7618 Liz Stachiw 212 555 2298 Tony Iannino 973 555 1295 The first case where Stromboli Pizza was removed
221. evious next gt lt Previous next gt Working with Files The Unix system recognizes only three basic types of files ordinary files directory files and special files An ordinary file is just that any file on the system that contains data text program instructions or just about anything else Directories are described later in this chapter As its name implies a special file has a special meaning to the Unix system and is typically associated with some form of I O A filename can be composed of just about any character directly available from the keyboard and even some that aren t provided that the total number of characters contained in the name is not greater than 1 255 If more than 255 characters are specified the Unix system simply ignores the extra characters 1 Modern Unix and Microsoft Windows systems support long filenames however some older Unix and Windows systems only allow much shorter filenames The Unix system provides many tools that make working with files easy Here we ll review many basic file manipulation commands Listing Files The 1s Command To see what files you have stored in your directory you can type the ls command ls READ ME names tmp This output indicates that three files called READ_ME names and tmp are contained in the current directory Note that the output of Ls may vary from system to system For example on many Unix systems ls produces multicolumn output when send
222. ew shell developed by David Korn at AT amp T for the Bourne shell would also run under the Korn the one on System V the Korn shell provides you with idea of the compatibility of the Korn shell with Bourne s the Bourne and Korn shells The main features added to the Korn shell are In the Korn shell and Bash if the is followed by a or a the value of the variable PWD or OLDPWD is substituted respectively PWD and OLDPWD are set by cd and are the full pathnames of the current and previous directories respectively and are not supported by the POSIX standard shell pwd usr spool uucppublic steve cd pwd users pat echo users pat echo usr spool uucppublic steve In addition to the preceding substitutions the shell also checks for a tilde after a colon and performs tilde substitution on that as well for PATH interpretation Order of Search It s worthwhile listing the order of searching the shell uses when you type a command name 1 The shell first checks to see whether the command is a reserved word such as for and do 2 If it s not a reserved word and is not quoted the shell next checks its alias list and if it finds a match performs the substitution If the alias definition ends in a space it attempts alias substitution on the next word The final result is then checked against the reserved word list and if it s not a reserved word the shell proceeds to step 3 3 N
223. example 2nd 3rd 4th 5th 6th 7th 8th 9th 10th shift command 2nd 3rd command history 2nd See also edit modes accessing 2nd accessing with fc command 2nd accessing with history command 2nd accessing with r command 2nd 3rd editing 2nd file location size of sizing 2nd command line command prompt changing 2nd command substitution construct 2nd 3rd 4th 5th 6th 7th 8th 9th backquote 2nd 3rd 4th expr command 2nd 3rd 4th 5th command line editing 2nd command line tools See commands commands 2nd See also command files programs period 2nd 3rd 4th 5th 6th 7th colon 2nd alias 2nd 3rd 4th 5th 6th 7th aliases defining 2nd 3rd 4th 5th removing awk 2nd break 2nd 3rd 4th 5th 6th E case 2nd 3rd pattern matching characters 2nd 3rd 4th pipe symbol 2nd 3rd syntax 2nd 3rd cat 2nd 3rd cd 2nd 3rd 4th 5th 6th 7th 8th 9th option 2nd command cycle 2nd command files 2nd comments 2nd 3rd 4th creating 2nd examples 2nd 3rd 4th execute permissions 2nd command history 2nd See also edit modes accessing 2nd accessing with fc command 2nd accessing with history command 2nd accessing with r command 2nd 3rd editing 2nd file location size of sizing 2nd command options 2nd 3rd command substitution construct 2nd 3rd 4th 5th 6th 7
224. ext the shell checks the command against its function list and executes it if found 4 The shell checks to see whether the command is a built in command such as cd and pwd 5 Finally the shell searches the PATH to locate the command 6 If the command still isn t found a command not found error message is issued nS Ga a Bg next gt Compatibility Summary Table 15 4 summarizes the compatibility of the POSIX standard shell the Korn shell and Bash with the features described in this chapter In this table an X denotes a supported feature UP an optional feature in the POSIX shell these are also known as User Portability features in the POSIX shell specification and POS a feature supported only by Bash when it is invoked with the name sh or with the posix command line option or after set o posix is executed Table 15 4 POSIX Shell Korn Shell and Bash Compatibility POSI X Shell Korn Shell Bash ENV file X X POS vi line edit mode X X X emacs line edit mode X X fc command X X X r command X II X I string X Functions X X X local variables X X autoload via FPATH X Integer expressions with X X Integer data type Aliases Arrays Job control cd cd old new username aaa integers in different bases xhexnumber Qoctalnumber UP UP X X X X X X X X X X X X X X X X X X rs ca Exercises 1 Using only shell built in commands write a function that prin
225. f This string contains an interpreted escape b n test nstring This string contains an interpreted escape test string printf A string S and a character c n hello A A string hello and a character A In the last printf c is used to display a single character If the corresponding argument is longer than one character only the first is displayed printf Just the first character c n abc The general format of a conversion specification is flags width precision type The type is the conversion specification character from Table 10 2 As you can see only the percent sign and type are required the other parameters are called modifiers and are optional Valid flags are and the space character left justifies the value being printed this will make more sense when we discuss the width modifier causes printf to precede integers with a or sign by default only negative integers are printed with a sign causes printf to precede octal integers with 0 and hexadecimal integers with Ox or OX for x or X respectively The space character causes printf to precede positive integers with a space and negative integers with a printf td n d n d n 10 10 20 10 10 20 printf d n d n d n 10 10 20 10 10 20 printf 0 x n 100 200 0144 O0xc8 As you can see using or space as the flag lines up columns of positive and negative numbers nicely The wi
226. f how quotes are handled by the shell is fascinating an entire chapter Chapter 6 Can Quote You on That is devoted to it grep takes its input from standard input if no filename is specified So you can use grep on the other side of a pipe to scan through the output of a command for something For example suppose that you want to find out whether the user jim is logged in You can use grep to search through who s output who grep jim jim tty16 Feb 20 10 25 Note that by not specifying a file to search grep automatically scans its standard input Naturally if the user jim were not logged in you simply would get back a new prompt because grep would not find jim in who s output who grep jim Regular Expressions and grep Let s take another look at the intro file cat intro The UNIX operating system was pioneered by Ken Thompson and Dennis Ritchie at Bell Laboratories in the late 1960s One of the primary goals in the design of the UNIX system was to create an environment that promoted efficient program development grep allows you to specify your pattern using regular expressions as in ed Given this information it means that you can specify the pattern tT he to have grep search for either a lower or uppercase T followed by the characters he So here s how to grep out all the lines containing the characters the or The grep tT he intro The UNIX operating system was pioneered by Ken in th
227. f the args program cat args echo Number of arguments passed is for arg do echo arg done args abc Number of arguments passed is 3 a b a args a b c Number of arguments passed is 2 a b The while Command The second type of looping command to be described in this chapter is the while The format of this command is while command do command command done command is executed and its exit status tested If it s zero the commands enclosed between the do and done are executed Then command is executed again and its exit status tested If it s zero the commands enclosed between the do and done are once again executed This process continues until command returns a nonzero exit status At that point execution of the loop is terminated Execution then proceeds with the command that follows the done Note that the commands between the do and done might never be executed if command returns a nonzero exit status the first time it s executed Here s a program called twhile that simply counts to 5 cat twhile i 1 while i le 5 do echo i i i 1 done twhile Run it 1 2 The variable i is used as the counting variable and is initially set equal to 1 Then the while loop is entered It continues execution as long as i is less than or equal to 5 Inside the loop the value of i is displayed at the terminal Then it is incremented by one The while loop is often used in conjunc
228. file And as far as cut is concerned tabs count as a single character when using the c option So cut extracts the first 15 characters from each line in the previous example giving the results as shown Given that the fields are separated by tabs you should use the f option to cut instead cut f1 phonebook Alice Chebba Barbara Swingle Jeff Goldberg Liz Stachiw Susan Goldberg Tony Iannino Much better Recall that you don t have to specify the delimiter character with the d option because cut assumes that a tab character is the delimiter by default But how do you know in advance whether fields are delimited by blanks or tabs One way to find out is by trial and error as shown previously Another way is to type the command sed n l file at your terminal If a tab character separates the fields t will be displayed instead of the tab sed n l phonebook Alice Chebba t973 555 2015 Barbara Swingle t201 555 9257 Jeff Goldberg t201 555 3378 Liz Stachiw t212 555 2298 Susan Goldberg t201 555 7776 Tony Iannino t973 555 1295 The output verifies that each name is separated from each phone number by a tab character sed is covered in more detail shortly u rs ca paste The paste command is sort of the inverse of cut Instead of breaking lines apart it puts them together The general format of the paste command is paste files where corresponding lines from each of the specified files are pasted toge
229. file is readable by the process s file file has nonzero length w file file is writable by the process x file file is executable L file file is a symbolic link The command f users steve phonebook tests whether the file users steve phonebook exists and is an ordinary file that is not a directory and not a special file The command r users steve phonebook tests whether the indicated file exists and is also readable by you The command s users steve phonebook tests whether the indicated file contains at least one byte of information in it This is useful for example if you create an error log file in your program and you want to see whether anything was written to it if s ERRFILE then echo Errors found cat ERRFILE fi A few more test operators when combined with the previously described operators enable you to make more complex types of tests The Logical Negation Operator The unary logical negation operator can be placed in front of any other test expression to negate the result of the evaluation of that expression For example r users steve phonebook returns a zero exit status true if users steve phonebook is not readable and f mailfile returns true if the file specified by mailfile does not exist or is not an ordinary file Finally l oxi 6X2 returns true if x1 is not identical to x2 and is obviously equivalent to
230. filel to file2 cat file2 This is in file2 This is in filel Recall that specifying more than one filename to cat results in the display of the first file followed immediately by the second file and so on cat filel This is in filel cat file2 This is in file2 cat filel file2 This is in filel This is in file2 cat filel file2 gt file3 Redirect it instead cat file3 This is in filel This is in file2 Now you can see where the cat command gets its name When used with more than one file its effect is to catenate the files together Incidentally the shell recognizes a special format of output redirection If you type gt file not preceded by a command the shell creates an empty that is zero character length file for you If file previously exists its contents will be lost Input Redirection Just as the output of a command can be redirected to a file so can the input of a command be redirected from a file And as the greater than character gt is used for output redirection the less than character lt is used to redirect the input of a command Of course only commands that normally take their input from standard input can have their input redirected from a file in this manner To redirect the input of a command you type the lt character followed by the name of the file that the input is to be read from So for example to count the number of lines in the file users you know that yo
231. finish execution You can try the wait command at your terminal sort big data gt sorted data amp Send it to the background 1 3423 Job number amp process id from the shell date Do some other work Wed Oct 2 15 05 42 EDT 2002 wait 3423 Now wait for the sort to finish When sort finishes prompt is returned The Variable If you have only one process running in the background then wait with no argument suffices However if you re running more than one command in the background and you want to wait on a particular one you can take advantage of the fact that the shell stores the process id of the last command executed in the background inside the special variable So the command wait waits for the last process sent to the background to complete execution As mentioned if you send several commands to the background you can save the value of this variable for later use with wait progl amp pidl prog2 amp pid2 wait pidl wait for progl to finish wait pid2 wait for prog2 to finish rs ca The trap Command When you press the Delete or Break key at your terminal during execution of a shell program normally that program is immediately terminated and your command prompt returned This may not always be desirable For instance you may end up leaving a bunch of temporary files that won t get cleaned up 2 Some Unix systems use Ctrl c rather than the Delete key for this purpose You can det
232. found the characters was in the first line that matched the indicated pattern Repeating the search recall that the ed command means to repeat the last search resulted in the display of the second line of the file because and matched the pattern The substitute command that followed specified that all occurrences of the character p followed by any single character followed by the character o were to be replaced by the characters XXX Matching the Beginning of the Line The Caret When the caret character is used as the first character in a regular expression it matches the beginning of the line So the regular expression George matches the characters George only if they occur at the beginning of the line ed intro 248 the Find the line that starts with the the design of the Unix system was to create an 1 s gt gt Insert gt gt at the beginning of each line 1 p gt gt The Unix operating system was pioneered by Ken gt gt Thompson and Dennis Ritchie at Bell Laboratories gt gt in the late 1960s One of the primary goals in gt gt the design of the Unix system was to create an gt gt environment that promoted efficient program gt gt development The preceding example shows how the regular expression can be used to match just the beginning of the line Here it is used to insert the characters gt gt at the start of each line A command such as 1 57 77 is commonly used to inser
233. from the loop and not from the program you can use the break command whose format is simply break When the break is executed control is sent immediately out of the loop where execution then continues as normal with the command that follows the done The Unix command true serves no purpose but to return an exit status of zero The command false also does nothing but return a nonzero exit status If you write while true do done the while loop will theoretically be executed forever because true always returns a zero exit status By the way the command also does nothing but return a zero exit status so an infinite loop can also be set up with while do done Because false always returns a nonzero exit status the loop until false do done will theoretically execute forever The break command is often used to exit from these sorts of infinite loops usually when some error condition or the end of processing is detected while true do cmd getcmd if cmd quit then break else processcmd cmd fi done Here the while loop will continue to execute the getcmd and processcmd programs until cmd is equal to quit At that point the break command will be executed thus causing the loop to be exited If the break command is used in the form break n the n innermost loops are immediately exited so in for file do while count lt 10 do if n error then break
234. gram 2nd vartest3 program 2nd vartest4 program 2nd vartest5 program verbose shell mode vi line edit mode 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th command mode commands 2nd 3rd 4th 5th 6th 2nd L and 3rd IO N gt E I N Qa In N 3 2 z S D jN 5 a e w r a N Qa WwW Qa lige p ir YON 2 S a T 2 jo E N 5 a Xx N a w a input mode vi shell mode 4 rrevious SYMBOL A B C D E F G H 1 J K L M N O P Q R S T U V W X Y Z w w operator w command vi 2nd w option wc command wait command 2nd variable 2nd example syntax 2nd waiting for processes 2nd 3rd 4th 5th wc command 2nd Web sites 2nd Cygwin Free Software Foundation Korn shell 2nd O Reilly and Associates Web site POSIX standard while command 2nd 3rd 4th 5th whitespace characters who command 2nd words counting wc command 2nd working directory printing 2nd writing sorted data to standard output 2nd to standard error 2nd Fe rrevious 4 rrevious SYMBOL A B C D E F G H 1 J K L M N O P Q R S T U V W X Y Z x x operator x command vi 2nd 3rd x conversion specification character X conversion specification character x option set command
235. hat the first program writes its output to standard output and the second program reads its input from standard input As another example of a pipe suppose that you wanted to count the number of files contained in your directory Knowledge of the fact that the Ls command displays one line of output per file enables you to use the same type of approach as before ls we l 10 The output indicates that the current directory contains 10 files It is also possible to form a pipeline consisting of more than two programs with the output of one program feeding into the input of the next Filters The term filter is often used in Unix terminology to refer to any program that can take input from standard input perform some operation on that input and write the results to standard output More succinctly a filter is any program that can be used between two other programs in a pipeline So in the previous pipeline wc is considered a filter Ls is not because it does not read its input from standard input As other examples cat and sort are filters whereas who date cd pwd echo rm mv and cp are not Fe previous NT lt Previous Jinext Standard Error In addition to standard input and standard output there is another place known as standard error This is where most Unix commands write their error messages And as with the other two standard places standard error is associated with your terminal by default In most cases
236. he PATH and HOME variables read only Subsequently attempting to assign a value to these variables causes the shell to issue an error message PATH bin usr bin readonly PATH PATH PATH users steve bin sh PATH is read only Here you see that after the variable PATH was made read only the shell printed an error message when an attempt was made to assign a value to it 2 To get a list of your read only variables type readonly p without any arguments 2 By default Bash produces output of the form declare r variable To get POSIX compliant output you must run Bash with the posix command line option or run the set command with the o posix option readonly p readonly PATH bin usr bin unset removes both exported and local shell variables You should be aware of the fact that the read only variable attribute is not passed down to subshells Also after a variable has been made read only in a shell there is no way to undo it ns Ga rs ca The unset Command Sometimes you may want to remove the definition of a variable from your environment To do so you type unset followed by the names of the variables x 100 echo x 100 unset x Remove x from the environment echo x You can t unset a read only variable Furthermore the variables IFS MAILCHECK PATH PS1 and PS2 cannot be unset Also some older shells do not support the unset command nS Ga Exercises
237. he command Examples hash rolo whoq Add rolo and whog to hash list hash Print hash list hash r Remove hash list The if Command General Format if command then command command fi command is executed and its exit status tested If it is zero the commands that follow up to the fi are executed Otherwise the commands that follow up to the fi are skipped Example if grep sys sysnames gt dev null then echo sys is a valid system name fL If the grep returns an exit status of zero which it will if it finds sys in the file sysnames the echo command is executed otherwise it is skipped The built in command test is often used as the command following the if Example if eq 0 then echo Usage 0 l file exit 1 fi An else clause can be added to the if to be executed if the command returns a nonzero exit status In this case the general format of the if becomes if command then command command else command command Ti If command returns an exit status of zero the commands that follow up to the else are executed and the commands between the else and the fi are skipped Otherwise command returns a nonzero exit status and the commands between the then and the else are skipped and the commands between the else and the fi are executed Example if z line then echo I couldn t find name else echo line Fi In the preceding example if Line has ze
238. he first few letters of the pipeline or string The following built in commands may be given a job_id as an argument kill fg bg and wait The special conventions and refer to the current and previous jobs respectively also refers to the current job The current job is the most recent job placed in the background or the job running in the foreground The previous job is the previous current job The convention string refers to the job whose name begins with string string refers to the job whose name contains string The jobs command may be used to list the status of all currently running jobs If the monitor option of the set command is turned on the shell prints a message when each job finishes If you still have jobs when you try to exit the shell a message is printed to alert you of this If you immediately try to exit again the shell exits The monitor option is enabled by default for interactive shells Stopping Jobs If the shell is running on a system with job control and the monitor option of the set command is turned on jobs that are running in the foreground may be placed in the background and vice versa Normally Ctrl z stops the current job The bg command puts a stopped job in the background The fg command brings a background or stopped job to the foreground Whenever a job in the background attempts to read from the terminal it is stopped until it is brought to the foreground Output from background jobs norma
239. he line at which point the is recognized by the shell as the pipe symbol The eval command is frequently used in shell programs that build up command lines inside one or more variables If the variables contain any characters that must be seen by the shell directly on the command line that is not as the result of substitution eval can be useful Command terminator amp 1 0 redirection lt gt and quote characters are among the characters that must appear directly on the command line to have any special meaning to the shell For the next example consider writing a program Last whose sole purpose is to display the last argument passed to it You needed to get at the last argument in the mycp program in Chapter 10 Reading and Printing Data There you did so by shifting all the arguments until the last one was left You can also use eval to get at it as shown cat last eval echo last one two three four four last Get the last file zoo report The first time the shell scans echo the backslash tells it to ignore the that immediately follows After that it encounters the special parameter so it substitutes its value on the command line The command now looks like this echo 4 the backslash is removed by the shell after the first scan When the shell rescans this line it substitutes the value of 4 and then executes echo This same technique could be used if you had a variable call
240. hell or you re using a system that doesn t have a POSIX compliant shell you can download the Korn shell executable for a variety of Unix systems for free from www research att com sw download If you only have access to Microsoft Windows systems but still want to try your hand at shell programming or you just want to get a taste of Unix install the Cygwin package from www cygwin com The base system includes Bash and you can also download other shells such as zsh The total feel of the system is remarkably like Unix even though it s running on Windows There s even an X Window System available to enhance the Unix look and feel Best of all the entire Cygwin package is free ns fa rs ca Books O Reilly amp Associates One of the best sources of books on Unix related topics is O Reilly and Associates www ora com Their books cover a wide variety of subjects and are available from their Web site from booksellers online and in book stores Their Web site also has many useful articles on Unix and Linux Two good references on Unix and Linux respectively Unix in a Nutshell System V Edition 3rd Edition A Robbins O Reilly amp Associates 1999 ISBN 1565924274 Linux in a Nutshell 3rd Edition E Siever S Spainhour J P Hekman and S Figgins O Reilly amp Associates 2000 ISBN 0596000251 Four good books on Perl programming from beginner to advanced Learning Perl 3rd Edition R L Schwartz and T P
241. hell mode mounset shell mode moving files mv command 2nd multiple characters matching 2nd 3rd 4th multiple commands entering 2nd entering on one line mv command moving files 2nd renaming files 2nd 3rd mycd function 2nd mycp program destination files checking 2nd 3rd echo escape characters 2nd final code listing 2nd 3rd 4th 5th 6th initial code listing 2nd 3rd read command 4 Previous 4 rrevious SYMBOL A B C D E F G H 1 J K L M N O P Q R S T U V W X Y Z n escape character n escape characters n command vi N command vi n option grep command 2nd sed command 2nd sort command 2nd n option set command naming files mv command 2nd 3rd naming conventions filenames pathnames 2nd pathname pathname full pathnames relative pathnames variables ne ne operator negation operators logical AND operator a 2nd logical negation operator 2nd logical OR operator o 2nd New KornShell Command and Programming Language The newgrp command 2nd newline characters nnn escape characters italic n s noclobber shell mode noexec shell mode noglob shell mode nolog shell mode nonzero exit status null command colon 2nd null values 2nd 3rd number pr
242. hen echo Usage mon user exit 1 fi user 1 Check every minute for user logging on until who grep user gt dev null do sleep 60 done When we reach this point the user has logged on echo user has logged on After checking that one argument was provided the program assigns 1 to user Then an until loop is entered This loop will be executed until the exit status returned by grep is zero that is until the specified user logs on As long as the user isn t logged on the body of the loop the sleep command is executed This command suspends execution of the program for one minute 60 seconds At the end of the minute the pipeline listed after the until is re executed and the process repeated When the until loop is exited signaling that the monitored user has logged on a message is displayed at the terminal to that effect mon sandy Time passes sandy has logged on Using the program as shown here is not very practical because it ties up your terminal until sandy logs on A better idea is to run mon in the background so that you can use your terminal for other work mon sandy amp Run it in the background 1 4392 Job number and process id nroff newmemo Do other work sandy has logged on Happens sometime later So now you can do other work and the mon program continues executing in the background until sandy logs 1 on or until you log off the system 1 Al your
243. her files besides sys caps through this same command sequence You could make a separate version of run for each such file or you could modify the run program so that you could specify the name of the file to be run on the command line That is you could change run so that you could type run new hire for example to specify that the file new hire is to be run through this command sequence or run sysS Caps to specify the file sys caps Whenever you execute a shell program the shell automatically stores the first argument in the special shell variable 1 the second argument in the variable 2 and so on These special variables more formally known as positional parameters are assigned after the shell has done its normal command line processing that is I O redirection variable substitution filename substitution and so on To modify the run program to accept the name of the file as an argument all you do to the program is change the reference to the file sys caps so that it instead references the first argument typed on the command line cat run tbl 1 nroff mm Tlp lp run new hire Execute it with new hire as the argument request id is lLaserl1 24 standard input Each time you execute the run program whatever word follows on the command line will be stored inside the first positional parameter by the shell In the example new hire will be stored in this parameter Substitution of positional parameters is id
244. here are several other ways to access your command history that are worth noting The history Command The operation of the history command differs between the Korn shell and Bash because it is not part of the POSIX standard The Korn shell history command writes your last 16 commands to standard output history 507 cd shell 508 cd ch15 509 vi int 510 ps 511 echo HISTSIZE 512 cat ENV 513 cp int int sv 514 history 515 exit 516 cd shell 517 cd ch16 518 viall 519 run n5 all 520 ps 521 lpr all out 522 history The numbers to the left are simply relative command numbers command number 1 would be the first or oldest command in your history Without any arguments the Bash history command lists your entire history as specified by the HISTSIZE variable to standard output If you just want to see the last few commands you must specify the number of commands to display as an argument history 10 513 cp int int sv 514 history 515 exit 516 cd shell 517 cd ch16 518 viall 519 run n5 all 520 ps 521 lpr all out 522 history 10 The fc Command The fc command allows you to start up an editor on one or more commands from your history or to simply write a list of history commands to your terminal In the latter form which is indicated by giving the l option to fc it is like typing in history only more flexible you can specify a range of commands to be listed or can get fewer or more than the last 16 com
245. here is to substitute the value of x in the second instance of x but not in the first Let s first assign a value to x x 1 Now try displaying the line without using any quotes echo lt lt lt echo x gt gt gt displays the value of x which is x syntax error lt unexpected The lt signals input redirection to the shell this is the reason for the error message If you put the entire message inside single quotes the value of x won t be substituted at the end If you enclose the entire string in double quotes both occurrences of x will be substituted Here are two different ways to do the quoting properly realize that there are usually several different ways to quote a string of characters to get the results you want echo lt lt lt lt lt lt echo x echo lt lt lt lt lt lt echo x echo x gt gt gt displays the value of x which is x gt gt gt displays the value of x which is 1 echo x gt gt gt displays the value of x which is x gt gt gt displays the value of x which is 1 In the first case everything is enclosed in double quotes and the backslash is used to prevent the shell from performing variable substitution in the first instance of x In the second case everything up to the last x is enclosed in single quotes If the variable x might have contained some filename substitution or whitespace characters a safer way of writing the echo would have be
246. hey ll never be executed exec can be used to close standard input and reopen it with any file that you want to read To change standard input to file you use the exec command in the form exec lt file Any commands that subsequently read data from standard input will read from file Redirection of standard output is done similarly The command exec gt report redirects all subsequent output written to standard output to the file report Note here that exec is not used to start up execution of a new program as previously described here it is used to reassign standard input or standard output If you use exec to reassign standard input and later want to reassign it someplace else you can simply execute another exec To reassign standard input back to the terminal you would write exec lt dev tty The same discussion applies to reassignment of standard output The and Constructs Sometimes you may want to group a set of commands together for some reason For example you may want to send a sort followed by execution of your plotdata program into the background for execution You can group a set of commands together by enclosing them in a set of parentheses or braces The first form causes the commands to be executed by a subshell the latter form by the current shell Here are some examples to illustrate how they work x 50 x 100 Execute this in a subshell echo x 50 Didn t change x 100 Exe
247. hile the commands between the do and done might never be executed if command returns a zero exit status the first time it s executed The until command is useful for writing programs that wait for a particular event to occur For example suppose that you want to see whether sandy is logged on because you have to give her something important You could send her electronic mail but you know that she usually doesn t get around to reading her mail until late in the day One approach is to use the on program from Chapter 8 Decisions Decisions to see whether sandy s logged on on sandy sandy is not logged on You could execute this program periodically throughout the day until sandy eventually logs on or you could write your own program to continually check until she does Let s call the program mon and have it take a single argument the name of the user you want to monitor Instead of having the program continually check for that user logging on we ll have it check only once every minute To do this you have to know about a command called sleep that suspends execution of a program for a specified number of seconds So the Unix command this isn t a shell built in sleep n suspends execution of the program for n seconds At the end of that interval the program resumes execution where it left off with the command that immediately follows the sleep cat mon Wait until a specified user logs on if ne 1 t
248. hoenix O Reilly amp Associates 2001 ISBN 0596001320 Perl in a Nutshell 2nd Edition S Spainhour E Siever and N Patwardhan O Reilly amp Associates 2002 ISBN 0596002416 Programming Perl 3rd Edition L Wall T Christiansen and J Orwant O Reilly amp Associates 2000 ISBN 0596000278 Advanced Perl Programming S Srinivasan O Reilly amp Associates 1997 ISBN 1565922204 A good book covering both the POSIX standard versions of awk and sed as well as the GNU versions Sed amp Awk 2nd Edition D Dougherty and A Robbins O Reilly amp Associates 1997 ISBN 1565922255 Sams and Que Learn the essentials of Unix shell programming from the ground up Sams Teach Yourself Shell Programming in 24 Hours 2nd Edition Sriranga Veeraraghaven Sams Publishing 2002 ISBN 0672323583 A good book for learning Unix and programming in C and Perl on a Unix system Sams Teach Yourself Unix in 24 Hours 3rd Edition Dave Taylor Sams Publishing 2001 ISBN 0672321270 This book offers a series of lectures written by several Unix experts who have years of experience to share with their audience Unix Unleashed Robin Anderson and Andy Johnston Sams Publishing 2001 ISBN 067232251X This book offers detailed information on a broad range of Red Hat Linux topics from installation to multimedia Red Hat Linux 8 Unleashed Billy Ball Sams Publishing 2002 ISBN 067232458xX Learn how to efficiently install Red Hat
249. home directories can vary from one Unix version to the next and even one user to the next let s assume that your home directory is called steve and that this directory is actually a subdirectory of a directory called users Therefore if you had the directories documents and programs the overall directory structure would actually look something like Figure 2 3 A special directory known as pronounced slash is shown at the top of the directory tree This directory is known as the root Figure 2 3 Hierarchical directory structure i ince pat steve ruth documents programs E memos proposals letters wb collect mon E plan dact sysA newhire no JSK AMG reply Whenever you are inside a particular directory called your current working directory the files contained within that directory are immediately accessible If you want to access a file from another directory you can either first issue a command to change to the appropriate directory and then access the particular file or you can specify the particular file by its pathname A pathname enables you to uniquely identify a particular file to the Unix system In the specification of a pathname successive directories along the path are separated by the slash character A pathname that begins with a slash character is known as a full pathname because it specifies a complete path from the root So for example the pathname users steve identifies the directory steve contained under the
250. hout editing it first The format fc s old new first causes the command first to be re executed after the string old in the command is replaced with new If first isn t specified the previous command is used and if old new isn t specified the command is not changed Examples fc l List the last 16 commands fc e vi sed Read the last sed command into vi fc 100 110 Read commands 100 to 110 into FCEDIT fc s Re execute the previous command fc s abc def 104 Re execute command 104 replacing abc with def The fg Command General Format fg job_id If job control is enabled the job specified by job_id is brought to the foreground If no argument is given the most recently suspended job or the job last sent to the background is brought to the foreground Example fg 2 The for Command General Format for var in word word word do command command done Execution of this command causes the commands enclosed between the do and done to be executed as many times as there are words listed after the in The first time through the loop the first word word is assigned to the variable var and the commands between the do and done executed The second time through the loop the second word listed word is assigned to var and the commands in the loop executed again This process continues until the last variable in the list word is assigned to var and the commands between the do and done executed At that
251. hy various flavors of the shell exist today including the older Bourne shell developed by Stephen Bourne the Korn shell developed by David Korn the Bourne again shell mainly used on Linux systems and the C shell developed by Bill J oy The Shell s Responsibilities Now you know that the shell analyzes each line you type in and initiates execution of the selected program But the shell also has other responsibilities as outlined in Figure 3 8 Figure 3 8 The shell s responsibilities interpreted programming language environment control Program Execution program execution variable and filename substitution VO redirection pipeline hookup The shell is responsible for the execution of all programs that you request from your terminal Each time you type in a line to the shell the shell analyzes the line and then determines what to do As far as the shell is concerned each line follows the same basic format program name arguments The line that is typed to the shell is known more formally as the command line The shell scans this command line and determines the name of the program to be executed and what arguments to pass to the program The shell uses special characters to determine where the program name starts and ends and where each argument starts and ends These characters are collectively called whitespace characters and are the space character the horizontal tab
252. iable This construct gives you the ability to count the number of characters stored inside a variable For example text The shell echo text 9 Note that some older shells do not support this feature Each of the parameter substitution constructs described in this section is summarized in Table A 3 in Appendix A Shell Summary fe revious next gt lt Previous next gt The so Variable Whenever you execute a shell program the shell automatically stores the name of the program inside the special variable 0 This can be used to advantage when you have two or more programs that are linked under different names and you want to know which one was executed It s also useful for displaying error messages because it removes the dependency of the filename from the program If the name of the program is referenced by 0 subsequently renaming the program will not require the program to be edited cat lu Look someone up in the phone book if ne 1 then echo Incorrect number of arguments echo Usage 0 name exit 1 fa name 1 grep name PHONEBOOK if ne O then echo I couldn t find name in the phone book f PHONEBOOK HOME phonebook export PHONEBOOK lu Teri Teri Zak 201 555 6000 lu Teri Zak Incorrect number of arguments Usage lu name mv lu lookup Rename it lookup Teri Zak See what happens now Incorrect number of arguments Usage lookup name
253. ifferent conversion specification characters Table 10 2 printf Conversion Specification Characters Character Use for Printing d Integers u Unsigned integers o Octal integers x Hexadecimal integers using a f X Hexadecimal integers using A F Cc Single characters S Literal strings b Strings containing backslash escape characters Percent signs The first five conversion specification characters are all used for displaying integers d displays signed integers and u displays unsigned integers u can also be used to display the positive representation of a negative number note that the result is machine dependent By default integers displayed as octal or hexadecimal numbers do not have a leading 0 or 0x but we ll show you how to enable this later in this section Strings are printed using s or b s is used to print strings literally without any processing of backslash escape characters b is used to force interpretation of the backslash escape characters in the string argument Here are a few printf examples printf The octal value for d is o n 20 20 The octal value for 20 is 24 printf The hexadecimal value for d is x n 30 30 The hexadecimal value for 30 is le printf The unsigned value for d is Su n 1000 1000 The unsigned value for 1000 is 4294966296 printf This string contains a backslash escape s n test nstring This string contains a backslash escape test nstring print
254. ify that one of the enclosed characters is to be matched So the regular expression tT he would match a lower or uppercase t followed immediately by the characters he ed intro 248 tT he Look for the or The The Unix operating system was pioneered by Ken Continue the search in the late 1960s One of the primary goals in Once again the design of the Unix system was to create an 1 s aeiouAEIOU g Delete all vowels 1 p Th nx prtng systm ws pnrd by Kn Thmpsn nd Dnns Rtch t BLL Lbrtrs n th lt 1960s n f th prmry gls n th dsgn f th nx systm ws t crt n nvrnmnt tht prmtd ffcnt prgrm dvlpmnt A range of characters can be specified inside the brackets This can be done by separating the starting and ending characters of the range by a dash So to match any digit character 0 through 9 you could use the regular expression 0123456789 or more succinctly you could simply write 0 9 To match an uppercase letter you write A Z And to match an upper or lowercase letter you write A Za z Here are some examples with ed ed intro 248 0 9 Find a line containing a digit in the late 1960s One of the primary goals in A Z Find a line that starts with an uppercase letter The Unix operating system was pioneered by Ken Again Thompson and Dennis Ritchie at Bell Laboratories 1 s A Z g Change all uppercase letters to s 1 p he nix operating system was pioneered by en h
255. ile to the variable copylist 5 If copylist is nonnull copy the files in it to to If this algorithm seems a bit fuzzy perhaps the program followed by a detailed explanation will help clear things up Note the modified command usage message cat mycp Copy a file final version numargs save this for later use filelist copylist Process the arguments storing all but the last in filelist while gt 1 do filelist filelist 1 shift done to 1 If less than two args or if more than two args and last arg is not a directory then issue an error message if numargs lt 2 o numargs gt 2 a d to then echo Usage mycp filel file2 echo mycp file s dir exit 1 fi Sequence through each file in filelist for from in filelist do See if destination file is a directory if d to then tofile to basename from else tofile to fi Add file to copylist if file doesn t already exist or if user says it s okay to overwrite if e tofile then echo tofile already exists overwrite yes no c read answer if answer yes then copylist copylist from fa else copylist copylist from fa done Now do the copy first make sure there s something to copy if n copylist then cp copylist to fi proceed with the copy Let s look at some sample
256. ill be exited At that point filelist will contain a space delimited list of all the files to be copied and 1 will contain the last argument which is the destination file or directory To see how this works consider execution of the while loop when the command is executed as mycp names progl prog2 lu bin Figure 10 1 depicts the changing values of the variables through each iteration of the loop The first line steve steve steve steve steve See what happened 543 Jul 19 14 10 lu 949 Jul 21 17 11 names 38 Jul 19 09 55 nu 498 Jul 21 17 11 progl 498 Jul 21 17 11 prog2 shows the state of the variables before the loop is entered Figure 10 1 Processing command line arguments 1 2 3 4 5 filelist 5 names progi prog2 lu bin null 4 progi prog2 lu bin names 3 prog2 lu bin names progi 2 lu bin names progi prog2 1 bin names progi proge lu After the loop is exited the last argument contained in 1 is stored in the variable to Next a test is made to ensure that at least two arguments were typed on the command line and if more than two were typed that the last argument is a directory If either condition isn t satisfied usage information is displayed to the user and the program exits with a status of 1 Following this a for loop is entered for the purpose of individually examining each file in the list to see whether it already exists If it does the user is prompted as before If the user wants to o
257. inds jack in who s output At that point the loop is terminated and the echo command that follows is executed The wait Command General Format wait job This command causes the shell to suspend its execution until the process identified as job finishes executing Job can be a process id number or a job_id If job is not supplied the shell waits for all child processes to finish executing If more than one process id is listed wait will wait for them all to complete wait is useful for waiting for processes to finish that have been sent to the background for execution Example sort large file gt sorted file amp sort in the background Continue processing wait Now wait for sort to finish plotdata sorted file The variable can be used to obtain the process id number of the last process sent to the background The while Command General Format while command do command command done command is executed and its exit status tested If it is zero the commands enclosed between the do and done are executed Then command is executed again and its status tested If it is zero the commands between the do and done are once again executed Execution of command and subsequent execution of the commands between the do and done continues until command returns a nonzero exit status at which point the loop is terminated Execution then continues with the command that follows the done Note that because command gets evalua
258. ine Enter edit mode Meaning Get previous command from history Get previous command from history Get next command from history Get next command from history Get the command number count from history the default is the oldest stored command Search history for the most recent command containing string if string is null the previous string will be used string is terminated by an Enter or a Ctrl j if string begins with search for line beginning with string string count l or count space count w count W count e count E count h count b count B count count fc count Fc Same as except that the search will be for the least recent command Repeat the last or command Repeat the last or command but reverse the direction of the search Move cursor right one character Move cursor right one alphanumeric word Move cursor right to next blank separated word Move cursor to end of word Move cursor to end of current blank separated word Move cursor left one character Move cursor left one word Move cursor left to previous blank separated word Move cursor to start of line Move cursor to first nonblank character Move cursor to end of line Move cursor to column count 1 is default Move cursor right to character c Move cursor left to character c count tc count Tc count c motion count d motion count P count p
259. ing its output to a terminal on others different colors may be used for different types of files You can always force single column output with the Ll option Displaying the Contents of a File The cat Command You can examine the contents of a file by using the cat command The argument to cat is the name of the file whose contents you want to examine cat names Susan Jeff Henry Allan Ken Counting the Number of Words in a File The wc Command With the wc command you can get a count of the total number of lines words and characters of information contained in a file Once again the name of the file is needed as the argument to this command wc names 5 5 27 names The wc command lists three numbers followed by the filename The first number represents the number of lines contained in the file 5 the second the number of words contained in the file in this case also 5 and the third the number of characters contained in the file 27 Command Options Most Unix commands allow the specification of options at the time a command is executed These options generally follow the same format letter That is a Command option is a minus sign followed immediately by a single letter For example to count just the number of lines contained in a file the option l that s the letter I is given to the wc command wc l names 5 names To count just the number of characters in a file the c option is specifi
260. ing sh x followed by the name of the program and its arguments This starts up a new shell to execute the indicated program with the x option enabled In this mode commands are printed at the terminal as they are executed preceded by a plus sign Let s try it out sh x ctype a Trace execution 1 ne 1 equals 1 char a Assignment of 1 to char echo a WC C numchars 2 wc returned 2 2 ne 1 That s why this test succeeded echo please type a single character please type a single character exit 1 The trace output indicates that wc returned 2 when echo char wc c was executed But why There seemed to be only one character in wc s input The truth of the matter is that two characters were actually given to wc the single character a and the invisible newline character that echo automatically prints at the end of each line So the program really should be testing for the number of characters equal to two the character typed plus the newline added by echo Go back to the ctype program and replace the if command that reads if numchars ne 1 then echo Please type a single character exit 1 fi with if numchars ne 2 then echo Please type a single character exit 1 fi and try it again ctype a lowercase letter ctype abc Please type a single character ctype 9 digit ctype K uppercase letter ctype special character ctype Usage c
261. ing to that chapter try the exercises that follow rs rs ca Exercises 1 Write a program called valid that prints yes if its argument is a valid shell variable name and no otherwise valid foo bar yes valid 123 no Hint Define a regular expression for a valid variable name and then enlist the aid of grep or sed 2 Write a program called t that displays the time of day in a m or p m notation rather than in 24 hour clock time Here s an example showing t run at night date Wed Aug 28 19 34 01 EDT 2002 f 7 21 pm Use the shell s built in integer arithmetic to convert from 24 hour clock time Then rewrite the program to use a case command instead Rewrite it again to perform arithmetic with the expr command Write a program called mysed that applies the sed script given as the first argument against the file given as the second If the sed succeeds that is exit status of zero replace the original file with the modified one So mysed 1 10d text will use sed to delete the first 10 lines from text and if successful will replace text with the modified file Write a program called isyes that returns an exit status of 0 if its argument is yes and 1 otherwise For purposes of this exercise consider y yes Yes YES and Y all to be valid yes arguments isyes yes echo isyes no Fr Ff echo Write the program using an if command and then rewrite it using a case co
262. ious version and note the changes that were made An additional if command was added to test whether the correct number of arguments was supplied If is not equal to 1 the program prints two messages otherwise the commands after the else clause are executed These commands are the same as appeared in the last version of on They assign 1 to user and then see whether user is logged on printing a message in either case Note that two fis are required because two if commands are used The indentation used goes a long way toward aiding the program s readability Make sure that you get into the habit of setting and following indentation rules in your programs on No arguments Incorrect number of arguments Usage on user on priscilla priscilla is not logged on on jo anne Incorrect number of arguments Usage on user One argument Two arguments rs ca The exit Command A built in shell command called exit enables you to immediately terminate execution of your shell program The general format of this command is exit n where n is the exit status that you want returned If none is specified the exit status used is that of the last command executed before the exit Be advised that executing the exit command directly from your terminal will log you off the system because it will have the effect of terminating execution of your login shell A Second Look at the rem Program exit is frequently used as a convenient
263. is command line it substitutes the value of the variable filename and also the value of the variable filenameX The shell thinks filenamexX is the full name of the variable because it s composed entirely of valid variable name characters To avoid this problem you can delimit the end of the variable name by enclosing the entire name but not the leading dollar sign in a pair of curly braces as in filename X This removes the ambiguity and the mv command then works as desired mv filename filename X Remember that the braces are necessary only if the last character of the variable name is followed by an alphanumeric character or an underscore Fe previous Ne lt T rs ca Built in Integer Arithmetic The POSIX standard shell provides a mechanism for performing integer arithmetic on shell variables called arithmetic expansion Note that some older shells do not support this feature The format for arithmetic expansion is expression where expression is an arithmetic expression using shell variables and operators Valid shell variables are those that contain numeric values leading and trailing whitespace is allowed Valid operators are taken from the C programming language and are listed in Appendix A Shell Summary The result of computing expression is substituted on the command line For example echo i 1 adds one to the value in the shell variable i and prints the result Notice that the variable i doesn t have
264. is simply export variables where variables is the list of variable names that you want exported For any subshells that get executed from that point on the value of the exported variables will be passed down to the subshell Here s a program called vartest3 to help illustrate the difference between local and exported variables cat vartest3 echo x x echo y y Assign values to the variables x and y in the login shell and then run vartest3 x 100 y 10 vartest3 x and y are both local variables so their values aren t passed down to the subshell that runs vartest3 Now let s export the variable y and try it again export y Make y known to subshells vartest3 x This time vartest3 knew about y because it is an exported variable Conceptually whenever a subshell is executed the list of exported variables gets copied down to the subshell whereas the list of local variables does not see Figure 11 4 Figure 11 4 Execution of vartest3 exported local x 100 variables variables gt lt ee i oe copied exported local variables vartest3 variables Pl subshel I gt Now it s time for another question What do you think happens if a subshell changes the value of an exported variable Will the parent shell know about it after the subshell has finished To answer this question here s a program called vartest4 cat vartest4 x 50 We ll assume that you haven t
265. is technique may not work on very large directories because you may exceed the limit on the length of the command line the precise length varies between Unix systems Working with such directories may cause problems when using filename substitution in other commands as well such as echo or for file in set echo 8 This is much faster than ls we l because the first method uses only shell built in commands In general your shell programs run much faster if you try to get as much done as you can using the shell s built in commands Other Options to set set accepts several other options each of them enabled by preceding the option with a and disabled by preceding it with a The x option that we have described here is perhaps the most commonly used Others are summarized in Table A 9 in Appendix A fe erevious Next rs ca The irs Variable There is a special shell variable called IFS which stands for Internal Field Separator The shell uses the value of this variable when parsing input from the read command output from command substitution the back quoting mechanism and when performing variable substitution If it s typed on the command line the shell treats it like a normal whitespace character that is as a word delimiter See what it s set to now echo IFS Well that wasn t very illuminating To determine the actual characters stored in there pipe the output from echo into the od
266. istall program 2nd 3rd lu program 2nd PHONEBOOK variable 2nd 3rd 4th 5th rem program 2nd 3rd 4th 5th 6th 7th 8th 9th rolo program 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th sample output 2nd 3rd 4th 5th 6th temporary files 2nd 3rd twhile 2nd vartest 2nd vartest2 2nd 2 vartest3 2n 2N vartest4 2n vartest5 prompts secondary prompts ps command 2nd PS1 variable 2nd 3rd PS2 variable 2nd 3rd PS4 variable pseudo ttys pwd command 2nd 3rd 4th 5th 6th PWD variable 4 rrevious SYMBOL A B C D E F G H 1 J K L M N O P Q R S T U V W X Y Z question mark filename substitution 2nd 3rd quote characters 2nd back quote 2nd 3rd 4th backslash 2nd 3rd backslash inside double quotes 2nd 3rd 4th line continuation character 2nd double quotes 2nd 3rd 4th 5th single quote 2nd 3rd 4th 5th 6th 7th Fe eevious 4 rrevious SYMBOL A B C D E F G H 1 J K L M N O P Q R S T U V W X Y Z r r escape character r r escape characters r r operator r command 2nd 3rd r command vi r option sort command 2nd read command 2nd 3rd exit status 2nd 3rd 4th 5th syntax 2nd read only variables 2nd creating 2nd reading files command reading data 2nd
267. it would a space or tab as an argument delimiter Then it passes the nine arguments to echo to be displayed echo never gets a chance to see that newline the shell gets to it first See Figure 6 5 Figure 6 5 echo address 39 East 12th Street arguments echo al eat ke New 10003 When the command echo address is used instead the shell substitutes the value of address as before except that the double quotes tell it to leave any embedded whitespace characters alone So in this case the shell passes a single argument to echo an argument that contains an embedded newline echo simply displays its single argument at the terminal Figure 6 6 illustrates this The newline character is depicted by the characters n Figure 6 6 echo address arguments i echo ae s nnn 39 East 12th Streetin New York N Y 10003 Double quotes can be used to hide single quotes from the shell and vice versa x Hello he said echo x Hello he said article Keeping the Logins from Lagging Bell Labs Record echo article Keeping the Logins from Lagging Bell Labs Record a previous next The Backslash Basically the backslash is equivalent to placing single quotes around a single character with a few minor exceptions The backslash quotes the single character that immediately follows it The general format is c where c is the character you want to quote Any special meaning normally a
268. iting 2nd 3rd listing 2nd 3rd looking up 2nd rolo program listing 2nd 3rd 4th 5th ENV files 2nd ENV variable 2nd environment See also variables defined ENV file 2nd subshells 2nd environmental control eof command vi eq eq operator equal sign 2nd 3rd erase command vi errexit shell mode errors standard error 2nd 3rd ESC command vi escape character escape characters echo command 2nd eval command 2nd 3rd 4th 5th 6th evaluating expressions expr command 2nd 3rd 4th 5th exec command 2nd 3rd 4th 5th execute permissions command files 2nd executing commands 2nd 3rd 4th asynchronous execution background execution scanning twice before execution 2nd 3rd 4th file contents in current shell 2nd 3rd 4th 5th functions 2nd terminating execution 2nd jobs background execution 2nd foreground execution 2nd loops background execution 2nd exercises chapter 1 2nd 3rd chapter 10 chapter 11 2nd 3rd chapter 12 2nd 3rd chapter 13 2nd chapter 14 2nd 3rd chapter 15 2nd 3rd chapter 4 chapter 5 2nd 3rd chapter 6 2nd chapter 7 2nd 3rd chapter 8 2nd 3rd chapter 9 2nd 3rd exit command 2nd 3rd 4th 5th 6th EXIT signal exit status 2nd 3rd variable 2nd 3rd 4th 5th 6th nonzero values read command 2nd 3rd 4th 5
269. itor noc lobber noexec noglob nolog Table A 10 Shell Modes Meaning Same as a Same as e The exit command must be used to leave the shell Same as m Same as C Same as n Same as f Don t put function definitions in the history nounset Same as U verbose Same as V vi The inline editor is set to vi xtrace Same as x The command set o without any following options has the effect of listing all shell modes and their settings The shell variable contains the current options setting Each word listed in args is set to the positional parameters 1 2 respectively If the first word might start with a minus sign it s safer to specify the option to set to avoid interpretation of that value If args is supplied the variable will be set to the number of parameters assigned after execution of the command Examples set set set set set VX name address phone 0 vi 0 verbose o noglob Print all command lines as they are read and each command and its arguments as it is executed Set 1 to name 2 to address and 3 to phone Set 1 to 1 Turn on vi mode Turn verbose mode off noglob on The shift Command General Format shift This command causes the positional parameters 1 2 n to be Shifted left one place That is 2 is assigned to 1 3 to 2 and n to n 1 is adjusted accordingly If th
270. kets as before The entire construct must be enclosed inside a pair of curly braces which is then preceded by a dollar sign echo array 0 hello echo array 1 some text echo array 2 users steve memos echo array hello As you can see from the preceding example if no subscript is specified element zero is used If you forget the curly braces when performing the substitution here s what happens echo array 1 hello 1 In the preceding example the value of array is substituted hel lo the value inside array 0 and then echoed along with 1 Note that because the shell does filename substitution after variable substitution the shell would attempt to match the pattern hello 1 against the files in your current directory The construct can be used as a subscript to substitute all the elements of the array on the command line with each element delimited by a single space character echo array hello some text users steve memos The construct array can be used to substitute the number of elements in the array array echo array 3 Note that the number reported here is the actual number of values stored inside the array not the largest subscript used to store an element inside the array array 10 foo echo array Display all elements hello some text users steve memos foo echo array Number of elements 4 You can declare an arr
271. l simply ignores you mary had a little larb_ Ctrl b mary had a little larb mary had a little larb Ctrl b mary had a little larb mary had a little larb Ctrl b mary had a little larb mary had a little larb Ctrl f mary had a little larb After the cursor is on the character you want to change you can use the Ctril d command to delete the current character mary had a little larb Ctrl d mary had a little lab Note that the b moved to the left when the r was deleted and is now the current character To add characters to the command line you simply type them in The characters are inserted before the current character mary had a little lab m mary had a little lamb mary had a little lamb m mary had a little lammb mary had a little lammb Ctrl h mary had a little lamb Note that the current erase character usually either or Ctri h will always delete the character to the left of the cursor The Ctrl a and Ctrl e commands may be used to move the cursor to the beginning and end of the command line respectively mary had a little lamb Ctrl a mary had a little lamb mary had a little lamb Ctrl e mary had a little lamb_ Note that the Ctrl e command places the cursor one space to the right of the last character on the line When you re not in emacs mode the cursor is always at the end of the line one space to the right of the last character typed in When you re at the end of the line anything you t
272. ld numbers as was used to specify character positions before for example f1 2 8 f1 3 f4 So to extract the names of all users of your system from etc passwd you could type the following cut d f1 etc passwd Extract field 1 root cron bin uucp asg steve other Given that the home directory of each user is in field 6 you can associate each user of the system with his or her home directory as shown cut d f1 6 etc passwd Extract fields 1 and 6 root cron bin uucp usr spool uucp asg steve users steve other If the cut command is used to extract fields from a file and the d option is not supplied cut uses the tab character as the default field delimiter The following depicts a common pitfall when using the cut command Suppose that you have a file called phonebook that has the following contents cat phonebook Alice Chebba 973 555 2015 Barbara Swingle 201 555 9257 Jeff Goldberg 201 555 3378 Liz Stachiw 212 555 2298 Susan Goldberg 201 555 7776 Tony Iannino 973 555 1295 If you just want to get the names of the people in your phone book your first impulse would be to use cut as shown cut c1 15 phonebook Alice Chebba 97 Barbara Swingle Jeff Goldberg 2 Liz Stachiw 212 Susan Goldberg Tony Iannino 97 Not quite what you want This happened because the name is separated from the phone number by a tab character and not blank spaces in the phonebook
273. le variable value For example to assign the value 1 to the shell variable count you simply write count 1 and to assign the value users steve bin to the shell variable my bin you simply write my bin users steve bin A few important points here First spaces are not permitted on either side of the equals sign Keep that in mind especially if you re in the good programming habit of inserting spaces around operators In the shell language you can t put those spaces in Second unlike most other programming languages the shell has no concept whatsoever of data types Whenever you assign a value to a Shell variable no matter what it is the shell simply interprets that value as a string of characters So when you assigned 1 to the variable count previously the shell simply stored the character 1 inside the variable count making no observation whatsoever that an integer value was being stored in the variable If you re used to programming in a language such as C or Pascal where all variables must be declared you re in for another readjustment Because the shell has no concept of data types variables are not declared before they re used they re simply assigned values when you want to use them As you ll see later in this chapter the shell does support integer operations on shell variables that contain strings that are also valid numbers through special built in operations Because the shell is an interpretive language you can assign
274. le users steve documents or relative to the current directory for example documents pat If CDPATH is non null and directory is not a full path the shell searches the colon delimited directory list in CDPATH for a directory containing directory Examples cd documents memos Change to documents memos directory cd Change to HOME directory An argument of causes the shell to make the previous directory the current directory The pathname of the new current directory is printed out Examples pwd usr lib uucp cd cd usr lib uucp The cd command sets the shell variable PWD to the new current directory and OLDPWD to the previous directory The continue command General Format continue Execution of this command from within a for while or until loop causes any commands that follow the continue to be skipped Execution of the loop then continues as normal If the format continue n is used the commands within the n innermost loops are skipped Execution of the loops then continue as normal The echo Command General Format echo args This command causes args to be written to standard output Each word from args is delimited by a blank space A newline character is written at the end If args is omitted the effect is to simply skip a line Certain backslashed characters have a special meaning to echo as shown in Table A 8 Table A 8 echo Escape Characters Character Prints a Alert b
275. lect and mon into the directory misc You can also use the mv command to change the name of a directory For example the following renames the directory programs to bin mv programs bin Linking Files The in Command In simplest terms the Ln command provides an easy way for you to give more than one name to a file The general form of the command is ln from to This links the file from to the file to Recall the structure of steve s programs directory from Figure 2 8 In that directory he has stored a program called wb Suppose that he decides that he d also like to call the program writeback The most obvious thing to do would be to simply create a copy of wb called writeback cp wb writeback The drawback with this approach is that now twice as much disk space is being consumed by the program Furthermore if steve ever changes wb he may forget to make a new copy of writeback resulting in two different copies of what he thinks is the same program By linking the file wb to the new name these problems are avoided Ln wb writeback Now instead of two copies of the file existing only one exists with two different names wb and writeback The two files have been logically linked by the Unix system As far as you re concerned it appears as though you have two different files Executing an Ls command shows the two files separately ls collect mon wb writeback Look what happens when you execute a
276. line 1 p perating system was pioneered by Ken nd Dennis Ritchie at Bell Laboratories e 1960s One of the primary goals in of the Unix system was to create an t that promoted efficient program ts 1 s 5 Delete the last 5 chars from each line 1 p perating system was pioneered b nd Dennis Ritchie at Bell Laborat e 1960s One of the primary goa of the Unix system was to crea t that promoted efficient pr E Note that the last line of the file didn t have five characters when the last substitute command was executed therefore the match failed on that line and thus was left alone recall that we specified that exactly five characters were to be deleted If a single number is enclosed in the braces followed immediately by a comma then at least that many occurrences of the previous regular expression must be matched So 5 matches at least five consecutive plus signs Once again if more than five exist the largest number is matched ed intro 248 1 s a zA Z 6 X g Change words at least 6 letters long to X 1 p The Unix X X was X by Ken X and X X at Bell X in the late 1960s One of the X goals in the X of the Unix X was to X an X that XX X X Saving Matched Characters It is possible to capture the characters matched within a regular expression by enclosing the characters inside backslashed parentheses These captured characters are stored in registers numbered 1 through 9 For
277. little trickier You ll see an example of nested command substitution later in this section You are not restricted to executing a single command between the parentheses Several commands can be executed if separated by semicolons Also pipelines can be used Here s a modified version of the nu program that displays the number of logged in users cat nu echo There are who wc l users logged in nu Execute it There are 13 users logged in Because single quotes protect everything the following output should be clear echo who wc l tells how many users are logged in who wc L tells how many users are logged in But command substitution is interpreted inside double quotes echo You have ls wc l files in your directory You have 7 files in your directory What causes those leading spaces before the 7 Remember that the shell is responsible for executing the command enclosed between the parentheses The only thing the echo command sees is the output that has been inserted by the shell Suppose that you re writing a shell program and want to assign the current date and time to a variable called now perhaps to display it later at the top of a report or log it into a file The problem here is that you somehow want to take the output from date and assign it to the variable Command substitution can be used for this now date Execute date and store the output in now echo now See wha
278. ll It is available in the standard Unix distributions from Sun HP and IBM and is the default shell on MIPS workstations Bash short for Bourne Again Shell was developed by Brian Fox for the Free Software Foundation It was also designed to be upward compatible with the System V Bourne shell and the POSIX standard shell and also contains many extensions from the Korn and C shells Bash is the standard shell on Linux systems Except for a few minor differences Bash and the Korn shell provide all the POSIX standard shell s features as well as many new ones To give you an idea of the compatibility of these shells with the POSIX standard all shell programs in the previous chapters work under both Bash and the Korn shell We ll note any nonstandard features that we discuss in this chapter and Table 15 4 at the end of this chapter lists the features supported by the different shells a e next gt Getting the Right Shell Most shells follow a convention that allows you to select a specific program to run a file If the first two characters on the first line of a file are the remainder of the line specifies an interpreter for the file So usr bin ksh specifies the Korn shell and usr bin bash specifies Bash If you use constructs specific to one shell you can use this feature to force that shell to run your programs avoiding compatibility problems Note that you can put any program you want here so a Perl program beginning
279. lly comes to the terminal If stty tostop is executed output from background jobs is disabled and a job writing to the terminal is stopped until it is brought to the foreground When the shell exits all stopped jobs are killed rs ia os fa Command Summary This section summarizes the shell s built in commands Actually some of these commands such as echo and test may not be built in to the shell but must be provided as a utility by a POSI X compliant system They are built in to Bash and the Korn shell and are so often used in shell scripts that we decided to list them here anyway The following commands are organized alphabetically for easy reference The Command General Format This is essentially a null command It is frequently used to satisfy the requirement that a command appear Example if who grep jack gt dev null then else echo jack s not logged in fi The command returns an exit status of zero The Command General Format file The dot command causes the indicated file to be read and executed by the shell just as if the lines from the file were typed at that point Note that file does not have to be executable only readable Also the shell uses the PATH variable to find file Example progdefs Execute commands in progdefs The preceding command causes the shell to search the current PATH for the file progdefs When it finds it it reads and executes the commands from the file
280. lowing people from Sams Phil Kennedy Wendy Ford and Scott Arant For the second revised edition of this book we d like to thank Kathryn Purdum our acquisitions editor Charlotte Clapp our project editor and Geneil Breeze our copy editor Fe previous NEXT 4 FRevious Next Chapter 2 A Quick Review of the Basics IN THIS CHAPTER Some Basic Commands Working with Files Working with Directories Filename Substitution Standard Input Output and I O Redirection Pipes Standard Error More on Commands Command Summary Exercises This chapter provides a review of the Unix system including the file system basic commands filename substitution O redirection and pipes Fe previous next gt ns i Some Basic Commands Displaying the Date and Time The date Command The date command tells the system to print the date and time date Sat Jul 20 14 42 56 EDT 2002 date prints the day of the week month day time 24 hour clock the system s time zone and year Throughout this book whenever we use boldface type like this it s to indicate what you the user types in Normal face type like this is used to indicate what the Unix system prints Italic type is used for comments in interactive sequences Every Unix command is ended with the pressing of the Enter key Enter says that you are finished typing things in and are ready for the Unix system to do its thing Finding Out Who s Logged I
281. ly to execute a bit faster Table 4 4 summarizes how to use tr for translating and deleting characters Bear in mind that tr works only on single characters So if you need to translate anything longer than a single character say all occurrences of unix to UNIX you have to use a different program such as sed instead tr Command tr op ae Tgl teagi ty tr a z A Z tr A Z N ZA M Eprat tr s 1 tr d I4 tr d 0 9 Table 4 4 tr Examples Description Translate all capital X s to small x s Translate all open parens to open braces all closed parens to closed braces Translate all lowercase letters to uppercase Translate uppercase letters A M to N Z and N Z to A M respectively Translate all tabs character in first pair of quotes to spaces Translate multiple spaces to single spaces Delete all formfeed octal 14 characters Delete all digits rs i grep grep allows you to search one or more files for particular character patterns The general format of this command is grep pattern files Every line of each file that contains pattern is displayed at the terminal If more than one file is specified to grep each line is also immediately preceded by the name of the file thus enabling you to identify the particular file that the pattern was found in Let s say that you want to find every occurrence of the word shell in the file ed cmd grep shell ed cmd files and is independent of th
282. m Ctrl p echo this is a test_ Every time Ctrl p is used the current line is replaced by the previous line from the command history echo this is a test_ Ctrl p cd tmp_ cd tmp_ Ctrl p pwd_ To execute the command being displayed just press Enter pwd_ Hit Enter tmp The Ctrl n command is the reverse of the Ctrl p command and is used to display the next command in the history The Ctrl r command is used to search through the command history for a command containing a string The Ctrl r is entered followed by the string to search for followed by the Enter key The shell then searches the command history for the most recently executed command that contains that string on the command line If found the command line is displayed otherwise the shell beeps the terminal When the Ctri r is typed the shell replaces the current line with R Ctrl r test Rtest_ The search is initiated when Enter is pressed Rtest_ Enter echo this is a test_ To execute the command that is displayed as a result of the search Enter must be pressed again echo this is a test_ Hit Enter again this is a test To continue the search through the command history you simply type Ctril r followed by an Enter Bash handles Ctrl r a little differently When you type Ctrl r Bash replaces the current line with reverse i search Ctrl r reverse i search _ As you type text the line is updated insid
283. mand the Unix system does not care whether the file specified as the 2 second argument already exists If it does the contents of the file will be lost called old_names exists executing the command For example if a file 2 Assuming that you have the proper permission to write to the file cp names old_names would copy the file names to old_names destroying the previous contents of old_names in the process Similarly the command mv names old names would rename names to old_ names even if the file old names existed prior to execution of the command Removing a File The rm Command To remove a file from the system you use the rm command The argument to rm is simply the name of the file to be removed rm hold it You can remove more than one file at a time with the rm command by simply specifying all such files on the command line For example the following would remove the three files wb collect and mon rm wb collect mon ns i a Working with Directories Suppose that you had a set of files consisting of various memos proposals and letters Further suppose that you had a set of files that were computer programs It would seem logical to group this first set of files into a directory called documents for example and the latter set of files into a directory called programs Figure 2 1 illustrates such a directory organization Figure 2 1 Example directory structure documents programs plan
284. mands listed For example the command fc l 510 515 writes commands 510 through 515 to standard output whereas the command fc n l 20 writes the last 20 commands to standard output not preceded by line numbers n Suppose that you ve just executed a long command line and then decide that it would be nice to turn that command line into a shell program called runx You can use fc to get the command from your history and 1 0 redirection to write that command to a file fc n l 1 gt runx That s the letter l followed by the number 1 fc is described in full detail in Appendix A The r Command A simple Korn shell command allows you to re execute previous commands using even a fewer number of keystrokes than described If you simply type in the r command the Korn shell re executes your last command date Thu Oct 24 14 24 48 EST 2002 r Re execute previous command date Thu Oct 24 14 25 13 EST 2002 When you type in the r command the Korn shell redisplays the previous command and then immediately executes it If you give the r command the name of a command as an argument the Korn shell re executes the most recent command line from your history that begins with the specified argument cat docs planA pwd users steve r cat Rerun last cat command cat docs planA Once again the Korn shell redisplays the command line from its history before automatically re executing it The final form of the
285. match usr spool uucppublic o usr spool uucppublic The last example shows that the rightmatch function should simply echo its first argument if it does not end with the specified pattern Write a function called Leftmatch that works similarly to the rightmatch function developed in Exercise 4 Its two arguments should be as follows leftmatch pattern value Here are some example uses leftmatch usr spool usr spool uucppublic uucppublic leftmatch s s main c main c Write a function called substring that uses the Leftmatch and rightmatch functions developed in Exercises 4 and 5 to remove a pattern from the left and right side of a value It should take three arguments as shown substring usr usr spool uucppublic uucppublic spool substring s s main c c main substring s S main c o Only left match main c substring x S main c o No matches s main c Modify the substring leftmatch and rightmatch functions developed in the previous exercises to take options that allow you to remove the largest possible matches of the specified pattern from the left or right side of the specified value a Chapter 13 Loose Ends IN THIS CHAPTER The eval Command The wait Command The trap Command More on I O Functions The type Command Exercises We ve put commands and features into this chapter that for one reason or another did not logically fit into earlier chapters There s no particul
286. mediately by the pressing of the Enter key followed by the closed quote on the next line No additional characters can be typed inside those quotes because they ll be stored inside IFS and then used by the shell Now let s change the IFS to something more visible like a colon IFS read xX yz 123 345 678 echo x 123 echo z 678 List one two three for x in list do echo x done one two three var a b c echo var a b c Because the IFS was changed to a colon when the line was read the shell divided the line into three words 123 345 and 678 which were stored into the three variables x y and z respectively In the next to last example the shell used the IFS when substituting the value of List in the for loop The last example shows that the shell doesn t use the IFS when performing variable assignment Changing the IFS is often done in conjunction with execution of the set command Line Micro Logic Corp Box 174 Hackensack NJ 07602 IFS set line echo How many parameters were set 3 for field do echo field done Micro Logic Corp Box 174 Hackensack NJ 07602 This technique is a powerful one it uses all built in shell commands which also makes it very fast An alternative approach might have been to echo the value of line into the tr command where all colons could have been translated into newlines an approach that would have been much slower This t
287. mes with contents as shown cat names Charlie Tony Emanuel Lucy Ralph Fred Tony You can see that the name Tony appears twice in the file You can use uniq to remove such duplicate entries uniq names Print unique lines Charlie Tony Emanuel Lucy Ralph Fred Tony Tony still appears twice in the preceding output because the multiple occurrences are not consecutive in the file and thus uniq s definition of duplicate is not satisfied To remedy this situation sort is often used to get the duplicate lines adjacent to each other The result of the sort is then run through uniq sort names uniq Charlie Emanuel Fred Lucy Ralph Tony So the sort moves the two Tony lines together and then uniq filters out the duplicate line recall that sort with the u option performs precisely this function The d Option Frequently you ll be interested in finding the duplicate entries in a file The d option to uniq should be used for such purposes It tells unig to write only the duplicated lines to out_file or standard output Such lines are written just once no matter how many consecutive occurrences there are sort names uniq d List duplicate lines Tony As a more practical example let s return to our etc passwd file This file contains information about each user on the system It s conceivable that over the course of adding and removing users from this file that perhaps the same usern
288. mmand This program can be useful when reading yes no responses from the terminal which you ll learn about in Chapter 10 Reading and Printing Data Use the date and who commands to write a program called conntime that prints the number of hours and minutes that a user has been logged on to the system assume that this is less than 24 hours 4 Previous NEXT Chapter 9 Round and Round She Goes IN THIS CHAPTER The for Command The until Command More on Loops The getopts Command Exercises In this chapter you ll learn how to set up program loops These loops will enable you to execute repeatedly a set of commands either a specified number of times or until some condition is met The three built in looping commands are e for e while e until You ll learn about each one of these loops in separate sections of this chapter es Ga rs ca The for Command The for command is used to execute a set of commands a specified number of times Its basic format is as shown for var in word word word do command command done The commands enclosed between the do and the done form what s known as the body of the loop These commands are executed for as many words as you have listed after the in When the loop is executed the first word wordy is assigned to the variable var and the body of the loop is then executed Next the second word in the list word is assigned to var and the body of the loop is exe
289. mp 2 usr spool Table 15 3 summarizes the various array constructs in the Korn shell and Bash Construct array i array array array array i val Table 15 3 Array Constructs Meaning Substitute value of element i Substitute value of first element array 0 Substitute value of all elements Substitute number of elements Store val into array i Previous next gt Job Control The shell provides facilities for controlling jobs A job is any command sequence For example who wc When a command is started in the background that is with amp the shell prints out the job number inside brackets as well as the process number who wc amp 1 832 When a job finishes the shell prints the message n sequence where n is the job number of the finished job and sequence is the text of the command sequence used to create the job The jobs command may be used to print the status of jobs that haven t yet finished jobs 3 Running make ksh amp 2 Running monitor amp 1 Running pic chapt2 troff gt aps out amp The and after the job number mark the current and previous jobs respectively The current job is the last job sent to the background and the previous job is the next to the last job sent to the background Several built in commands may be given a job number or the current or previous job as arguments The shell s built in kill
290. n Here the first argument air specifies the three acceptable options to the command and option specifies the variable that getopts will use as previously described The getopts command permits options to be stacked together on the command line This is done by following a single minus sign with one or more consecutive options letters For example our foo command can be executed like this foo a r i or like this foo ari using this stacking feature The getopts command also handles the case where an option must be followed by an argument For example the new t option to be added to the mon command requires a following argument To handle options that take arguments getopts requires that at least one whitespace character separate the option from the argument Furthermore such options cannot be stacked To indicate to getopts that an option takes a following argument you write a colon character after the option letter on the getopts command line So our mon program which takes m and t options should call getopts like this getopts mt option If getopts doesn t find an argument after an option that requires one it stores a question mark inside the specified variable and writes an error message to standard error Otherwise it stores the actual argument inside a special variable called OPTARG One final note about getopts Another special variable called OPTIND is used by the command This variable is initially set
291. n The who Command The who command can be used to get information about all users currently logged in to the system who pat tty29 Jul 19 14 40 ruth tty37 Jul 19 10 54 steve tty25 Jul 19 15 52 Here three users are logged in pat ruth and steve Along with each user id the tty number of that user and the day and time that user logged in is listed The tty number is a unique identification number the Unix system gives to each terminal or network device that a user has logged into The who command also can be used to get information about yourself who am i pat tty29 Jul 19 14 40 who and who am i are actually the same command who In the latter case the am and i are arguments to the who command Echoing Characters The echo Command The echo command prints or echoes at the terminal whatever else you happen to type on the line there are some exceptions to this that you ll learn about later echo this is a test this is a test echo why not print out a longer line with echo why not print out a longer line with echo echo A blank line is displayed echo one two three four five one two three four five You will notice from the preceding example that echo squeezes out extra blanks between words That s because on a Unix system the words are important the blanks are merely there to separate the words Generally the Unix system ignores extra blanks you ll learn more about this in the next chapter fe pr
292. n command construct 2nd 3rd 4th 5th 6th 7th 8th 9th backquote 2nd 3rd 4th expr command 2nd 3rd 4th 5th summary of commands 2nd suspending jobs 2nd 3rd 4th symbolic links 2nd Previous SYMBOL A B C D E F G H 1 J K L M N O P Q R S T U V W X Y Z t t escape character t t escape characters t option sort command 2nd 3rd tapping signals 2nd temporary files 2nd 3rd TERM signal TERM variable 2nd terminal type 2nd terminating function execution 2nd jobs 2nd loops 2nd test command 2nd 3rd 4th alternative format 2nd file operators 2nd 3rd integer operators 2nd 3rd logical AND operator a 2nd logical negation operator 2nd logical OR operator 0 2nd parentheses 2nd string operators 2nd 3rd 4th 5th 6th 7th 8th syntax 2nd testing conditions in if commands file operators 2nd 3rd integer operators 2nd 3rd logical AND operator a 2nd logical negation operator 2nd logical OR operator o 2nd parentheses 2nd string operators 2nd 3rd 4th 5th 6th 7th 8th test command syntax 2nd 3rd text See also lines characters extracting 2nd 3rd 4th 5th 6th 7th 8th 9th translating from standard input 2nd 3rd 4th 5th 6th 7th 8th 9th editing sed command 2nd 3rd 4th 5th 6th 7th 8th
293. n your shell displays your command prompt and waits for you to type your first command J ust before it does that however your login shell executes two special files on the system The first is etc profile This file is set up by the system administrator and usually does things like checking to see whether you have mail Where do you think the You have mail message comes from setting your default file creation mask your umask assigning values to some standard exported variables and anything else that the administrator wants to have executed whenever a user logs in The second file that gets automatically executed is profile in your home directory Your system administrator may have given you a default profile file when you got your account See what s in it now cat HOME profile PATH bin usr bin usr lbin export PATH Here you see a small profile file that simply sets the PATH and exports it You can change your profile file to include any commands that you want executed whenever you log in You can even put commands in your profile file that override settings usually environment variables made in etc profile Note that the commands in etc profile and profile are executed by your login shell as if you typed in etc profile profile as soon as you logged in which means that changes made to your environment remain after the programs are executed Here s a sample profile that sets your PATH to
294. n asked to type in the name This avoids the RE error that grep issues if it s given a null first argument Now let s look at the individual programs that rolo executes Each of the original programs has been changed to accommodate the new entry format and also to be more user friendly ns Ga add Program to add someone to the phonebook file echo Type in your new entry echo When you re done type just a single Enter on the line first entry while true do echo gt gt c read line if n line then entry entry line if z first then first line fi else break fi done echo entry gt gt PHONEBOOK sort o PHONEBOOK PHONEBOOK echo echo first has been added to the phone book This program adds an entry to the phone book It continually prompts the user to enter lines until a line with just an Enter is typed that is a null line Each line that is entered is concatenated to the variable entry with the special character used to logically separate one line from the next When the while loop is exited the new entry is added to the end of the phone book and the file is sorted ne Ga fe erevious Next lu Look someone up in the phone book name 1 grep name PHONEBOOK gt tmp matches if s tmp matches then echo I can t find name in the phone book else Display each of the matching entries while read line
295. n here sed has facilities that enable you to loop build text in a buffer and combine many commands into a single editing script Table 4 2 shows some more examples of sed commands Table 4 2 sed Examples sed Command Description sed 5d Delete line 5 sed Tt est d Delete all lines containing Test or test sed n 20 25p text Print only lines 20 through 25 from text sed 1 10s unix UNIX g intro Change unix to UNIX wherever it appears in the first 10 lines of intro sed jan s 1 5 Change the first 1 to 5 on all lines containing jan sed s data Delete the first three characters from each line of data sed s data Delete the last 3 characters from each line of data sed n l text Print all lines from text showing nonprinting characters as nn e nn is the octal value of the character and tab characters as a nex gt os fa tr The tr filter is used to translate characters from standard input The general form of the command is tr from chars to chars where from chars and to chars are one or more single characters Any character in from chars encountered on the input will be translated into the corresponding character in to chars The result of the translation is written to standard output In its simplest form tr can be used to translate one character into another Recall the file intro from earlier in this chapter cat intro The UNIX operating system was pioneered by Ken Thompson and Den
296. n ls l ls l total 5 rWXr Xr x 1 steve DP3725 358 Jun 25 13 31 collect rwWXr xr x 1 steve DP3725 1219 Jun 25 13 31 mon rWXr Xr x 2 steve DP3725 89 Jun 25 13 30 wb rWXr Xr x 2 steve DP3725 89 Jun 25 13 30 writeback The number right before steve is 1 for collect and mon and 2 for wb and writeback This number is the number of links to a file normally 1 for nonlinked nondirectory files Because wb and writeback are linked this number is 2 for these files This implies that you can link to a file more than once You can remove either of the two linked files at any time and the other will not be removed rm writeback ls l total 4 rWXr Xr x 1 steve DP3725 358 Jun 25 13 31 collect rWXr xr x 1 steve DP3725 1219 Jun 25 13 31 mon rWXr Xr x 1 steve DP3725 89 Jun 25 13 30 wb Note that the number of links on wb went from 2 to 1 because one of its links was removed Most often ln is used to link files between directories For example suppose that pat wanted to have access to steve s wb program Instead of making a copy for himself subject to the same problems described previously or including steve s programs directory in his PATH described in detail in Chapter 11 Your Environment he can simply link to the file from his own program directory for example pwd users pat bin pat s program directory ls l total 4 rWXr Xr x 1 pat DP3822 1358 Jan 15 11 01 lcat rwXr xr x 1 pat DP3822 504
297. n the list 3 is assigned to i the third time through the loop and the echo command executed This results in 3 being displayed at the terminal At that point no more words are left in the list so execution of the for command is then complete and the shell displays your command prompt to let you know it s done Recall the run program from Chapter 7 Passing Arguments that enabled you to run a file through tbl nroff and lp cat run tbl 1 nroff mm Tlp lp If you wanted to run the files memol through memo4 through this program you could type the following at the terminal for file in memol memo2 memo3 memo4 gt do gt run file gt done request id is laser1 33 standard input request id is laser1 34 standard input request id is laser1 35 standard input request id is laser1 36 standard input The four words memol memo2 memo3 and memo4 will be assigned to the variable file in order and the run program will be executed with the value of this variable as the argument Execution will be just as if you typed in the four commands run memol request id is laser1 33 standard input run memo2 request id is laser1 34 standard input run memo3 request id is laserl1 35 standard input run memo4 request id is laserl1 36 standard input Incidentally the shell permits filename substitution in the list of words in the for meaning that the previous loop could have also been written this way
298. ncluding arrays data typing and built in arithmetic operations ns Ga Chapter 4 Tools of the Trade IN THIS CHAPTER Regular Expressions cut te ct W s D Oo Qa M wn e 3 ct Cc H Exercises This chapter provides detailed descriptions of some commonly used shell programming tools Covered are cut paste sed tr grep uniq and sort The more proficient you become at using these tools the easier it will be to write shell programs to solve your problems In fact that goes for all the tools provided by the Unix system rs ca Regular Expressions Before getting into the tools you need to learn about regular expressions Regular expressions are used by several different Unix commands including ed sed awk grep and to a more limited extent vi They provide a convenient and consistent way of specifying patterns to be matched The shell recognizes a limited form of regular expressions when you use filename substitution Recall that the asterisk specifies zero or more characters to match the question mark specifies any single character and the construct specifies any character enclosed between the brackets The regular expressions recognized by the aforementioned programs are far more sophisticated than those recognized by the shell Also be advised that the asterisk and the question mark are treated differently by these programs than by the shell Throughout this section we assume f
299. nd syntax standard consists of a set of rules as outlined in the Utility Argument Syntax section of the POSIX standard Also note that the old version of mon could have been executed as follows mon sandy mail steve amp 1 5522 to achieve the same net result as adding the m option Two last points before leaving the discussion of mon First you ll probably always want to run this program in the background It would be nice if mon itself could take care of that Later you ll see how to do it Second the program always sends mail to steve not very nice if someone else wants to run it A better way is to determine the user running the program and then send him or her the mail if the m option is selected But how do you do that One way is to execute the who command with the am i options and get the user name that comes back This tells you who s logged on to the terminal that the program was run from You can then use cut to extract the username from who s output and use that name as the recipient of the mail All this can be done in the last if command of mon if it s changed to read as shown if eq 1 then echo user has logged on else runner who am i cut cl1 8 echo user has logged on mail runner fi Now the program can be run by anyone and the mail will be properly sent rs ca More on Loops Breaking Out of a Loop Sometimes you may want to make an immediate exit from a loop To just exit
300. ndard input will get an end of file condition as if Ctrl d were typed sort data gt out amp Send the sort to the background 1 1258 Process id date Your terminal is immediately available to do other work Sat Jul 20 14 45 09 EDT 2002 When a command is sent to the background the Unix system automatically displays two numbers The first is called the command s job number and the second the process id In the preceding example 1 was the job number and 1258 the process id The job number is used by some shell commands that you ll learn more about in Chapter 15 The process id uniquely identifies the command that you sent to the background and can be used to obtain status information about the command This is done with the ps command The ps Command The ps command gives you information about the processes running on the system ps without any options prints the status of just your processes If you type in ps at your terminal you ll get a few lines back describing the processes you have running ps PID TTY TIME COMMAND 195 01 0 21 sh The shell 1353 01 0 00 ps This ps command 1258 01 0 10 sort The previous sort The ps command prints out four columns of information PID the process id TTY the terminal number that the process was run from TIME the amount of computer time in minutes and seconds that process has used and COMMAND the name of the process The sh process in the preceding example is the shell that was sta
301. ne book put commands from rolo here None of the commands inside the original add Lu rem or rolo programs would have to be changed These first three programs are turned into functions by including them inside rolo sandwiched between the function header and the closing curly brace Note that defining them as functions this way now makes them inaccessible as standalone commands Removing a Function Definition To remove the definition of a function from the shell you use the unset command with the f option This is the same command you use to remove the definition of a variable to the shell unset f nu nu sh nu not found The return Command If you execute an exit command from inside a function its effect is not only to terminate execution of the function but also of the shell program that called the function If you instead want to just terminate execution of the function you can use the return command whose format is return n The value n is used as the return status of the function If omitted the status returned is that of the last command executed This is also what gets returned if you don t execute a return at all in your function The return status is in all other ways equivalent to the exit status You can access its value through the shell variable and you can also test it in if while and until commands Fe previous NEXT previous Next The type Command When you type in th
302. ne book Please select one of the above 1 3 2 Enter name to be added El Coyote Enter number 212 555 3232 rolo Try it again Would you like to 1 Look someone up 2 Add someone to the phone book 3 Remove someone from the phone book Please select one of the above 1 3 1 Enter name to look up Coyote El Coyote 212 555 3232 rolo Once again Would you like to 1 Look someone up 2 Add someone to the phone book 3 Remove someone from the phone book Please select one of the above 1 3 4 Bad choice When an invalid choice is entered the program simply displays Bad choice and then terminates A friendlier approach would be to reprompt the user until a proper choice is made This can be done by enclosing the entire program inside an until loop that will be executed until a valid selection is made To determine when a valid choice has been made we can test a variable in the until that won t be assigned a value in the program until either 1 2 or 3 is selected by the user Another change to make to rolo involves the way it will be used Because the most common operation performed will be one of lookup there will probably be a tendency on the part of the user to avoid typing rolo then making selection 1 and then typing the name to be found when instead he or she can still type in lu name directly Given all this it might be a good idea to allow rolo to take command line arguments If any arguments are ty
303. ng function execution jobs 2nd 3rd 4th 5th 6th loops 2nd stream editor sed d option 2nd n option 2nd examples 2nd 3rd 4th 5th syntax streams stream editor sed d option 2nd n option 2nd examples 2nd 3rd 4th 5th syntax string operators 2nd 3rd 4th 5th 6th 7th 8th strings string operators 2nd 3rd 4th 5th 6th 7th 8th subscripts subshells 2nd 3rd 4th 5th passing variables to 2nd 3rd 4th subsitution parameter substitution 2nd substituting variables 2nd substituting filenames 2nd 3rd 4th asterisk 2nd 3rd 4th question mark 2nd brackets 2nd examples 2nd multiple character matching 2nd 3rd 4th single character matching 2nd 3rd variables 2nd 3rd substitution parameter substitution 2nd parameter construct variable construct 2nd parameter pattern construct parameter pattern construct parameter pattern construct parameter value construct parameter value construct parameter value construct parameter value construct parameter colon value construct 2nd 3rd parameter colon value construct 2nd 3rd parameter colon value construct 2nd 3rd parameter colon value construct 2nd 3rd parameter construct 2nd 3rd pattern matching contructs 2nd 3rd 4th tilde substitution 2nd 3rd 4th 5th substitutio
304. ng example we assume that the user steve has been granted read access to pat s phone book file 4 Previous Next Chapter 12 More on Parameters IN THIS CHAPTER Parameter Substitution The 0 Variable The set Command The IFS Variable The readonly Command The unset Command Exercises In this chapter you ll learn some more about parameters Technically parameters include the arguments passed to a program the positional parameters the special shell variables such as and and ordinary variables also Known as keyword parameters Positional parameters cannot be assigned values directly however they can be reassigned values with the set command Keyword parameters are assigned values simply by writing variable value The format is a bit more general than that shown actually you can assign several keyword parameters at once using the format variable value variable value as the following example illustrates x 100 y 200 z 50 echo x y z 100 200 50 fe erevious Next ns on Parameter Substitution In the simplest form to have the value of a parameter substituted you simply precede the parameter with a dollar sign as in i or 9 parameter If there s a potential conflict caused by the characters that follow the parameter name you can enclose the name inside curly braces as in mv file file x This command would add an x to the end of the filename specified by fil
305. nis Ritchie at Bell Laboratories in the late 1960s One of the primary goals in the design of the UNIX system was to create an environment that promoted efficient program development The following shows how tr can be used to translate all letter e s to x s tr e x lt intro Thx UNIX opxrating systxm was pionxxrxd by Kxn Thompson and Dxnnis Ritchix at Bxll Laboratorixs in thx latx 1960s Onx of thx primary goals in thx dxsign of thx UNIX systxm was to crxatx an xnvironmxnt that promotxd xfficixnt program dxvxlopmxnt The input to tr must be redirected from the file intro because tr always expects its input to come from standard input The results of the translation are written to standard output leaving the original file untouched Showing a more practical example recall the pipeline that you used to extract the usernames and home directories of everyone on the system cut d f1 6 etc passwd root cron bin uucp usr spool uucp asg steve users steve other You can translate the colons into tab characters to produce a more readable output simply by tacking an appropriate tr command to the end of the pipeline cut d f1 6 etc passwd tr root cron bin f uucp usr spool uucp asg steve users steve other Enclosed between the single quotes is a tab character even though you can t see it just take our word for it It must be enclosed in quotes to keep it from the
306. nly command line If you subsequently try to assign a value to a readonly variable the shell issues an error message readonly variables are useful for ensuring that you don t accidentally overwrite the value of a variable They re also good for ensuring that other people using a shell program can t change the values of particular variables for example their HOME directory or their PATH The readonly attribute is not passed down to subshells readonly with a p option prints a list of your readonly variables Example readonly DB users steve database Assign value to DB and make it readonly DB foo Try to assign it a value sh DB is read only Error message from the shell echo DB But can still access its value users steve database The return Command General Format return n This command causes the shell to stop execution of the current function and immediately return to the caller with an exit status of n If n is omitted the exit status returned is that of the command executed immediately prior to the return The set Command General Format set options args This command is used to turn on or off options as specified by options It is also used to set positional parameters as specified by args Each single letter option in options is enabled if the option is preceded by a minus sign or disabled if preceded by a plus sign Options can be grouped as in set fx which enables the f and x options
307. not setting it to a default value as in PHONEBOOK HOME phonebook This says that if PHONEBOOK is set to some value leave it alone otherwise set it to HOME phonebook Note that the preceding example could not stand alone as a command because after the substitution was performed the shell would attempt to execute the result PHONEBOOK PHONEBOOK HOME phonebook sh users steve phonebook cannot execute To use this construct as a standalone command the null command is often employed If you write PHONEBOOK HOME phonebook the shell still does the substitution it evaluates the rest of the command line yet executes nothing the null command PHONEBOOK PHONEBOOK HOME phonebook echo PHONEBOOK See if it got assigned users steve phonebook PHONEBOOK foobar Shouldn t change it echo PHONEBOOK users steve phonebook It didn t parameter value If parameter is not null the shell substitutes its value otherwise the shell writes value to standard error and then exits don t worry if it s done from your login shell you won t be logged off If value is omitted the shell writes the message prog parameter parameter null or not set Here s an example from the terminal PHONEBOOK PHONEBOOK No PHONEBOOK file No PHONEBOOK file PHONEBOOK Don t give a value sh PHONEBOOK parameter null or not set With this construct y
308. nto the misc directory under the same names To copy a file from another directory into your current one and give it the same name use the fact that the current directory can always be referenced as pwd users steve misc cp programs collect The preceding command copies the file collect from the directory programs into the current directory users steve misc Moving Files Between Directories You recall that the mv command can be used to rename a file However when the two arguments to this command reference different directories the file is actually moved from the first directory into the second directory For example first change from the home directory to the documents directory cd documents Suppose that now you decide that the file plan contained in the memos directory is really a proposal and not a memo So you want to move it from the memos directory into the proposals directory The following would do the trick mv memos plan proposals plan As with the cp command if the source file and destination file have the same name only the name of the destination directory need be supplied mv memos plan proposals Also like the cp command a group of files can be simultaneously moved into a directory by simply listing all files to be moved before the name of the destination directory pwd users steve programs mv wb collect mon misc This would move the three files wb col
309. nts Executing trap with no arguments results in the display of any traps that you have changed trap echo logged off at date gt gt HOME logoffs 0 trap List changed traps trap echo logged off at date gt gt HOME logoffs EXIT Ctrl d Log off login steve Log back in Password cat HOME logoffs See what happened logged off at Wed Oct 2 15 11 58 EDT 2002 A trap was set to be executed whenever signal 0 was received by the shell This signal is generated whenever the shell is exited Because this was set in the login shell the trap will be taken when you log off The purpose of this trap is to write the time you logged off into the file HOME Logof fs The command is enclosed in single quotes to prevent the shell from executing date when the trap is defined The trap command is then executed with no arguments which results in the display of the changed action to be taken for signal O EXIT Next steve logs off and then back on again to see whether the trap works Displaying the contents of HOME lLogoffs verifies that the echo command was executed when steve logged off Ignoring Signals If the command listed for trap is null the specified signal will be ignored when received For example the command trap 2 specifies that the interrupt signal is to be ignored You might want to ignore certain signals when performing some operation that you don t want interrupted Note that the first argument
310. o make the output a bit more informative cat stats echo The current date and time is date echo echo The number of users on the system is who wc l echo echo Your current working directory is pwd stats Execute it The current date and time is Wed Jul 10 12 00 27 EDT 2002 The number of users on the system is 13 Your current working directory is users steve documents proposals Recall that echo without any arguments simply skips a line in the display Shortly you ll see how to have the message and the data displayed on the same line like this The current date and time is Wed Jul 10 12 00 27 EDT 2002 Comments The shell programming language would not be complete without a comment statement A comment is a way for you to insert remarks or comments inside the program that otherwise have no effect on its execution Whenever the shell encounters the special character at the start of a word it takes whatever characters follow the to the end of the line as comments and simply ignores them 2 If the starts the line the entire line is treated as a comment by the shell Here are examples of valid comments 2 Note that the may be your default erase character If so to enter the character into an editor such as ed or vi you ll have to escape it by preceding it with a Alternatively you can change your erase character to something else with the stty command Here is an entire commenta
311. octal dump command with the b byte display option echo IFS od b 0000000 040 011 012 012 0000004 The first column of numbers shown is the relative offset from the start of the input The following numbers are the octal equivalents of the characters read by od The first such number is 040 which is the ASCII value of the space character It s followed by 011 the tab character and then by 012 the newline character The next character is another newline this was written by the echo These characters for IFS come as no surprise they re the whitespace characters we ve talked about throughout the book You can change your IFS to any character or characters you want This is useful when you want to parse a line of data whose fields aren t delimited by the normal whitespace characters For example we noted that the shell normally strips any leading whitespace characters from the beginning of any line that you read with the read command You can change your IFS to just a newline character before the read is executed which has the effect of preserving the leading whitespace because the shell won t consider it a field delimiter read line Try it the old way Here s a line echo line Here s a line IFS gt read line Here s a line echo line Here s a line Set it to a just a newline Try it again Leading spaces preserved To change the IFS to just a newline an open quote was typed followed im
312. ocuments ls documents letters memos proposals To take a look at the subdirectory memos you follow a similar procedure Ls documents memos dact plan If you specify a nondirectory file argument to the ls command you simply get that filename echoed back at the terminal Ls documents memos plan documents memos plan An option to the ls command enables you to determine whether a particular file is a directory among other things The l option the letter I provides a more detailed description of the files in a directory If you were currently in steve s home directory as indicated in Figure 2 6 the following would illustrate the effect of supplying the l option to the ls command ls l total 2 drwxr xr x 5 steve DP3725 80 Jun 25 13 27 documents drwxr xr x 2 steve DP3725 96 Jun 25 13 31 programs The first line of the display is a count of the total number of blocks 1 024 bytes of storage that the listed files use Each successive line displayed by the ls l command contains detailed information about a file in the directory The first character on each line tells whether the file is a directory If the character is d it is a directory if itis itis an ordinary file finally if itis b c l or p it is a special file The next nine characters on the line tell how every user on the system can access the particular file These access modes apply to the file s owner the first three characters othe
313. of new items added to the menu Because individual entries can be rather long an editing option has been added to allow you to edit a particular entry Formerly the only way to change an entry was to first remove it and then add a new one a strategy that was perfectly acceptable when the entries were small Another option allows for listing of the entire phone book With this option just the first and last lines of each entry are displayed This assumes that the user follows some convention such as putting the name on the first line and the number on the last The entire menu selection process was placed inside a while loop so that rolo will continue to display menus until the exit option is picked from the menu A trap command is executed before the loop is entered This trap specifies that a continue command is to be executed if signal number 2 is received So if the user presses the Delete key in the middle of an operation such as listing the entire phone book the program won t exit but will abort the current operation and simply continue with the loop This will result in the redisplay of the menu Because entries can now span as many lines as desired the action performed when add is selected has been changed Instead of asking for the name and number rolo executes the add program to get the entry from the user For the lookup change and remove options a check is made to ensure that the user doesn t simply press the Enter key whe
314. of time that has been used by the shell and by all its child processes For each two numbers are listed first the accumulated user time and then the accumulated system time Note that times does not report the time used by built in commands Example times Print time used by processes 1m5s 2m9s 1 min 5 secs user time 2 mins 9 secs system time 8m22 23s 6m22 01s Time used by child processes The trap Command General Format trap commands signals This command tells the shell to execute commands whenever it receives one of the signals listed in signals The listed signals can be specified by name or number trap with no arguments prints a list of the current trap assignments If the first argument is the null string as in trap signals the signals in signals are ignored when received by the shell If the format trap signals is used processing of each signal listed in signals is reset to the default action Examples trap echo hangup gt gt ERRFILE exit 2 Log message and exit on hangup trap rm TMPFILE exit 1 2 15 remove TMPFILE on signals l 2 or 15 trap 2 Ignore interrupts trap 2 Reset default processing of interrupts Table A 12 lists values that can be specified in the signal list Table A 12 Signal Numbers and Names for trap Signal Signal Name Generated for 0 EXIT Exit from the shell 1 HUP Hangup 2 INT Interrupt for example Delete key Ctrl c 3 QUIT Quit 6 ABRT Abort
315. ogram 2nd number2 program 2nd numbers exit status 2nd variable 2nd 3rd 4th 5th 6th nonzero values zero job numbers line numbering program 2nd signal numbers 2nd 3rd 4 rrevious SYMBOL A B C D E F G H 1 J K L M N O P Q R S T U V W X Y Z o 0 option set command o conversion specification character o option sort command 2nd o option set command O Reilly and Associates recommended reading 2nd Web site operators 2nd file operators 2nd 3rd integer operators 2nd 3rd logical AND operator a 2nd logical negation operator 2nd logical OR operator o 2nd string operators 2nd 3rd 4th 5th 6th 7th 8th options command 2nd 3rd OR operators logical OR operator o 2nd ordinary files output redirection 2nd 3rd 4th 5th 6th 7th 8th output See I O input output 4 rrevious 4 rrevious SYMBOL A B C D E F G H 1 J K L M N O P Q R S T U V W X Y Z P command vi p command vi p option export command parameter substitution 2nd parameter construct variable construct 2nd parameter pattern construct parameter pattern construct parameter pattern construct parameter value construct parameter value construct paramete
316. om a file or the terminal Here s a program called words that counts the number of words typed on a line using the shell s definition of a word cat words Count words ona line read line set line echo words Run it Here s a line for you to count 7 The program stores the line read in the shell variable Line and then executes the command set line This causes each word stored in line to be assigned to the positional parameters The variable is also set to the number of words assigned which is the number of words on the line The Option Try typing in a line to words that begins with a and see what happens words 1 5 4 words 1 bad option s After the line was read and assigned to Line the command set line was executed After the shell did its substitution the command line looked like this set 1 5 4 When set executed it saw the and thought that an option was being selected thus explaining the error message Another problem with words occurs if you give it a line consisting entirely of whitespace characters or if the line is null words Just Enter is pressed CDPATH users steve usr spool EDITOR bin vi HOME users steve IFS LOGNAME steve MAIL usr spool mail steve MATLCHECK 600 PATH bin usr bin users steve bin PHONEBOOK users steve phonebook PS1 PS2 gt PwWD users steve misc SHELL usr bin sh TERM xterm TMOUT 0 TZ EST
317. om the file memo and passing it the names of seven users who are to receive the mail Notice that Tony receives the same mail twice because he s listed twice in the names file You can remove any duplicate entries from the file by using sort with the u option remove duplicate lines rather than cat to ensure that each person only receives mail once mail sort u names lt memo It s worth noting that the shell does filename substitution after it substitutes the output from commands Enclosing the commands inside double quotes prevents the shell from doing the filename substitution on this output if desired Command substitution is often used to change the value stored in a shell variable For example if the shell variable name contains someone s name and you want to convert every character in that variable to uppercase you could use echo to get the variable to tr s input perform the translation and then assign the result back to the variable name Ralph Kramden name echo name tr a z A Z Translate to uppercase echo name RALPH KRAMDEN The technique of using echo in a pipeline to write data to the standard input of the following command is a simple yet powerful technique it s used often in shell programs The next example shows how cut is used to extract the first character from the value stored in a variable called filename filename users steve memos firstchar echo filename cut cl
318. ompson and ennis itchie at ell aboratories in the late 1960s ne of the primary goals in the design of the nix system was to create an environment that promoted efficient program development As you ll learn shortly the asterisk is a special character in regular expressions However you don t need to put a backslash before the asterisk in the replacement string of the substitute command In general regular expression characters such as and are only meaningful in the search string and have no special meaning when they appear in the replacement string If a caret appears as the first character after the left bracket the sense of the match is inverted H For example the regular expression 1 Recall that the shell uses the for this purpose A Z matches any character except an uppercase letter Similarly A Za z matches any nonalphabetic character ed intro 248 1 s a zA Z g Delete all nonalphabetic characters 1 p TheUnixoperatingsystemwaspioneeredbyKen ThompsonandDennisRitchieatBellLaboratories InthelatesOneoftheprimarygoalsin ThedesignoftheUnixsystemwastocreatean Environmentthatpromotedefficientprogram development Matching Zero or More Characters The Asterisk You know that the asterisk is used by the shell in filename substitution to match zero or more characters In forming regular expressions the asterisk is used to match zero or more occurrences of the preceding character
319. on XX means match at least one consecutive X Similarly XXX means match at least two consecutive X s There is a more general way to specify a precise number of characters to be matched by using the construct min max where min specifies the minimum number of occurrences of the preceding regular expression to be matched and max specifies the maximum For example the regular expression X 1 10 matches from one to ten consecutive X s As stated before whenever there is a choice the largest pattern is matched so if the input text contains eight consecutive X s at the beginning of the line that is how many will be matched by the preceding regular expression As another example the regular expression A Za z 4 7 matches a sequence of alphabetic letters from four to seven characters long ed intro 248 1 s A Za Z 4 7 X q 1 p The X Xng X was Xed by Ken Xn and X X at X XX in the X 1960s One of the X X in the X of the X X was to X an XX X Xd Xnt X XX A few special cases of this special construct are worth noting If only one number is enclosed between the braces as in 10 that number specifies that the preceding regular expression must be matched exactly that many times So a zA Z 7 matches exactly seven alphabetic characters and A 10 matches exactly ten characters no matter what they are ed intro 248 1 s 10 Delete the first 10 chars from each
320. ore options remain on the command line that is if the next command line argument does not begin with a getopts returns a nonzero exit status The special variable OPTIND is also used by getopts It is initially set to 1 and is adjusted each time getopts returns to indicate the number of the next command line argument to be processed The argument can be placed on the command line to specify the end of the command line arguments getopts supports stacked arguments as in repx iau which is equivalent to repx i a u Options that take following arguments may not be stacked If the format getopts options var args is used getopts parses the arguments specified by args rather than the command line arguments Example usage Usage foo r 0 outfile infile while getopts ro opt do case opt in r rflag 1 0 oflag 1 ofile 0OPTARG echo usage exit 1 esac done if OPTIND gt then echo Needs input file echo usage exit 2 Ta shift OPTIND 1 ifile 1 The hash Command General Format hash commands This command tells the shell to look for the specified commands and to remember what directories they are located in If commands is not specified a list of the hashed commands is displayed If the format hash r is used the shell removes all commands from its hash list Next time any command is executed the shell uses its normal search methods to find t
321. orks for numbers up to 12 digits long including sign cat while read numberl number2 do printf 12d 12d n numberl number2 done cat data 1234 7960 593 595 395 304 3234 999 394 493 align data 1234 7960 593 595 395 304 3234 999 394 493 In Chapters 12 14 and 15 you ll see more uses for printf But first try your hand at the following exercises a ns on Exercises 1 What happens to mycp if one or more of the files to be copied doesn t exist Can you make any suggestions to better handle the situation 2 What happens to mycp if one of the filenames contains a character that has a special meaning to the shell such as or 3 Write a program called mymv that does with the mv command what mycp does with the cp command How many changes did you have to make to mycp to produce this new program 4 Modify mycp to prompt for arguments if none are supplied A typical execution of the modified version should look like this mycp Source file name voucher Destination file name voucher sv Make sure that the program allows one or both of the files to be specified with filename substitution characters 5 Adda n option to mycp that suppresses the normal check for the existence of the destination files 6 Modify mycp to use sed instead of the while loop to process the arguments typed on the command line 7 Modify the rem program used by rolo so that if multiple entries are fo
322. ory c List the files that contain Move history exec c makemove c testch c Because grep conveniently lists the files one per line you can pipe the output from grep l into wc to count the number of files that contain a particular pattern grep l Move history c we l 3 So the preceding says that precisely three C program files reference the variable Move history What are you counting if you use grep without the l option The n Option If the n option is used with grep each line from the file that matches the specified pattern is preceded by its relative line number in the file From previous examples you saw that the file testch c was one of the three files that referenced the variable Move history the following shows how you can pinpoint the precise lines in the file that reference the variable grep n Move history testch c Precede matches with line numbers 13 GLOBAL MOVE Move history 100 0 197 Move history Number_half_moves 1 from move from 198 Move history Number half moves 1 to move to As you can see Move history is used on lines 13 197 and 198 in testch c rs i ns ca sort You re familiar with the basic operation of sort sort names Charlie Emanuel Fred Lucy Ralph Tony Tony By default sort takes each line of the specified input file and sorts it into ascending order Special characters are sorted according to the internal encoding of the charac
323. ou can easily check to see whether a set of variables needed by a program are all set and not null as in TOOLS EXPTOOLS TOOLBIN parameter value This one substitutes value if parameter is not null otherwise it substitutes nothing traceopt T echo options traceopt trace mode options trace mode traceopt echo options traceopt trace mode options The value part for any of the constructs in this section can be a command substitution it s executed by the shell only if its value is to be used In WORKDIR DBDIR pwd WORKDIR is assigned the value of DBDIR if it s not null otherwise the pwd command is executed and the result assigned to WORKDIR pwd is executed only if DBDIR is null Pattern Matching Constructs The POSIX standard shell provides four parameter substitution constructs that perform pattern matching Note that some older shells do not support this feature In each case the construct takes two arguments a variable name or parameter number and a pattern The shell searches through the contents of the specified variable to match the supplied pattern If the pattern is matched the shell substitutes the value of the variable on the command line with the matching portion of the pattern deleted If the pattern is not matched the entire contents of the variable are substituted on the command line In any case the contents of the variable remain unchanged The
324. ou may not want to see the actual lines that match a pattern but may be interested in knowing only the names of the files that contain the pattern For example suppose that you have a set of C programs in your current directory these filenames end with the characters c and you want to know which files use a variable called Move history The following example shows one way of finding the answer grep Move history c Find Move history in all C source files exec c MOVE Move history 200 0 exec c cpymove amp Move history Number half moves 1 exec c undo move amp Move history Number half moves 1 exec c cpymove amp last move amp Move history Number half moves 1 exec c convert _move amp Move history Number half moves 1 exec convert _move S amp Move history i 1 exec c convert _move amp Move history Number half _moves 1 makemove c IMPORT MOVE Move history makemove c if Move history j from BOOK i j from OR makemove c Move history j to BOOK i j to testch c GLOBAL MOVE Move history 100 0 testch c Move history Number half moves 1 from move from testch c Move history Number half moves 1 to move to Sifting through the preceding output you discover that three files exec c makemove c and testch c use the variable The l option to grep gives you just a list of files that contain the specified pattern not the matching lines from the files grep l Move hist
325. pability to substitute portions of the current directory s path with something else Bash and the POSIX standard shell do not support this feature The format is cd old new cd attempts to replace the first occurrence of the string old in the current directory s path with the string new pwd usr spool uucppublic pat cd pat steve Change pat to steve and cd usr spool uucppublic steve cd prints out name of new directory pwd Confirm location usr spool uucppublic steve Tilde Substitution If a word on a command line begins with the tilde character the shell scans the rest of the word and performs the following substitutions If the tilde is the only character in the word or if the character following the tilde is a slash the value of the HOME variable is substituted echo users pat grep Korn shell chapter9 ksh The Korn shell is a new shell developed by David Korn at AT amp T for the Bourne shell would also run under the Korn the one on System V the Korn shell provides you with idea of the compatibility of the Korn shell with Bourne s the Bourne and Korn shells The main features added to the Korn shell are If the rest of the word up to a slash is a user s login name in etc passwd the tilde and the user s login name are substituted with the HOME directory of that user echo steve users steve echo pat users pat grep Korn pat shell chapter9 ksh The Korn shell is a n
326. ped rolo can assume that a lookup is being requested and just call Lu directly So if the user wants to perform a quick lookup he or she can type rolo followed by the name On the other hand if the user wants to see the menu typing just rolo causes the program to display its menu and prompt for a choice The preceding two changes looping until a valid choice is selected and doing a quick lookup were added to version 2 of rolo that is shown next cat rolo rolo rolodex program to look up add and remove people from the phone book version 2 If arguments are supplied then do a lookup if ne 0 then lu exit fi validchoice set it null Loop until a valid selection is made until do n validchoice Display menu echo Would you like to 1 Look someone up 2 Add someone to the phone book 3 Remove someone from the phone book Please select one of the above 1 3 c Read and process selection read choice echo case choice in 1 echo Enter name to look up c read name lu name validchoice TRUE 2 echo Enter name to be added c read name echo Enter number c read number add name number validchoice TRUE 3 echo Enter name to be removed c read name rem name validchoice TRUE echo Bad choice esac done If is nonzero Lu is called directly with the arguments typ
327. period at the start of the path bin usr bin For security reasons it s generally not a good idea to have your current directory searched before the 2 system ones This is to avoid the so called Trojan horse problem Someone stores her own version of a command such as su the command that changes you to another user in a directory she can write into and waits for another user to change to that directory and run su If the PATH specifies that the current directory be searched first then the horsed version of su will be executed This version will get the password that is typed and then print out Sorry The user will think he just typed the wrong password The period for specifying the current directory is optional for example the path bin usr bin is equivalent to the previous one however throughout this text we ll specify the current directory with a period for clarity You can always override the PATH variable by specifying a path to the file to be executed For example if you type bin date the shell goes directly to bin to execute date The PATH in this case is ignored as it is if you type in bin lu or rolo This last case says to execute the program rolo in the current directory So now you understand why you couldn t execute rolo from your HOME directory users steve bin wasn t included in your PATH and so the shell couldn t find rolo This is a simple matter to rectify You can simply add thi
328. phonebook See what happened Alice Chebba 973 555 2015 Barbara Swingle 201 555 9257 Liz Stachiw 212 555 2298 Susan Goldberg 201 555 7776 Susan Topple 212 555 4932 Tony Iannino 973 555 1295 Stromboli Pizza 973 555 9478 Stromboli Pizza was quoted so that the shell passed it along to add as a single argument what would have happened if it wasn t quoted After add finished executing Lu was run to see whether it could find the new entry and it did The cat command was executed to see what the modified phonebook file looked like The new entry was added to the end as intended Unfortunately the new file is no longer sorted This won t affect the operation of the Lu program but you can add a sort to the add program to keep the file sorted after new entries are added cat add Add someone to the phonebook file version 2 echo 1 2 gt gt phonebook sort o phonebook phonebook Recall that the o option to sort specifies where the sorted output is to be written and that this can be the same as the input file add Billy Bach 201 555 7618 cat phonebook Alice Chebba 973 555 2015 Barbara Swingle 201 555 9257 Billy Bach 201 555 7618 Liz Stachiw 212 555 2296 Stromboli Pizza 973 555 9478 Susan Goldberg 201 555 7776 Susan Topple 212 555 4932 Tony Iannino 973 555 1295 So each time a new entry is added the phonebook file will get re sorted rs ca A Program to Remove Someone from the Phone Book No s
329. point execution of the for loop is terminated Execution then continues with the command that immediately follows the done The special format for var do done indicates that the positional parameters 1 2 are to be used in the list and is equivalent to for var in do done Example nroff all of the files in the current directory for file in do nroff Tlp file lp done The getopts Command General Format getopts options var This command processes command line arguments options is a list of valid single letter options If any letter in options is followed by a that option takes a following argument on the command line which must be separated from the option by at least one whitespace character Each time getopts is called it processes the next command line argument If a valid option is found getopts stores the matching option letter inside the specified variable var and returns a zero exit status If an invalid option is specified that is one not listed in options getopts stores a inside var and returns with a zero exit status It also writes an error message to standard error If an option takes a following argument getopts stores the matching option letter inside var and stores the following command line argument inside the special variable OPTARG If no arguments are left on the command line getopts stores a inside var and writes an error message to standard error If no m
330. r it s as if it wasn t even typed This construct is most often used for typing long commands over multiple lines The Backslash Inside Double Quotes We noted earlier that the backslash is one of the three characters interpreted by the shell inside double quotes This means that you can use the backslash inside these quotes to remove the meaning of characters that otherwise would be interpreted inside double quotes that is other backslashes dollar signs back quotes newlines and other double quotes If the backslash precedes any other character inside double quotes the backslash is ignored by the shell and passed on to the program echo x x echo is the backslash character is the backslash character x 5 echo The value of x is x The value of x is 5 In the first example the backslash precedes the dollar sign interpreted by the shell inside double quotes So the shell ignores the dollar sign removes the backslash and executes echo In the second example the backslash precedes a space not interpreted by the shell inside double quotes So the shell ignores the backslash and passes it on to the echo command The last example shows the backslash used to enclose double quotes inside a double quoted string As an exercise in the use of quotes let s say that you want to display the following line at the terminal lt lt lt echo x gt gt gt displays the value of x which is x The intention
331. r at cursor to c a Enter input mode and enter text after the current character 1 Enter input mode and insert text before the current character k Get previous command from history j Get next command from history string Search history for the most recent command containing string if string is null the previous string will be used Fe previous If wext u Next The Line Edit Mode After turning on the emacs line editor you probably won t even notice anything different because you can type in and execute commands the same way as before set o emacs echo hello hello pwd users pat To use the line editor you enter emacs commands emacs commands are either control characters that is characters typed in by holding down the Ctrl key and pressing another character or they are characters preceded by the ESCAPE or Esc key You may enter emacs commands anytime you want there are no separate modes like the vi line editor Note that emacs commands are not followed by an Enter We cover only a few of them here for a complete list of commands refer to the documentation for Bash or the Korn shell First let s look at moving the cursor around The Ctri b command moves the cursor to the left and the Ctrl f command moves it to the right Try this out by pressing Ctri b and Ctrl f a few times The cursor should move around on the line If you try to move the cursor past the left or right side of the line the shel
332. r of arguments passed is 3 a b args a b c Number of arguments passed is 2 In the second case even though a b was passed as a single argument to args the in the for command was replaced by the shell with a b c which is three words Thus the loop was executed three times Whereas the shell replaces the value of with 1 2 if you instead use the special shell variable it will be replaced with 1 2 The double quotes are necessary around because without them this variable behaves just like Go back to the args program and replace the with cat args echo Number of arguments passed is for arg in do echo arg done Now try it args abc Number of arguments passed is 3 a b c args a b c Number of arguments passed is 2 ab args Try it with no arguments Number of arguments passed is 0 In the last case no arguments were passed to the program So the variable was replaced by nothing The net result is that the body of the loop was not executed at all The for Without the List A special notation is recognized by the shell when writing for commands If you write for var do command command done note the absence of the in the shell automatically sequences through all the arguments typed on the command line just as if you had written for var in do command command done Here s the third and last version o
333. r the value of nosuch A variable that contains no value is said to contain the null value It is the default case for variables that you never store values in When the shell performs its variable substitution any values that are null are completely removed from the command line without a trace wc nosuch l nosuch nosuch names 7 names The shell scans the command line substituting the null value for the variable nosuch After the scan is completed the line effectively looks like this wc l names which explains why it works Sometimes you may want to explicitly set a variable null in a program This can be done by simply assigning no value to the variable as in dataflag Alternatively you can list two adjacent pairs of quotes after the So dataflag and dataflag both have the same effect of assigning the null value to dataflag Be advised that the assignment dataflag is not equivalent to the three previous ones because it assigns a single space character to dataf lag that s different from assigning no characters to it Filename Substitution and Variables Here s a puzzle for you If you type will the shell store the character into the variable x or will it store the names of all the files in your current directory into the variable x Let s try it out and see Us What files do we have addresses intro lotsaspaces names nu numbers phonebook stat x echo x addr
334. r users in the same group as the file s owner the next three characters and finally to all other users on the system the last three characters They tell whether the user can read from the file write to the file or execute the contents of the file The ls l command lists the link count see Linking Files The In Command later in this chapter the owner of the file the group owner of the file how large the file is that is how many characters are contained in it and when the file was last modified The information displayed last on the line is the filename itself ls l programs total 4 rWXr Xr x 1 steve DP3725 358 Jun 25 13 31 collect rWXr xXr x 1 steve DP3725 1219 Jun 25 13 31 mon rWXr Xr x 1 steve DP3725 89 Jun 25 13 30 wb The dash in the first column of each line indicates that the three files collect mon and wb are ordinary files and not directories Creating a Directory The mkdir Command To create a directory the mkdir command must be used The argument to this command is simply the name of the directory you want to make For example assume that you are still working with the directory structure depicted in Figure 2 7 and that you want to create a new directory called misc on the same level as the directories documents and programs If you were currently in your home directory typing the command mkdir misc would achieve the desired effect mkdir misc Now if you execute an ls command yo
335. r value construct parameter value construct parameter colon value construct 2nd 3rd parameter colon value construct 2nd 3rd parameter colon value construct 2nd 3rd parameter colon value construct 2nd 3rd parameter construct 2nd 3rd pattern matching contructs 2nd 3rd 4th parameters 2nd 3rd See also arguments variables 2nd See also variables keyword parameters assigning values to 2nd parameter substitution 2nd 3rd 4th parameter construct variable construct 2nd parameter pattern construct parameter pattern construct parameter pattern construct parameter value construct parameter value construct parameter value construct parameter value construct parameter colon value construct 2nd 3rd parameter colon value construct 2nd 3rd parameter colon value construct 2nd 3rd parameter colon value construct 2nd 3rd parameter construct 2nd 3rd pattern matching contructs 2nd 3rd 4th positional parameters 2nd reassigning values to 2nd 3rd parent directories parent process ids PPIDs parent processes parentheses 2nd passing variables to subshells 2nd 3rd 4th passing arguments 2nd 3rd 4th variable 2nd variable 2nd n variable 2nd phonebook file example 2nd 3rd 4th 5th 6th 7th 8th 9th 10th
336. ra Swingle 212 555 2298 Liz Stachiw 201 555 7776 Susan Goldberg 973 555 1295 Tony Iannino The names and the phone numbers are separated from each other in the phonebook file by a single tab character The regular expression VG Ma says to match all the characters up to the first tab that s the character between the and the and assign them to register 1 and to match all the characters that follow the tab character and assign them to register 2 The replacement string 2 4 specifies the contents of register 2 followed by a space followed by the contents of register 1 So when ed applies the substitute command to the first line of the file Alice Chebba 973 555 2015 it matches everything up to the tab Alice Chebba and stores it into register 1 and everything after the tab 973 555 2015 and stores it into register 2 Then it substitutes the characters that were matched the entire line with the contents of register 2 973 555 2015 followed by a space followed by the contents of register 1 Alice Chebba 973 555 2015 Alice Chebba As you can see regular expressions are powerful tools that enable you to match complex patterns Table 4 1 summarizes the special characters recognized in regular expressions Table 4 1 Regular Expression Characters Notation Meaning any character beginning of line end of line zero or more occurrences of previous regular expression Example wood x IN
337. racters caret 2nd 3rd command vi underscore 2nd backquote 2nd 3rd 4th semicolon construct 2nd 3rd 4th 5th 6th construct 2nd 3rd 4th braces pipe symbol 2nd 3rd construct 2nd 3rd 4th pattern matching pipe construct 2nd 3rd 4th 5th 6th 7th 8th 9th tilde tilde substitution 2nd 3rd 4th 5th command vi 0 command vi 2nd 3rd O exit status 0 signal 0 signal number 1 signal 1 signal number 14 signal 15 signal 15 signal number 2 signal 2 signal number 3 signal 6 signal 9 signal 4 rrevious SYMBOL A B C D E F G H 1 J K L M N O P Q R S T U V W X Y Z a escape character a command vi 2nd 3rd A command vi a option set command ABRT signal access modes accessing command history 2nd fc command 2nd history command 2nd r command 2nd 3rd active jobs printing list of 2nd add program 2nd 3rd 4th 5th addi program 2nd Advanced Linux Programming Advanced Perl Programming alias command 2nd 3rd 4th 5th 6th 7th aliases defining 2nd 3rd 4th 5th removing allexport shell mode ALRM signal ampersand amp amp construct 2nd 3rd 4th ampersand amp AND operators logical AND operator a 2nd apostrophe 2nd 3rd 4th 5th 6th 7th
338. rguments to set you ll get an alphabetized list of all the variables that exist in your environment be they local or exported set Show me all variables CDPATH users steve usr spool EDITOR bin vi HOME users steve IFS LOGNAME steve MAIL usr spool mail steve MATLCHECK 600 PATH bin usr bin users steve bin PHONEBOOK users steve phonebook PS1 PS2 gt PwWD users steve misc SHELL usr bin sh TERM xterm TMOUT 0 TZ EST5EDT cmd wc x Using set to Reassign Positional Parameters There is no way to directly assign a value to a positional parameter for example 1 100 does not work These parameters are initially set on execution of the shell program The only way they may be changed is with the shift or the set commands If words are given as arguments to set on the command line those words will be assigned to the positional parameters 1 2 and so forth The previous values stored in the positional parameters will be lost forever So set abc assigns a to 1 b to 2 and c to 3 also gets set to 3 set one two three four echo 1 2 3 4 one two three four echo This should be 4 4 echo What does this reference now one two three four for arg do echo arg done one two three four So after execution of the set everything seems to work consistently and the for loop without a list set is often used in this fashion to parse data read fr
339. rmat case value in pat command command command pat gt command command command pat command command command esac The word value is successively compared against pat patz pat until a match is found The commands that appear immediately after the matching pattern are then executed until a double semicolon is encountered At that point execution of the case is terminated If no pattern matches value none of the commands inside the case are executed The pattern matches anything and is often used as the last pattern in a case as the catchall case The shell metacharacters match zero or more characters match any single character and match any single character enclosed between the brackets can be used in patterns The character can be used to specify a logical ORing of two patterns as in pat pat which means to match either pat or patz Examples case 1 in L Lopt TRUE w wopt TRUE C copt TRUE echo Unknown option esac case choice in 1 9 valid TRUE echo Please choose a number from 1 9 esac The cd Command General Format cd directory Execution of this command causes the shell to make directory the current directory If directory is omitted the shell makes the directory specified in the HOME variable the current directory If the shell variable CDPATH is null directory must be a full directory path for examp
340. ro length the echo command that displays the message I couldn t find name is executed otherwise the echo command that displays the value of line is executed A final format of the if command is useful when more than a two way decision has to be made Its general format is if command then command command elif command then command command elif command then command command else command command fi command command Command are evaluated in order until one of the commands returns an exit status of zero at which point the commands that immediately follow the then up to another elif else or fi are executed If none of the commands returns an exit status of zero the commands listed after the else if present are executed Example if choice a then add elif choice d then delete elif choice 1 then list else echo Bad choice error TRUE fi The jobs Command General Format jobs The list of active jobs is printed If the l option is specified detailed information about each job including its process id is listed as well If the p option is specified only process ids are listed If an optional job_id is supplied to the jobs command just information about that job is listed Example sleep 100 amp 1 1104 jobs 1 Running Sleep 100 amp The kitt Command General Format kill signal job The kill command sends the signal signal
341. rogram that you saw earlier in this chapter can be rewritten to use a case statement rather than the if elif Here is such a version of the program This time we took advantage of the fact that date with the H option writes a two digit hour to standard output cat greetings Program to print a greeting case version hour date H case hour in 1 01 echo Good morning 1 2 7 echo Good afternoon echo Good evening esac The two digit hour obtained from date is assigned to the shell variable hour Then the case statement is executed The value of hour is compared against the first pattern 0 1 01 which matches any value that starts with a zero followed by any character midnight through 9 00 a m or any value that starts with a one and is followed by a zero or one 10 00 or 11 00 a m The second pattern 1 2 7 matches a value that starts with a one and is followed by any one of the digits two through seven noon through 5 00 p m The last case the catchall matches anything else 6 00 p m through 11 00 p m date Wed Aug 28 15 45 12 EDT 2002 greetings Good afternoon rs i a fo The Null Command This seems about as good a time as any to talk about the shell s built in null command The format of this command is simply and the purpose of it is you guessed it to do nothing So what good is it Well in most cases it s used to satisfy the r
342. rrect number of arguments Usage rem name rem Susan More than one match please qualify further rem Susan Topple rem Susan Topple I couldn t find Susan Topple in the phone book She s history Now you have a fairly robust rem program It checks for the correct number of arguments printing the proper usage if the correct number isn t supplied it also checks to make sure that precisely one entry is removed from the phonebook file ns Ga rs i The case Command The case command allows you to compare a single value against other values and to execute one or more commands when a match is found The general format of this command is case value in pat command command command paty command command command pat command command command esac The word value is successively compared against the values pat patz pat until a match is found When a match is found the commands listed after the matching value up to the double semicolons are executed After the double semicolons are reached execution of the case is terminated If a match is not found none of the commands listed in the case is executed As an example of the use of the case the following program called number takes a single digit and translates it to its English equivalent cat number Translate a digit to English if ne 1 then echo Usage number digit exit 1 fi case 1 in 0 echo
343. rs ca Exercises 1 Modify rolo so that upper and lowercase letters are not distinguished when doing a lookup in the phone book 2 Adda m command line option to rolo to send mail to the person who follows on the command line Have rolo look up the person in the phone book and then look for the string mail mailaddr in the matching entry where mailaddr is the person s mail address Then start up an editor as in change mode to allow the user to enter the mail message When the editing is complete mail the message to the user If no mail address is found in the phone book prompt for it Also add a mail option to the menu so that it can be selected interactively Prompt for the name of the person to send mail to 3 After adding the m option add a f option to specify that the mail message is to be taken from the file that follows on the command line So rolo m tony f memo should look up tony and mail him the contents of the file memo 4 Can you think of other ways to use rolo For example can it be used as a small general purpose database program for example for storing recipes or employee data 5 Modify rolo to use the following convention instead of the exported PHONEBOOK variable the file rolo in each rolo user s home directory contains the pathname to that user s phone book file for example cat HOME rolo users steve misc phonebook Then add an option to rolo to allow you to look up someone in another
344. rs todd progl shortcut sweeney How many arguments do you think were passed to the echo program one or four Because we said that the shell is the one that performs the filename substitution the answer is four When the shell analyzes the line echo it recognizes the special character and substitutes on the command line the names of all files in the current directory it even alphabetizes them for you echo mrs todd progl shortcut sweeney Then the shell determines the arguments to be passed to the command So echo never sees the asterisk As far as it s concerned four arguments were typed on the command line see Figure 3 11 Figure 3 11 Execution of echo echo arguments shortcut 1 0 Redirection It is the shell s responsibility to take care of input and output redirection on the command line It scans the command line for the occurrence of the special redirection characters lt gt or gt gt also lt lt as you ll learn in Chapter 13 Loose Ends When you type the command echo Remember to tape Law and Order gt reminder the shell recognizes the special output redirection character gt and takes the next word on the command line as the name of the file that the output is to be redirected to In this case the file is reminder If reminder already exists and you have write access to it the previous contents are lost if you don t have write access to it the shell gives you an error message
345. rst and second argument tr s lt lotsaspaces This is an example of a file that contains a lot of blank spaces The options to tr in effect say translate space characters to space characters replacing multiple spaces in the output by a single space The d Option tr can also be used to delete single characters from a stream of input The general format of tr in this case is tr d from chars where any single character listed in from chars will be deleted from standard input In the following example tr is used to delete all spaces from the file intro tr d lt intro TheUNIXoperatingSystemwaspioneeredbyKen ThompsonandDennisRitchieatBellLaboratories inthelate1960s 0neoftheprimarygoalsin thedesignoftheUNIXSystemwastocreatean environmentthatpromotedefficientprogram development Of course you probably realize that you could have also used sed to achieve the same results sed s g intro TheUNIXoperatingsystemwaspioneeredbyKen ThompsonandDennisRitchieatBellLaboratories inthelate1960s O0neoftheprimarygoalsin thedesignoftheUNIXsystemwastocreatean environmentthatpromotedefficientprogram development This is not atypical for the Unix system there s almost always more than one approach to solving a particular problem In the case we just saw either approach is satisfactory that is tr or sed however tr is probably a better choice in this case because it is a much smaller program and like
346. rted when you logged in and it has used 21 seconds of computer time Until the command is finished it shows up in the output of the ps command as a running process Process number 1353 in the preceding example is the ps command that was typed in and 1258 is the sort from the preceding example When used with the f option ps prints out more information about your processes including the parent process id PPID the time the processes started STIME and the command arguments ps f UID PID PPID C STIME TTY TIME COMMAND steve 195 1 0 10 58 29 tty01 0 21 sh steve 1360 195 43 14 54 48 tty0l 0 01 ps f steve 1258 195 14 45 04 tty01 3 17 sort data rs ca Command Summary Table 2 2 summarizes the commands reviewed in this chapter In this table file refers to a file file s to one or more files dir to a directory and dir s to one or more directories Table 2 2 Command Summary Command Description cat file s Display contents of file s or standard input if not supplied cd dir Change working directory to dir cp file file Copy file to file cp file s dir Copy file s into dir date Display the date and time echo args Display args ln file file Link file to file ln file s dir Link file s into dir ls file s List file s ls dir s List files in dir s or in current directory if dir s is not specified mkdir dir s Create directory dir s mv file file Move file to file simply rename it if both
347. ry IN THIS APPENDIX Startup Commands Comments Parameters and Variables Command Re entry Quoting Filename Substitution I O Redirection Exported Variables and Subshell Execution Functions Job Control Command Summary This appendix summarizes the main features of the standard POSIX shell as per IEEE Std 1003 1 2001 a fo Startup The shell can be given the same options on the command line as can be specified with the set command In addition the following options can be specified c commands commands are executed i The shell is interactive Signals 2 3 and 15 are ignored S Commands are read from standard input Fe previous wet ns ca Commands The general format of a command typed to the shell is command arguments where command is the name of the program to be executed and arguments are its arguments The command name and the arguments are delimited by whitespace characters normally the space tab and newline characters changing the variable IFS affects this Multiple commands can be typed on the same line if they re separated by semicolons Every command that gets executed returns a number known as the exit status zero is used to indicate success and nonzero indicates a failure The pipe symbol can be used to connect the standard output from one command to the standard input of another as in who wc l The exit status is that of the last command in the pipeline
348. ry file If the size of the file is zero no match was found and an appropriate message is issued Otherwise for each matching entry the program displays the entry and asks the user whether that entry is to be removed This provides reassurance to the user that the entry the user intends to remove is the same one that the program intends to remove even in the single match case After a y has been typed to the program a break command is executed to exit from the loop Outside the loop the program tests the value of answer to determine how the loop was exited If its value is not equal to y then the user doesn t want to remove an entry after all for whatever reason Otherwise the program proceeds with the removal by greping out all lines but the desired one and here the pattern specified to grep is made to match only entire lines by anchoring it to the start and end of the line change Change an entry in the phone book name 1 Get matching entries and save in temp file grep name PHONEBOOK gt tmp matches if s tmp matches then echo I can t find name in the phone book exit 1 fi Display matching entries one at a time and confirm change while read line do display line echo Change this entry y n c read answer lt dev tty if answer y then break fi done lt tmp matches rm tmp matches if answer y then exit fi Start
349. ry line who wc l count the number of users Test to see if the correct arguments were supplied Comments are useful for documenting commands or sequences of commands whose purposes may not be obvious or are sufficiently complex so that if you were to look at the program again in a week you might forget why they re there or what they do J udicious use of comments can help make shell programs easier to debug and to maintain both by you and by someone else who may have to support your programs Let s go back to the stats program and insert some comments cat stats stats prints date number of users logged on and current working directory echo The current date and time is date echo echo The number of users on the system is who wc l echo echo Your current working directory is pwd The extra blank lines cost little in terms of program space yet add much in terms of program readability They re simply ignored by the shell fe erevious Next rs ca Variables Like virtually all programming languages the shell allows you to store values into variables A shell variable begins with an alphabetic or underscore _ character and is followed by zero or more alphanumeric or underscore characters To store a value inside a shell variable you simply write the name of the variable followed immediately by the equals sign followed immediately by the value you want to store in the variab
350. s cat vars BOOK users steve book UUPUB usr spool uucppublic DOCS users steve docs memos DB usr2 data You know that if you execute vars the values assigned to these variables will not be accessible by you after this program has finished executing because vars will be run in a subshell vars echo BOOK The Command Luckily there is a shell built in command called pronounced dot whose general format is file and whose purpose is to execute the contents of file in the current shell That is commands from file are executed by the current shell just as if they were typed at that point A subshell is not spawned to execute the program The shell uses your PATH variable to find file just like it does when executing other programs vars Execute vars in the current shell echo BOOK users steve book Hoorah Because a subshell isn t spawned to execute the program any variable that gets assigned a value stays even after execution of the program is completed It follows then that if you have a program called db that has the following commands in it cat db DATA usr2 data RPTS DATA rpts BIN DATA bin cd DATA executing db with the dot command defines the three variables DATA RPTS and BIN in the current shell and then changes you to the DATA directory pwd usr2 data This last example brings up an interesting point of discussion If you re one of those Unix users
351. s sed prints just those lines from standard input that contain those characters The following example shows how sed can be used to display just the lines that contain a particular string sed n UNIX p intro Just print lines containing UNIX The UNIX operating system was pioneered by Ken the design of the UNIX system was to create an Deleting Lines To delete entire lines of text use the d command By specifying a line number or range of numbers you can delete specific lines from the input In the following example sed is used to delete the first two lines of text from intro sed 1 2d intro Delete lines 1 and 2 in the late 1960s One of the primary goals in the design of the UNIX system was to create an environment that promoted efficient program development Remembering that by default sed writes all lines of the input to standard output the remaining lines in text that is lines 3 through the end simply get written to standard output By preceding the d command with a string of text you can use sed to delete all lines that contain that text In the following example sed is used to delete all lines of text containing the word UNIX sed UNIX d intro Delete all lines containing UNIX Thompson and Dennis Ritchie at Bell Laboratories in the late 1960s One of the primary goals in environment that promoted efficient program development The power and flexibility of sed goes far beyond what we ve show
352. s and Subshell Execution Functions Job Control Command Summary Appendix B For More Information Online Documentation Documentation on the Web Books Inde Fe Previous Next gt x Previous MMiNext Copyright Copyright 2003 by Sams Publishing All rights reserved No part of this book shall be reproduced stored in a retrieval system or transmitted by any means electronic mechanical photocopying recording or otherwise without written permission from the publisher No patent liability is assumed with respect to the use of the information contained herein Although every precaution has been taken in the preparation of this book the publisher and author assume no responsibility for errors or omissions Nor is any liability assumed for damages resulting from the use of the information contained herein Library of Congress Catalog Card Number 2002115932 Printed in the United States of America First Printing March 2003 06 05 04034321 Trademarks All terms mentioned in this book that are known to be trademarks or service marks have been appropriately capitalized Sams Publishing cannot attest to the accuracy of this information Use of a term in this book should not be regarded as affecting the validity of any trademark or service mark Warning and Disclaimer Every effort has been made to make this book as complete and as accurate as possible but no warranty or fitness is implied The
353. s associated with standard input 1 with standard output and 2 with standard error Fe previous Next rs ca Exported Variables and Subshell Execution Commands other than the shell s built in commands are normally executed in a new shell called a subshell Subshells cannot change the values of variables in the parent shell and they can only access variables from the parent shell that were exported to them either implicitly or explicitly by the parent If the subshell changes the value of one of these variables and wants to have its own subshells know about it it must explicitly export the variable before executing the subshell When the subshell finishes execution any variables that it may have set are inaccessible by the parent The Construct If one or more commands are placed inside parentheses those commands will be executed in a subshell The Construct If one or more commands are placed inside curly braces those commands will be executed by the current shell With this construct and the construct O can be redirected and piped into and out of the set of enclosed commands and the set can be sent to the background for execution by placing an amp at the end For example progl prog2 prog3 2 gt errors amp submits the three listed programs to the background for execution with standard error from all three programs redirected to the file errors More on Shell Variables A shell variable can
354. s because that s what we want to display The first one is easy because it can be directly referenced as 1 To get the last one you use eval as you Saw in Chapter 13 Loose Ends The command eval echo has the effect of displaying the value of the last positional parameter The command eval printf 40 40s s n I gets evaluated to printf 40 40s s n Steve s Ice Cream 4 using the entry shown previously as the example and then the shell rescans the line to substitute the value of 4 before executing printf Fe previous wext i fo Sample Output Now it s time to see how rolo works We ll start with an empty phone book and add a few entries to it Then we ll list all the entries look up a particular one and change one using the default editor ed remember that the variable EDITOR can always be set to a different editor and then exported To conserve space we ll show only the full menu that rolo displays the first time PHONEBOOK users steve misc book export PHONEBOOK rolo Start it up users steve misc book does not exist Should I create it for you y n y Would you like to 1 Look someone up 2 Add someone to the phone book 3 Remove someone from the phone book 4 Change an entry in the phone book 5 List all names and numbers in the phone book 6 Exit this program Please select one of the above 1 6 2 Type in your new entry When you re done
355. s directory to your PATH PATH bin usr bin users steve bin Now any program in users steve bin can be executed by you from anywhere pwd Where am I users steve rolo Liz grep can t open phonebook This time the shell finds rolo and executes it but grep can t find the phonebook file Look back at the rolo program and you ll see that the grep error message must be coming from lu Take another look at lu cat users steve bin Lu Look someone up in the phone book version 3 if ne 1 then echo Incorrect number of arguments echo Usage lu name exit 1 Ler grep name phonebook grep is trying to open the phonebook file in the current directory which is users steve that s where the program is being executed from the current directory has no relation to the directory in which the program itself resides The PATH only specifies the directories to be searched for programs to be executed and not for any other types of files So phonebook must be precisely located for Lu There are several ways to fix this problem a problem which by the way exists with the rem and add programs as well One approach is to have the lu program change directory to users steve bin before it does the grep That way grep finds phonebook because it exists in the current directory cd users steve bin grep 1 phonebook This approach is a good one to take when you re doing a lot of work wi
356. s fixed a fact that you can use to your advantage when writing greetings because this means that the time will always appear in character positions 12 through 19 Actually for this program you really only need the hour displayed in positions 12 and 13 So to get the hour from date you can write date cut c12 13 10 Now the task of writing the greetings program is straightforward cat greetings Program to print a greeting hour date cut c12 13 if hour ge O a hour le 11 then echo Good morning else if hour ge 12 a hour le 17 then echo Good afternoon else echo Good evening fi Fa If hour is greater than or equal to 0 midnight and less than or equal to 11 up to 11 59 59 Good morning is displayed If hour is greater than or equal to 12 noon and less than or equal to 17 up to 5 59 59 p m Good afternoon is displayed If neither of the preceding two conditions is satisfied Good evening is displayed greetings Good morning As noted the nested if command sequence used in greetings is so common that a special elif construct is available to more easily express this sequence The general format of this construct is if command then command command elif command then command command elif command then command command else command command fi command command command are executed in turn and their exit statuses tested
357. s the same general format as cp and mv meaning that you can link a bunch of files at once into a directory using the format ln files directory Removing a Directory The rmdir Command You can remove a directory with the rmdir command The stipulation involved in removing a directory is that no files be contained in the directory If there are files in the directory when rmdir is executed you will not be allowed to remove the directory To remove the directory misc that you created earlier the following could be used rmdir users steve misc Once again the preceding command works only if no files are contained in the misc directory otherwise the following happens rmdir users steve misc rmdir users steve misc not empty If this happens and you still want to remove the misc directory you would first have to remove all the files contained in that directory before reissuing the rmdir command As an alternate method for removing a directory and the files contained in it you can use the r option to the rm command The format is simple rm r dir where dir is the name of the directory that you want to remove rm removes the indicated directory and all files including directories in it rs i ns on Filename Substitution The Asterisk One powerful feature of the Unix system that is actually handled by the shell is filename substitution Let s say that your current directory has these files in it ls
358. s to be informed by mail exit 1 fi user 1 Check every minute for user logging on until who grep user gt dev null do sleep 60 done When we reach this point the user has logged on if mailopt FALSE then echo user has logged on else echo user has logged on mail steve fa The first test checks to see whether the m option was supplied If it was the characters TRUE are assigned to the variable mailopt and shift is executed to shift out the first argument moving the name of the user to be monitored to 1 and decrementing If the m option wasn t specified as the first argument the characters FALSE are assigned to mailopt Execution then proceeds as in the previous version However this time when the loop is exited a test is made to see whether the m option was selected If it wasn t the message is written to standard output otherwise it s mailed to steve mon sandy m Usage mon m user m means to be informed by mail mon m sandy amp 1 5435 vi newmemo Work continues you have mail mail From steve Wed Aug 28 17 44 46 EDT 2002 sandy has logged on d Of course we could have written mon to accept the m option as either the first or second argument but that goes against the recommended command syntax standard which specifies that all options should 2 precede any other types of arguments on the command line 2 The comma
359. s to remove all aliases The unset Command General Format unset names This causes the shell to erase definitions of the variables or functions listed in names Read only variables cannot be unset The v option to unset specifies that a variable name follows whereas the f option specifies a function name If neither option is used it is assumed that variable name s follow Example unset dblist files Remove definitions of variables dblist and files The until Command General Format until command do command command done command is executed and its exit status tested If it is nonzero the commands enclosed between the do and done are executed Then command is executed again and its status tested If it is nonzero the commands between the do and done are once again executed Execution of command and subsequent execution of the commands between the do and done continues until command returns a zero exit status at which point the loop is terminated Execution then continues with the command that follows the done Note that because command gets evaluated immediately on entry into the loop the commands between the do and done may never be executed if it returns a zero exit status the first time Example sleep for 60 seconds until jack logs on until who grep jack gt dev null do sleep 60 done echo jack has logged on The preceding loop continues until the grep returns a zero exit status that is f
360. shell and give tr a chance to see it The octal representation of a character can be given to tr in the format nnn where nnn is the octal value of the character For example the octal value of the tab character is 11 If you are going to use this format be sure to enclose the character in quotes The tr command tro NLI translates all colons to tabs just as in the preceding example Table 4 3 lists characters that you ll often want to specify in octal format Table 4 3 Octal Values of Some ASCII Characters Character Octal Value Bell 7 Backspace 10 Tab 11 Newline 12 Linefeed 12 Formfeed 14 Carriage Return 15 Escape 33 In the following example tr takes the output from date and translates all spaces into newline characters The net result is that each field of output from date appears on a different line date tr 12 Translate spaces to newlines Sun Jul 28 19 13 46 EDT 2002 tr can also take ranges of characters to translate For example the following shows how to translate all lowercase letters in intro to their uppercase equivalents tr a z A Z lt intro THE UNIX OPERATING SYSTEM WAS PIONEERED BY KEN THOMPSON AND DENNIS RITCHIE AT BELL LABORATORIES IN THE LATE 1960S ONE OF THE PRIMARY GOALS IN THE DESIGN OF THE UNIX SYSTEM WAS TO CREATE AN ENVIRONMENT THAT PROMOTED EFFICIENT PROGRAM DEVELOPMENT The character ranges a z and A Z are enclosed in quotes to keep the
361. space Now you have one file rolosubs that contains the source for the three programs lu add and rem which can be sent to someone else using mail mail tony aisystems com lt rolosubs Mail the archive mail tony aisystems com Mail tony a message Tony I mailed you a shell archive containing the programs lu add and rem rolo itself will be sent along shortly Pat Ctrl d When tony receives the file in his mail he can extract the three programs simply by running the shell on the file after having first removed some header lines that mail sticks at the beginning of the file sh rolosubs Extracting lu Extracting add Extracting rem ls lu add rem add lu rem The shar program used to create the rolosubs archive file is simple cat shar Program to create a shell archive from a set of files echo echo To restore type sh archive echo for file do echo echo echo Extracting file echo cat gt file lt lt THE END OF DATA cat file echo THE END OF DATA done Refer to the contents of the rolosubs file when studying the operation of this shar program Remember shar actually creates a shell program More sophisticated archiving programs allow entire directories to be archived and also check to make sure that no data is lost in the transmission see Exercises 2 and 3 at the end of this chapter The Unix sum command can be used to generate a checksum for a program This che
362. suffix testitest test should result in testltest being renamed to test1 Hint Use sed and command substitution rs i a Chapter 8 Decisions Decisions IN THIS CHAPTER Exit Status The test Command The else Construct The exit Command The elif Construct The case Command The Null Command The amp amp and Constructs Exercises This chapter introduces a statement that is present in almost all programming languages if It enables you to test a condition and then change the flow of program execution based on the result of the test The general format of the if command is if command then command command Ti where command is executed and its exit status is tested If the exit status is zero the commands that follow between the then and the fi are executed otherwise they are skipped Fe previous NEXT u Next Exit Status Whenever any program completes execution under the Unix system it returns an exit status back to the system This status is a number that usually indicates whether the program successfully ran By convention an exit status of zero indicates that a program succeeded and nonzero indicates that it failed Failures can be caused by invalid arguments passed to the program or by an error condition detected by the program For example the cp command returns a nonzero exit status if the copy fails for some reason for example if it can t create the destination file or if
363. t so it s a good idea to get into the habit of enclosing it in quotes because it often contains whitespace Characters in the format string that are not preceded by a percent sign are written to standard output One or more characters preceded by a percent sign are called conversion specifications and tell printf how the corresponding argument should be displayed So for each percent sign in the format string there should be a corresponding argument except for the special conversion specification which causes a single percent sign to be displayed Here s a simple example of printf printf This is a number d n 10 This is a number 10 printf doesn t add a newline character to its output like echo however printf understands the same escape characters that echo does refer to Table 10 1 earlier in this chapter so adding n to the end of the format string causes the prompt to appear on the next line Although this is a simple case that could easily be handled by echo it helps to illustrate how the conversion specification d is interpreted by printf When the format string is scanned by printf it outputs each character in the string without modification until it sees the percent sign then it reads the d and recognizes that the d should be replaced by the next argument which must be an integer number After that argument 10 is sent to standard output printf sees the n and outputs a newline Table 10 2 summarizes the d
364. t 2nd 3rd parameter colon value construct 2nd 3rd parameter colon value construct 2nd 3rd parameter colon value construct 2nd 3rd parameter construct 2nd 3rd variable construct italicize variable 2nd 0 variable 2nd 3rd array construct percent sign 2nd amp ampersand amp amp construct 2nd 3rd 4th 5th 6th 7th 8th 9th lt redirection operator lt amp redirection operator lt amp characters lt amp redirection operator lt lt redirection characters 2nd 3rd 4th lt lt redirection operator lt left angle bracket redirection operator single quote 2nd 3rd 4th 5th 6th 7th construct parentheses 2nd construct 2nd 3rd 4th 5th backslash 2nd 3rd backslash inside double quotes 2nd 3rd 4th line continuation character 2nd double quotes 2nd 3rd 4th 5th semicolon semicolon command vi asterisk 2nd 3rd 4th 5th 6th filename substitution 2nd 3rd 4th 5th command vi plus sign command vi hyphen option cd command 2nd option set command 2nd 3rd 4th option set command a logical AND operator 2nd o logical OR operator 2nd period 2nd 3rd command 2nd 3rd 4th 5th 6th 7th command 2nd 3rd 4th 5th pathname pathname construct 2nd 3rd 4th precision modifier
365. t concludes our introduction to writing commands and using variables The next chapter goes into detail on the quoting mechanisms in the shell Fe previous NEXT a Exercises 1 Which of the following are valid variable names XXXXXX E 12345 HOMEDIR file name _ date file_name x0 9 filel Slimit 2 Suppose that your HOME directory is users steve and that you have subdirectories as shown in the following figure users pal gt steve ruth documents programs memos proposals letters wb collect mon plan dact sysA new hire no JSK AMG reply Assuming that you just logged in to the system and executed the following commands docs users steve documents let docs letters prop docs proposals write the commands in terms of these variables to a List the contents of the documents directory b Copy all files from the letters directory to the proposals directory c Move all files whose names contain a capital letter from the letters directory to the current directory d Count the number of files in the memos directory What would be the effect of the following commands a ls let b cat prop sys A gt gt let no JSK c echo let d cp let no JSK progs e cd prop 3 Write a program called nf to display the number of files in your current directory Type in the program and test it out 4 Write a program called whos to display a sorted list of the logged in users Just display the usern
366. t got assigned Wed Aug 28 14 47 26 EDT 2002 When you write now date the shell realizes that the entire output from date is to be assigned to now Therefore you don t need to enclose date inside double quotes Even commands that produce more than a single line of output can be stored inside a variable filelist 1ls echo filelist addresses intro lotsaspaces names nu numbers phonebook stat What happened here You end up with a horizontal listing of the files even though the newlines from Ls were stored inside the filelist variable take our word for it The newlines got eaten up when the value of filelist was substituted by the shell in processing the echo command line Double quotes around the variable will preserve the newlines echo filelist addresses intro Llotsaspaces names nu numbers phonebook stat To store the contents of a file into a variable you can use cat namelist cat names echo names Charlie Emanuel Fred Lucy Ralph Tony Tony If you want to mail the contents of the file memo to all the people listed in the names file who we ll assume here are users on your system you can do the following mail cat names lt memo Here the shell executes the cat and inserts the output on the command line so it looks like this mail Charlie Emanuel Fred Lucy Ralph Tony Tony lt memo Then it executes mail redirecting its standard input fr
367. t just the usernames from the output of who You already know how to do that with the cut command who cut c1 8 root ruth steve pat Alternatively you can use sed to delete all the characters from the first blank space that marks the end of the username through the end of the line by using a regular expression in the edit command who sed s root ruth steve pat The sed command says to substitute a blank space followed by any characters up to the end of the line with nothing that is delete the characters from the first blank to the end of the line from each line of the input The n Option We pointed out that sed always writes each line of input to standard output whether or not it gets changed Sometimes however you ll want to use sed just to extract some lines from a file For such purposes use the n option This option tells sed that you don t want it to print any lines unless explicitly told to do so This is done with the p command By specifying a line number or range of line numbers you can use sed to selectively print lines of text So for example to print just the first two lines from a file the following could be used sed n 1 2p intro Just print the first 2 lines The UNIX operating system was pioneered by Ken Thompson and Dennis Ritchie at Bell Laboratories If instead of line numbers you precede the p command with a string of characters enclosed in slashe
368. t relates to writing and running programs ns ca Local Variables Type the following program called vartest into your computer cat vartest echo x vartest consists of a solitary echo command that displays the value of the variable x surrounded by colons Now assign any value you want to the variable x from your terminal x 100 Here we chose 100 Question What do you think will be displayed when vartest is now executed Answer vartest vartest doesn t know about the value of x Therefore its value is null The variable x that was assigned the value 100 in the login shell is known as a local variable The reason why it has this name will become clear shortly Here s another example This program is called vartest2 cat vartest2 x 50 echo x x 100 vartest2 Execute it 50 Now the question is What s the value of x echo x 100 So you see that vartest2 didn t change the value of x that you set equal to 100 in your login shell Subshells The behavior exhibited by vartest and vartest2 is due to the fact that these two programs are run as subshells by your login shell A subshell is for all intents and purposes an entirely new shell executed by your login shell to run the desired program So when you ask your login shell to execute vartest it starts up a new Shell to execute the program Whenever a new shell runs it runs in its own environment with its own set of local variables
369. t s what we ll use throughout the remainder of the book Integer Operators test has an assortment of operators for performing integer comparisons Table 8 2 summarizes these operators Operator int inti int int inti int eq ints ge int gt int le int lt int ne int Table 8 2 test Integer Operators Returns TRUE exit status of 0 if int is equal to intz int is greater than or equal to into int is greater than intz int is less than or equal to into int is less than intz int is not equal to int For example the operator eq tests to see whether two integers are equal So if you had a shell variable called count and you wanted to see whether its value was equal to zero you would write count eq O Other integer operators behave similarly so choice tests to see whether the variable choice is less than 5 the command index ne max tests to see whether the value of index is not equal to the value of max and finally ae ne 0 tests to see whether the number of arguments passed to the command is not equal to zero The test command interprets the value as an integer when an integer operator is used and not the shell so these comparisons work regardless of the shell variable s type Let s reinforce the difference between test s string and integer operators by taking a look at a few examples x1 005 x2 10 x1 5
370. t spaces at the start of each line in this case five spaces would be inserted Matching the End of the Line The Dollar Sign Just as the is used to match the beginning of the line so is the dollar sign used to match the end of the line So the regular expression contents matches the characters contents only if they are the last characters on the line What do you think would be matched by the regular expression Would this match a period character that ends a line No This matches any single character at the end of the line including a period recalling that the period matches any character So how do you match a period In general if you want to match any of the characters that have a special meaning in forming regular expressions you must precede the character by a backslash to remove that special meaning So the regular expression matches any line that ends in a period and the regular expression sie matches any line that starts with one good for searching for nroff commands in your text ed intro 248 INe 7 Search for a line that ends with a period development 1 s gt gt Add gt gt to the end of each line 1 p The Unix operating system was pioneered by Ken gt gt Thompson and Dennis Ritchie at Bell Laboratories gt gt in the late 1960s One of the primary goals in gt gt the design of the Unix system was to create an gt gt environment that promoted efficient program gt gt
371. t the command prompt it doesn t You should also set and export inside your profile file a variable called SHELL cat profile SHELL usr bin ksh export SHELL This variable is used by certain applications Such as vi to determine what shell to start up when you execute a shell escape In such cases you want to make sure that each time you start up a new shell you get the shell you want and not an older Bourne shell rs rs ca Command Line Editing Line edit mode is a feature of the shell that allows you to edit a command line using built in commands that mimic those found in two popular screen editors The POSIX standard shell provides the capability to mimic vi however both Bash and the Korn shell also support an emacs line edit mode We list the complete set of vi commands in Table A 4 in Appendix A Shell Summary If you ve used either of these screen editors you ll find that the built in line editors in the shell are faithful reproductions of their full screen counterparts If you ve never used a screen editor don t be intimidated This capability is one of the most useful features in the shell In fact after learning how to use one of the Shell s built in editors you ll be able to learn vi or emacs with little effort To turn on a line edit mode you use the set command with the o mode option where mode is either vi or emacs set o vi Turn on vi mode Note that you can put this in your profile or ENV
372. t up a new shell usr bin sh The HOME directory is set to usr2 data and then the variables BIN RPTS and DATA are defined relative to this HOME a good idea in case you ever have to move the directory structure somewhere else all you d have to change in the program is the variable HOME Next the PATH is modified to include the database bin directory and the CDPATH variable is set to search the current directory the HOME directory and the RPTS directory which presumably contains subdirectories After exporting these variables which as you recall must be done to put the values of these variables into the environment of subsequently spawned subshells the standard shell usr bin sh is started From that point on this new shell processes commands typed in from the terminal When Ctrl d is typed to this shell control returns to db which in turn returns control to your login shell db Run it DB echo HOME usr2 data DB cd rpts Try out CDPATH usr2 data rpts It works DB ps See what processes are running PID TTY TIME COMMAND 123 13 0 40 sh Your login shell 761 13 0 01 sh Subshell running db 765 13 0 01 sh New shell run from db 769 13 0 03 ps DB Ctrl d Done for now echo HOME users steve Back to normal The execution of db is depicted in Figure 11 7 where we ve shown only the exported variables of interest not necessarily all that exist in the environment Figure 11 7 Executing db
373. tation but are not required by the standard Examples y 22 33 z y y y 1 rs ca Filename Substitution After parameter substitution and command substitution is performed on the command line the shell looks for the special characters and If they re not quoted the shell searches the current directory or another directory if preceded by a and substitutes the names of all files that match these names are first alphabetized by the shell If no match is found the characters remain untouched Note that filenames beginning with a must be explicitly matched so echo won t display your hidden files echo will The filename substitution characters are summarized in Table A 6 Table A 6 Filename Substitution Characters Character s Meaning Matches any single character Matches zero or more characters chars Matches any single character in chars the format C C gt can be used to match any character in the range C through Co inclusive for example A Z matches any uppercase letter chars Matches any single character not in chars a range of characters may be specified previously fe erevious Next gt previous next I O Redirection When scanning the command line the shell looks for the special redirection characters lt and gt If found they are processed and removed with any associated arguments from the command line Table A 7 summarizes the different t
374. tead the format n must be used So to directly access argument 10 you must write 10 in your program rs fi rs ca The shift Command The shift command allows you to effectively left shift your positional parameters If you execute the command shift whatever was previously stored inside 2 will be assigned to 1 whatever was previously stored in 3 will be assigned to 2 and so on The old value of 1 will be irretrievably lost When this command is executed the number of arguments variable is also automatically decremented by one cat tshift Program to test the shift echo shift echo shift echo shift echo shift echo shift echo tshift abcde sab cde 4bcde BC ae 2de le If you try to shift when there are no variables to shift that is when already equals zero you ll get an error message from the shell the error will vary from one shell to the next prog shift bad number where prog is the name of the program that executed the offending shift You can shift more than one place at once by writing a count immediately after shift as in shift 3 This command has the same effect as performing three separate shifts shift shift shift The shift command is useful when processing a variable number of arguments You ll see it put to use when you learn about loops in Chapter 9 Round and Round She Goes Ss Gy Exercises a
375. ted immediately on entry into the loop the commands between the do and done may never be executed if it returns a nonzero exit status the first time Example fill up the rest of the buffer with blank lines while lines le maxlines do echo gt gt BUFFER Lines lines 1 done a Appendix B For More Information IN THIS APPENDIX e Online Documentation Documentation on the Web e Books Many sources of information on the Unix system are available however we have selectively listed some titles and Web sites here of particular value to shell programmers All Web sites and URLs are valid as of the publication of this book but as is often the case on the Internet some may not be available by the time you read this There is one reference that you cannot do without This is the Unix documentation for your particular system It gives detailed descriptions on the syntax and various options for each of the commands Fs previous Next rs ca Online Documentation If a printed version of your system s documentation isn t available you can use the man command to get information referred to as the man pages by Unix users about any specific Unix command The format is man command Some systems have an interactive documentation command called info To invoke it simply type info After it starts up just type h for a tutorial Note that some smaller systems may not have online documentation due to disk spa
376. ters For example on a machine that encodes characters in ASCII the space character is represented internally as the number 32 and the double quote as the number 34 This means that the former would be sorted before the latter Note that the sorting order is implementation dependent so although you are generally assured that sort will perform as expected on alphabetic input the ordering of numbers punctuation and special characters is not always guaranteed We will assume we re working with the ASCII character set in all our examples here sort has many options that provide more flexibility in performing your sort We ll just describe a few of the options here The u Option The u option tells sort to eliminate duplicate lines from the output sort u names Charlie Emanuel Fred Lucy Ralph Tony Here you see that the duplicate line that contained Tony was eliminated from the output The r Option Use the r option to reverse the order of the sort sort r names Reverse sort Tony Tony Ralph Lucy Fred Emanuel Charlie The o Option By default sort writes the sorted data to standard output To have it go into a file you can use output redirection sort names gt sorted names Alternatively you can use the o option to specify the output file Simply list the name of the output file right after the o sort names o sorted names This sorts names and writes the results to sorte
377. test command is passed the three arguments name with its value substituted of course and julio test then tests to see whether the first argument is identical to the third argument and returns a zero exit status if it is and a nonzero exit status if it is not The exit status returned by test is then tested If it s zero the commands between then and fi are executed in this case the single echo command is executed If the exit status is nonzero the echo command is skipped It s good programming practice to enclose shell variables that are arguments to test inside a pair of double quotes to allow variable substitution This ensures that test sees the argument in the case where its value is null For example consider the following example name Set name null test name julio sh test argument expected Because name was null only two arguments were passed to test and julio because the shell substituted the value of name before parsing the command line into arguments In fact after name was substituted by the shell it was as if you typed the following test julio When test executed it saw only two arguments see Figure 8 1 and therefore issued the error message Figure 8 1 test name julio with name null ll pace oe By placing double quotes around the variable you ensure that test sees the argument because quotes act as a placeholder when the argument is null test name julio echo
378. th question mark 2nd brackets 2nd examples 2nd multiple character matching 2nd 3rd 4th single character matching 2nd 3rd variables 2nd 3rd filenames filename substitution 2nd 3rd 4th asterisk 2nd 3rd 4th question mark 2nd brackets 2nd examples 2nd multiple character matching 2nd 3rd 4th single character matching 2nd 3rd variables 2nd 3rd files See also directories profile 2nd 3rd archive files creating 2nd 3rd 4th 5th command files 2nd comments 2nd 3rd 4th creating 2nd examples 2nd 3rd 4th execute permissions 2nd command history file See command history copying checking destination files 2nd 3rd cp command 2nd 3rd 4th echo escape characters 2nd final code listing 2nd 3rd 4th 5th 6th initial code listing 2nd 3rd read command counting words in wc command 2nd deleting rm command 2nd destination files displaying contents of cat command 2nd duplicating lines in 2nd 3rd 4th 5th 6th 7th 8th editing sed command 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th ENV files 2nd executing contents in current shell 2nd 3rd 4th 5th extracting characters from cut command 2nd 3rd 4th 5th 6th 7th 8th 9th 10th file descriptors file operators 2nd 3rd filename substitution 2nd 3rd 4th asterisk 2nd 3rd 4th question
379. th zero exiting loops break command 2nd 3rd programs 2nd expansion arithmetic expansion 2nd 3rd export command 2nd 3rd p option exported variables 2nd 3rd 4th 5th 6th 7th exporting variables 2nd 3rd 4th 5th 6th 7th 8th 9th 10th expressions arithmetic expressions 2nd 3rd 4th evaluating expr command 2nd 3rd 4th 5th expressions regular expressions 2nd construct 2nd 3rd 4th L construct 2nd 3rd construct 2nd 3rd 4th asterisk 2nd 3rd 4th caret 2nd dollar sign 2nd 3rd grep command l option 2nd n option 2n v option 2n examples 2nd 3rd 4th 5th 6th 7th a es syntax period 2nd 3rd summary of 2nd extracting characters from files cut command 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 4 rrevious SYMBOL A B C D E F G H 1 1 K L M N O P Q R S T U V W X Y Z f f escapecharacte f f escapecharacter f f operator F command vi f option cut command 2nd 3rd 4th 5th f option set command false command fc command 2nd 3rd 4th 5th 6th 7th FCEDIT variable 2nd fg command 2nd 3rd 4th 5th 6th fields Input Field Separator IFS variable 2nd 3rd 4th 5th skipping during sorts 2nd file descriptors file operators 2nd 3rd filename substitution 2nd 3rd 4th asterisk 2nd 3rd 4
380. th 6th man mkdir 2nd multiple commands entering on one line mv moving files 2nd renaming files 2nd 3rd newgrp 2nd null command colon 2nd paste d option 2n s option 2n examples 2nd 3rd a a syntax perl printf 2nd 3rd 4th 5th 6th 7th 8th conversion specification characters 2nd conversion specification modifiers 2nd examples 2nd 3rd 4th 5th 6th syntax printing information about ps 2nd pwd 2nd 3rd 4th 5th 6th r 2nd 3rd read 2nd 3rd readonly 2nd 3rd 4th return 2nd 3rd 4th returning information about 2nd type command 2nd rm 2nd rmdir 2nd scanning twice before execution 2nd 3rd 4th sed om d option 2n n option 2n examples 2nd 3rd 4th 5th ou syntax sending to background 2nd 3rd set 2nd 3rd 4th 5th 6th 7th 8th option 2nd 3rd 4th x option 2nd executing without arguments 2nd reassigning positional parameters with 2nd 3rd sh x option 2nd 3rd 4th shift 2nd 3rd 4th 5th sort 2nd In option 2nd n option 2nd 0 option 2nd r option 2nd toption 2nd 3rd u option 2nd summary of 2nd test 2nd 3rd 4th alternative format 2nd file operators 2nd 3rd integer operators 2nd 3rd logical AND operator a 2nd logical negation operator 2nd logical OR operator 0 2nd parentheses 2nd string operators 2nd 3r
381. th 8th 9th backquote 2nd 3rd 4th expr command 2nd 3rd 4th 5th command line editing 2nd continue 2nd 3rd 4th continuing to next line cp 2nd 3rd 4th cut d option 2nd 3rd 4th 5th foption 2nd 3rd 4th 5th examples 2nd 3rd 4th syntax date 2nd 3rd echo 2nd 3rd 4th 5th 6th 7th escape characters 2nd emacs line edit mode 2nd entering 2nd 3rd entering multiple 2nd eval 2nd 3rd 4th 5th 6th exec 2nd 3rd 4th 5th executing asynchronous execution executing in background exit 2nd 3rd 4th 5th 6th exit status export 2nd 3rd p option expr 2nd 3rd 4th 5th false fc 2nd 3rd 4th 5th 6th 7th fg 2nd 3rd 4th 5th 6th for 2nd 3rd variable variable 2nd 3rd examples 2nd 3rd 4th 5th for without the list 2nd syntax 2nd general format of getopts 2nd 3rd 4th 5th 6th 7th 8th 9th 10th grep 2nd l option 2n n option 2n v option 2n examples 2nd 3rd 4th 5th 6th 7th gt Qa es syntax grouping 2nd 3rd 4th 5th 6th 7th hash 2nd history 2nd if 2nd 3rd 4th 5th amp amp construct 2nd 3rd 4th construct 2nd 3rd 4th elif construct 2nd 3rd 4th 5th else construct 2nd 3rd 4th exit status 2nd 3rd 4th 5th 6th 7th syntax 2nd testing conditions in See test command info w jobs 2nd 3rd kill 2nd 3rd 4th In 2nd 3rd 4th 5th 6th Is 2nd 3rd 4th 5
382. th different files in a particular directory simply cd to the directory first and then you can directly reference all the files you need A second approach is to simply list a full path to phonebook in the grep command grep 1 users steve bin phonebook But suppose that you want to let others use your rolo program and associated Lu add and rem programs You can give them each their own copy and then you ll have several copies of the identical program on the system programs that you ll probably have to maintain And what happens if you make a small change to rolo Are you going to update all their copies as well A better solution might be to keep 3 just one copy of rolo but to give other users access to it 3 This can be done by giving them execute permission on all the directories leading to rolo as well as read and execute permissions on the programs themselves They can always copy your programs at that point but you won t have to maintain them If you change all the references of phonebook to explicitly reference your phone book everyone else who uses your rolo program will be using your phone book and not his own One way to solve the problem is to require that everyone have a phonebook file in his home directory this way if the program references the file as HOME phonebookw it will be relative to the home directory of the person running the program Let s try this approach Define a variable inside rolo called PHON
383. the file etc passwd cat etc passwd root 0 0 The Super User usr bin ksh cron 1 1 Cron Daemon for periodic tasks bin 3 3 The owner of system files uucp 5 5 usr spool uucp usr lib uucp uucico asg 6 6 The Owner of Assignable Devices steve 203 100 users steve usr bin ksh other 4 4 Needed by secure program etc passwd is the master file that contains the usernames of all users on your computer system It also contains other information such as your user id number your home directory and the name of the program to start up when you log in Getting back to the cut command you can see that the data in this file does not align itself the same way who s output does So getting a list of all the possible users of your system cannot be done using the c option to cut One nice thing about the format of etc passwd however is that fields are delimited by a colon character So although each field may not be the same length from one line to the next you know that you can count colons to get the same field from each line The d and f options are used with cut when you have data that is delimited by a particular character The format of the cut command in this case becomes cut ddchar ffields file where dchar is the character that delimits each field of the data and fields specifies the fields to be extracted from file Field numbers start at 1 and the same type of formats can be used to specify fie
384. the name of a file minimizes the possibility of another process using the same file So you can replace the two lines from rem with these grep v name phonebook gt tmp phonebook mv tmp phonebook phonebook to circumvent any potential problems Each person running rolo will run it as a different process so the temporary file used in each case will be different The Exit Status from read read always returns an exit status of zero unless an end of file condition is detected on the input If the data is coming from the terminal this means that Ctrl d has been typed If the data is coming from a file it means that there s no more data to read from the file Knowing about the exit status returned by read makes it easy to write a loop that will read any number of lines of data from a file or from the terminal The next program called addi reads in lines containing pairs of integers Each pair of numbers is summed and the result written to standard output cat addi add pairs of integers on standard input while read nl n2 do echo n1 n2 done The while loop is executed as long as the read command returns an exit status of zero that is as long as there s still data to be read Inside the loop the two values read from the line presumably integers no error checking is done here are summed and the result written to standard output by echo addi 10 25 35 5 12 123 3 126 Ctrl d It goes witho
385. the preceding program with an if else instead of using the exit hint look at the last version of on Whether you use the exit or an if else is up to you Sometimes the exit is a more convenient way to get out of the program quickly particularly if it s done early in the program rs The euif Construct As your programs become more complex you may find yourself needing to write nested if statements of the following form if command then else command command if command then else command command if command then else fi command command command command fi fi This type of command sequence is useful when you need to make more than just a two way decision as afforded by the if else construct In this case a multiway decision is made with the last else clause executed if none of the preceding conditions is satisfied As an example suppose that you wanted to write a program called greetings that would print a friendly Good morning Good afternoon or Good evening whenever you logged on to the system For purposes of the example consider any time from midnight to noon to be the morning noon to 6 00 p m the afternoon and 6 00 p m to midnight the evening To write this program you have to find out what time it is date serves just fine for this purpose Take another look at the output from this command date Wed Aug 29 10 42 01 EDT 2002 The format of date s output i
386. ther to form single lines that are then written to standard output The dash character can be used in files to specify that input is from standard input Suppose that you have a set of names in a file called names cat names Tony Emanuel Lucy Ralph Fred Suppose that you also have a file called numbers that contains corresponding phone numbers for each name in names cat numbers 307 555 5356 212 555 3456 212 555 9959 212 555 7741 212 555 0040 You can use paste to print the names and numbers side by side as shown paste names numbers Paste them together Tony 307 555 5356 Emanuel 212 555 3456 Lucy 212 555 9959 Ralph 212 555 7741 Fred 212 555 0040 Each line from names is displayed with the corresponding line from numbers separated by a tab The next example illustrates what happens when more than two files are specified cat addresses 55 23 Vine Street Miami 39 University Place New York 17 E 25th Street New York 38 Chauncey St Bensonhurst 17 E 25th Street New York paste names addresses numbers Tony 55 23 Vine Street Miami 307 555 5356 Emanuel 39 University Place New York 212 555 3456 Lucy 17 E 25th Street New York 212 555 9959 Ralph 38 Chauncey St Bensonhurst 212 555 7741 Fred 17 E 25th Street New York 212 555 0040 The d Option If you don t want the fields separated by tab characters you can specify the d option with the format
387. tion on a Loop You can also perform 1 0 redirection on the entire loop Input redirected into the loop applies to all commands in the loop that read their data from standard input Output redirected from the loop to a file applies to all commands in the loop that write to standard output fori in1234 gt do gt echo i gt done gt loopout Redirect loop s output to loopout cat loopout You can override redirection of the entire loop s input or output by explicitly redirecting the input and or output of commands inside the loop To force input or output of a command to come from or go to the terminal use the fact that dev tty always refers to your terminal In the following loop the echo command s output is explicitly redirected to the terminal to override the global output redirection applied to the loop for file do echo Processing file file gt dev tty done gt output echo s output is redirected to the terminal while the rest goes to the file output Naturally you can also redirect the standard error output from a loop simply by tacking on a 2 gt file after the done while endofdata ne TRUE do done 2 gt errors Here output from all commands in the loop writing to standard error will be redirected to the file errors Piping Data Into and Out of a Loop A command s output can be piped into a loop and the entire output from a loop can be piped into another command in the expected manner Here
388. tion with the shift command to process a variable number of arguments typed on the command line The next program called prargs prints each of the command line arguments one per line cat prargs Print command line arguments one per line while ne 0 do echo 1 shift done prargs abc a b prargs a b c a b c prargs addresses intro lotsaspaces names nu numbers phonebook stat prargs No arguments While the number of arguments is not equal to zero the value of 1 is displayed and then a shift executed Recall that this shifts down the variables that is 2 to 1 3 to 2 and so on and also decrements When the last argument has been displayed and shifted out will equal zero at which point execution of the while will be terminated Note that if no arguments are given to prargs as was done in the last case the echo and shift are never executed because is equal to zero as soon as the loop is entered fe previous nex gt rs ca The untit Command The while command continues execution as long as the command listed after the while returns a zero exit status The until command is similar to the while only it continues execution as long as the command that follows the until returns a nonzero exit status As soon as a zero exit status is returned the loop is terminated Here is the general format of the until until command do command command done Like the w
389. to be preceded by a dollar sign That s because the shell knows that the only valid items that can appear in arithmetic expansions are operators numbers and variables If the variable is not defined or contains a NULL string its value is assumed to be zero So if we have not assigned any value yet to the variable a we can still use it in an integer expression echo a Variable a not set echo a a 1 Equivalent toa 0 1 1 echo a 1 Now a contains 1 Note that assignment is a valid operator and the value of the assignment is substituted in the second echo command in the preceding example Parentheses may be used freely inside expressions to force grouping as in echo i i 10 j If you want to perform an assignment without echo or some other command you can move the assignment before the arithmetic expansion So to multiply the variable i by 5 and assign the result back to i you can write s o 2 3 Note that spaces are optional inside the double parentheses but are not allowed when the assignment is outside them Finally to test to see whether i is greater than or equal to O and less than or equal to 100 you can write result i gt 0 amp amp i lt 100 which assigns result 1 if the expression is true and 0 if it s false 1 100 200 10 j i lt 1000 If i is lt 1000 set j 0 otherwise 1 echo i j 2000 0 A is 2000 so j was set to 0 Tha
390. to do in Chapter 4 Tools of the Trade where we talked about regular expressions Because who lists each username in column one of each output line we can anchor the pattern to match the beginning of the line by preceding the pattern with the character who grep user gt dev null But that s not enough grep still matches a line like bobby tty07 Jul 8 09 35 if you ask it to search for the pattern bob What you need to do is also anchor the pattern on the right Realizing that who ends each username with one or more spaces the pattern A duser LLI now only matches lines for the specified user Let s try the new and improved version of on cat on determine if someone is logged on version 2 user 1 if who grep user gt dev null then echo user is logged on Ta who Who s on now root console Jul 8 10 37 barney tty03 Jul 8 12 38 fred tty04 Jul 8 13 40 joanne tty07 Jul 8 09 35 tony tty19 Jul 8 08 30 lulu tty23 Jul 8 09 55 on Lulu lulu is logged on on ann Try this again on What happens if we don t give any arguments If no arguments are specified user will be null grep will then look through who s output for lines that start with a blank why It won t find any and so just a command prompt will be returned In the next section you ll see how to test whether the correct number of arguments has been supplied to a program and if not take some action
391. to one and is updated each time getopts returns to reflect the number of the next command line argument to be processed Here is the third version of mon that uses the getopts command to process the command line arguments It also incorporates the previously noted change to send mail to the user running the program cat mon Wait until a specified user logs on version 3 Set up default values mailopt FALSE interval 60 process command options while getopts mt option do case option in m mailopt TRUE t interval 0PTARG echo Usage mon m t n user echo m means to be informed by mail echo t means check every n secs exit 1 esac done Make sure a user name was specified if OPTIND gt then echo Missing user name exit 2 fi shiftcount OPTIND 1 shift shiftcount user 1 Check for user logging on until who grep user gt dev null do sleep interval done When we reach this point the user has logged on if mailopt FALSE then echo user has logged on else runner who am i cut cl1 8 echo user has logged on mail runner fi mon m Missing user name mon x fred Illegal option mon illegal option x Usage mon m t n user m means to be informed by mail t means check every n secs mon m t 600 ann amp Check every 10 min for ann 1 5792 When the
392. to whatever you want In the following examples the underline _ represents the cursor A command line will be shown followed by one or more keystrokes followed by what the line looks like after applying the keystrokes before keystrokes after First let s look at moving the cursor around The H key moves the cursor to the left and the L key moves it to the right Try this out by entering command mode and pressing the H and L keys a few times The cursor should move around on the line If you try to move the cursor past the left or right side of the line the shell beeps at you mary had a little larb Esc mary had a little larb mary had a little larb h mary had a little larb mary had a little larb h mary had a little larb mary had a little larb l mary had a little larb After the cursor is on the character you want to change you can use the x command to delete the current character X it out mary had a little larb X mary had a little lab Note that the b moved to the left when the r was deleted and is now the current character To add characters to the command line you can use the i and a commands The i command inserts characters before the current character and the a command adds characters after the current character Both of these commands put you back into input mode you must press Esc again to go back to command mode mary had a little lab im mary had a little lamb mary had a little lamb m
393. tor the change program won t abort The editor is then started to allow the user to edit the entry After the user makes his changes writes the file and quits the editor control is given back to change The old entry is then removed from the phone book with grep and the modified entry is converted into the special internal format with tr and tacked onto the end An extra newline character must be added here to make sure that a real newline is stored in the file after the entry This is done with an echo with no arguments The phone book file is then sorted and the temporary files removed os oa os oo listall list all of the entries in the phone book IFS to be used in set command below echo while read line do Get the first and last fields presumably names and numbers set line display 1st and last fields in reverse order eval printf 40 40s S n S1 n NING LSH n done lt PHONEBOOK The listall program lists all entries in the phone book printing just the first and last lines of each entry The internal field separator characters IFS is set to a to be used later inside the loop Each line from the phone book file is then read and assigned to the variable Line The set command is used to assign each field to the positional parameters The trick now is to get the value of the first and last positional parameter
394. tput be from the following commands echo error echo Is 5 4 gt 18 echo x echo What is your name echo y echo Would you like to play a game echo y echo echo echo echo echo echo echo z we l echo z we l echo symbol z Iwc l echo _ now_ echo hello symbol out echo earn echo I don t understand symbol 2 Write the commands to remove all the space characters stored in the shell variable text Be sure to assign the result back to text First use tr to do it and then do the same thing with sed 3 Write the commands to count the number of characters stored in the shell variable text Then write the commands to count all the alphabetic characters Hint Use sed and wc What happens to special character sequences such as n if they re stored inside text 4 Write the commands to assign the unique lines in the file names to the shell variable namelist 4 Previous Next Chapter 7 Passing Arguments IN THIS CHAPTER e The shift Command e Exercises Shell programs become far more useful after you learn how to process arguments passed to them In this chapter you ll learn how to write shell programs that take arguments typed on the command line Recall the program run that you wrote in Chapter 5 And Away We Go to run the file sys caps through tbl nroff and lp cat run tbl sys caps nroff mm Tlp lp Suppose that you need to run ot
395. tring out as many commands as you want on the line as long as each command is delimited by a semicolon Sending a Command to the Background Normally you type in a command and then wait for the results of the command to be displayed at the terminal For all the examples you have seen thus far this waiting time is typically short maybe a second or two However you may have to run commands that require many seconds or even minutes to execute In those cases you ll have to wait for the command to finish executing before you can proceed further unless you execute the command in the background If you type in a command followed by the ampersand character amp that command will be sent to the background for execution This means that the command will no longer tie up your terminal and you can then proceed with other work The standard output from the command will still be directed to your terminal however in most cases the standard input will be dissociated from your terminal If the command does try to read any input from standard input it will be stopped and will wait for you to bring it to the foreground we ll discuss this in more detail in Chapter 15 Interactive and Nonstandard Shell Features 31 3 Note that the capability to stop a command when it reads from standard input may be missing on non Unix implementations of the shell or on older shells that do not conform to the POSIX standard On these implementations any read from sta
396. ts all filenames in a specified directory hierarchy Its output should be similar to the output of the find command myfind users pat users pat users pat bin users pat bin ksh users pat bin lLf users pat bin pic users pat chapt1 users pat chapt1 intro users pat rje users pat rje filel Hint Bash and Korn shell functions can be recursive 2 Write a shell function called octal that converts octal numbers given as command line arguments to decimal numbers and prints them out one per line octal 10 11 12 8 9 10 Hint for Korn shell users If you assign a decimal number to a variable when it s declared for example typeset i d 10 0 assignments to this variable from other bases are converted to decimal first Modify the cdh function to filter out multiple occurrences of the same directory for example cdh l 0 users pat cdh cdh cdh l 0 users pat Modify the cdh function to set the prompt PS1 to show the current directory for example users pat cdh tmp tmp cdh users pat Modify the cdh function to allow the user to specify a partial name of a directory in the history file preceded by a dash etc cdh l 0 users pat 1 tmp 2 users steve 3 usr spool uucppublic 4 usr local bin 5 etc etc cdh pub usr spool uucppublic cdh bin usr local bin 6 Bash users only Add the Korn shell s cd old new feature to the cdh function Appendix A Shell Summa
397. ttached to that character is removed Here is an example echo gt syntax error newline or unexpected echo gt gt In the first case the shell sees the gt and thinks that you want to redirect echo s output to a file So it expects a filename to follow Because it doesn t the shell issues the error message In the next case the backslash removes the special meaning of the gt so it is passed along to echo to be displayed x echo x x In this case the shell ignores the that follows the backslash and as a result variable substitution is not performed Because a backslash removes the special meaning of the character that follows can you guess what happens if that character is another backslash Right it removes the special meaning of the backslash echo Naturally you could have also written echo Using the Backslash for Continuing Lines As mentioned at the start of this section c is basically equivalent to c One exception to this rule is when the backslash is used as the very last character on the line Lines one gt two Single quotes tell shell to ignore newline echo lines one two Lines one Try it with a instead gt two echo lines onetwo The shell treats a backslash at the end of the line as a line continuation It removes the newline character that follows and also does not treat the newline as an argument delimite
398. ttle lambda Ww had a little lambda had a little lambda 2w had a little lambda had a little lambda 3b had a little lambda At any time you can press Enter and the current line will be executed as a command had a little lambda Hit Enter ksh had not found After a command is executed you are placed back in input mode Accessing Commands from Your History So far you ve learned how to edit the current line You can use the vi commands k and j to retrieve commands from your history The k command replaces the current line on your terminal with the previously entered command putting the cursor at the beginning of the line Let s assume that these commands have just been entered pwd users pat cd tmp echo this is a test this is a test Now go into command mode and use k to access them Esc k echo this is a test Every time k is used the current line is replaced by the previous line from the command history echo this is a test k cd tmp cd tmp k pwd To execute the command being displayed just press the Enter key pwd Hit Enter tmp The j command is the reverse of the k command and is used to display the next command in the history The command is used to search through the command history for a command containing a string If the is entered followed by a string the shell searches backward through its history to find the most recently executed command that contains th
399. tus if file is a block special file file is a character special file file is a directory file exists file is an ordinary file file has its set group id SGID bit set file is a symbolic link file has its sticky bit set file is a symbolic link file is a named pipe file is readable by the process file is a socket file has nonzero length fd is open file descriptor associated with a terminal 1 is default u file w file x file String Operators string n string Zz string string strings string strings file has its set user id SUID bit set file is writable by the process file is executable string is not null string is not null and string must be seen by test string is null and string must be seen by test string is identical to string string is not identical to strings Integer Comparison Operators int eq ints int ge int int gt int int le int int lt int int ne int int is equal to int int is greater than or equal to int int is greater than int int is less than or equal to intz int is less than intz int is not equal to int Boolean Operators expr expr is FALSE otherwise returns TRUE expr a expr expr is TRUE and exprz is TRUE expr 0 expr2 expr is TRUE or expr is TRUE The times Command General Format times Execution of this command causes the shell to write to standard output the total amount
400. tutorials twhile program 2nd type command 2nd 3rd 4th types integer 2nd types data TZ variable 2nd 4 rrevious SYMBOL A B C D E F G H 1 J K L M N O P Q R S T U V W X Y Z u command vi U command vi u conversion specification character u option sort command 2nd u option set command umask command 2nd unalias command 2nd underscore _ 2nd uniq command C option d option 2nd 3rd examples 2nd 3rd syntax Unix C Shell Field Guide The Unix in a Nutshell colon System V Edition Unix Programmer s Manual Volume II Unix Programming Environment The 2nd Unix Unleashed unset command 2nd 3rd 4th until command 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th examples 2nd 3rd 4th 5th 6th 7th 8th syntax 2nd User s Reference Manual System Administrator s Reference Manual users logged in users displaying 2nd utilities See also commands shell defined utilities See commands 4 Previous 4 rrevious SYMBOL A B C D E F G H 1 J K L M N O P Q R S T U V W X Y Z v escape character v option grep command 2nd v option set command values assigning to keyword parameters 2nd assigning to variables 2nd 3rd comparing with case command command syntax 2nd 3rd pattern m
401. ty Topics in C Programming and Unix Shell Programming rs iy 4 PREvious NEXT We Want to Hear from You As the reader of this book you are our most important critic and commentator We value your opinion and want to know what we re doing right what we could do better what areas you d like to see us publish in and any other words of wisdom you re willing to pass our way You can email or write me directly to let me know what you did or didn t like about this book as well as what we can do to make our books stronger Please note that cannot help you with technical problems related to the topic of this book and that due to the high volume of mail receive might not be able to reply to every message When you write please be sure to include this book s title and author as well as your name and phone or email address will carefully review your comments and share them with the author and editors who worked on the book Email feedback samspublishing com Mail Mark Taber Associate Publisher Sams Publishing 201 West 103rd Street Indianapolis IN 46290 USA Ss Ga rs ca Reader Services For more information about this book or others from Sams Publishing visit our Web site at www samspublishing com Type the ISBN excluding hyphens or the title of the book in the Search box to find the book you re looking for rs i a Chapter 1 Introduction It s no secret that the Unix operating system has emerged
402. type char Now it seems to work just fine What do you think happens if you use ctype without enclosing the in quotes In Chapter 12 More on Parameters you ll learn how you can turn this trace feature on and off at will from inside your program Before leaving the ctype program here s a version that avoids the use of wc and handles everything with the case cat ctype Classify character given as argument version 2 if ne 1 then echo Usage ctype char exit 1 fi Now classify char making sure only one was typed char 1 case char in 0 9 echo digit a z echo lowercase letter A Z echo uppercase letter echo special character 4 echo Please type a single character esac The matches any single character If this pattern is matched the character is a special character If this pattern isn t matched more than one character was typed so the catchall case is executed to print the message ctype u lowercase letter ctype gt special character ctype xx Please type a single character Back to the case The symbol has the effect of a logical OR when used between two patterns That is the pattern pat patz specifies that either pat or pat is to be matched For example lt 1 tist matches either the value l or list and dmd 5620 tty5620 matches either dmd or 5620 or tty5620 The greetings p
403. type just a single Enter on the line gt gt Steve s Ice Cream gt gt 444 6th Avenue gt gt New York City 10003 gt gt 212 555 3021 gt gt Steve s Ice Cream has been added to the phone book Would you like to Please select one of the above 1 6 2 Type in your new entry When you re done type just a single Enter on the line gt gt YMCA gt gt 973 555 2344 gt gt YMCA has been added to the phone book Would you like to Please select one of the above 1 6 2 Type in your new entry When you re done type just a single Enter on the line gt gt Maureen Connelly gt gt Hayden Book Companu gt gt 10 Mulholland Drive gt gt Hasbrouck Heights N J 07604 gt gt 201 555 6000 gt gt Maureen Connelly has been added to the phone book Would you like to Please select one of the above 1 6 2 Type in your new entry When you re done type just a single Enter on the line gt gt Teri Zak gt gt Hayden Book Company gt gt see Maureen Connelly for address gt gt 201 555 6060 gt gt Teri Zak has been added to the phone book Would you like to Please select one of the above 1 6 5 Maureen Connelly 201 555 6000 Steve s Ice Cream 212 555 3021 Teri Zak 201 555 6060 YMCA 973 555 2344 Would you like to Please select one of the above 1 6 1 Enter name to look up Maureen Maureen Connelly Hayden Book Companu 10 Mulholland Drive
404. u can execute the command wc l users wc l users 2 users Or you can count the number of lines in the file by redirecting the standard input of the wc command from the file users wc l lt users 2 Note that there is a difference in the output produced by the two forms of the wc command In the first case the name of the file users is listed with the line count in the second case it is not This points out the subtle distinction between the execution of the two commands In the first case wc knows that it is reading its input from the file users In the second case it only knows that it is reading its input from standard input The shell redirects the input so that it comes from the file users and not the terminal more about this in the next chapter As far as wc is concerned it doesn t know whether its input is coming from the terminal or from a file rs i Frevious J Next gt Pipes As you will recall the file users that was created previously contains a list of all the users currently logged in to the system Because you know that there will be one line in the file for each user logged in to the system you can easily determine the number of users logged in by simply counting the number of lines in the users file who gt users wc l lt users 5 This output would indicate that currently five users were logged in Now you have a command sequence you can use whenever you want to know how many users ar
405. u may use some kind of networked windowing system for example X Window System to start up a terminal emulation program for example xterm For each physical terminal port on a system a program called getty will be active This is depicted in Figure 3 2 Figure 3 2 The getty process Unix system kernel The Unix system more precisely a program called init automatically starts up a getty program on each terminal port whenever the system is allowing users to log in getty determines the baud rate displays the message login at its assigned terminal and then just waits for someone to type in something As soon as someone types in some characters followed by Enter the getty program disappears but before it goes away it starts up a program called login to finish the process of logging in see Figure 3 3 It also gives login the characters you typed in at the terminal characters that presumably represent your login name Figure 3 3 login started on sue s terminal login sue Password Unix system kernel When Login begins execution it displays the string Password at the terminal and then waits for you to type your password After you have typed it Login then proceeds to verify your login name and password against the corresponding entry in the file etc passwd This file contains one line for each user of the system That line specifies among other things the login name home directory and program to st
406. u should get the new directory listed ls documents misc programs The directory structure now appears as shown in Figure 2 8 Figure 2 8 Directory structure with newly created misc directory sleve documents programs misg memos proposals letters wh collact mon ee es plan dact sys A newhire noJSK AMG reply Copying a File from One Directory to Another The cp command can be used to copy a file from one directory into another For example you can copy the file wb from the programs directory into a file called wbx in the misc directory as follows cp programs wb misc wbx Because the two files are contained in different directories it is not even necessary that they be given different names cp programs wb misc wb When the destination file has the same name as the source file in a different directory of course it is necessary to specify only the destination directory as the second argument cp programs wb misc When this command gets executed the Unix system recognizes that the second argument is the name of a directory and copies the source file into that directory The new file is given the same name as the source file You can copy more than one file into a directory by listing the files to be copied before the name of the destination directory If you were currently in the programs directory the command cp wb collect mon misc would copy the three files wb collect and mon i
407. unctions to determine time zone information The simplest setting for TZ is a time zone name of three or more alphabetic characters followed by a number that specifies the number of hours that must be added to the local time to arrive at Coordinated Universal Time also known as Greenwich Mean Time This number can be positive local time zone is west of 0 longitude or negative local time zone is east of 0 longitude For example Eastern Standard Time can be specified as TZ EST5 The date command calculates the correct time based on this information and also uses the time zone name in its output TZ EST5 date Wed Sep 18 15 24 09 EST 2002 TZ xyz3 date Wed Sep 18 17 24 28 xyz 2002 A second time zone name can follow the number if this time zone is specified daylight savings time is assumed to apply date automatically adjusts the time in this case when daylight saving is in effect and is assumed to be one hour earlier than standard time If a number follows the daylight saving time zone name this value is used to compute the daylight savings time from the Coordinated Universal Time in the same way as the number previously described So the following TZ settings are quivalent TZ EST5EDT TZ EST5EDT6 The TZ variable is usually set in either the etc profile file or your profile file If not set an implementation specific default time zone is used typically Coordinated Universal Time rs fa os fa Exercises 1 Writ
408. und the program will prompt the user for the entry to be removed Here s a sample session rolo Please select one of the above 1 3 3 Enter name to be removed Susan More than one match please select the one to remove Susan Goldberg Remove y n n Susan Topple Remove y n y 8 Modify rolo so that the menu is redisplayed after each selection is made and processed To allow the user to get out of this add another selection to the menu to exit from the program 9 What happens to the rolo program if just an Enter is given as the name for the add look up or remove options 10 Modify lu to use printf to print the name and phone number so that they line up in columns for names up to 40 characters in length Hint use cut f and the fact that the fields in the phonebook are separated by tabs rs Chapter 11 Your Environment IN THIS CHAPTER Local Variables Exported Variables PS1 and PS2 HOME James Your PATH Your Current Directory More on Subshells Your profile File The TERM Variable The TZ Variable Exercises When you log on to the system you re effectively given your own copy of the shell program This shell maintains what s known as your environment an environment that is distinct from other users on the system This environment is maintained from the moment you log on until the moment you log off In this chapter you ll learn about this environment in detail and you ll see how i
409. ur quotes when we defined the dir alias Recalling that the shell performs parameter substitution inside double quotes the shell evaluated PWD at the time the alias was defined This means that for all intents and purposes the dir alias was defined as though we typed in the following alias dir echo steve The solution is to use single rather than double quotes when defining the dir alias to defer the parameter substitution until the time the alias is executed alias dir echo PWD Define alias pwd Where are we users steve dir Execute alias steve cd letters Change directory dir Execute alias again letters cd usr spool One more try dir spool Now the alias works just fine If an alias ends with a space the word following the alias is also checked for alias substitution For example alias nohup bin nohup nohup ll causes the shell to perform alias checking on the string Ll after replacing nohup with bin nohup Quoting a command prevents alias substitution For example Tt ksh ll command not found The format alias name causes the value of the alias name to be listed and the alias command without arguments causes all aliases to be listed The following aliases are automatically defined when the Korn shell starts up autoload typeset fu functions typeset f history fc l integer typeset 1 local typeset nohup nohup r fc
410. urn home rolo Liz Quick lookup No phonebook file in users steve Forgot to move it mv users steve bin phonebook rolo Liz Try again Liz Stachiw 212 555 2298 rolo Try menu selection Would you like to 1 Look someone up 2 Add someone to the phone book 3 Remove someone from the phone book Please select one of the above 1 3 2 Enter name to be added Teri Zak Enter number 201 555 6000 rolo Teri Teri Zak 201 555 6000 rolo lu and add seem to be working fine rem should also be tested to make sure that it s okay as well If you still want to run lu rem or add standalone you can do it provided that you first define PHONEBOOK and export it PHONEBOOK HOME phonebook export PHONEBOOK lu Harmon I couldn t find Harmon in the phone book If you do intend to run these programs standalone you d better put checks in the individual programs to ensure that PHONEBOOK is set to some value rs ca Your Current Directory Your current directory is also part of your environment Take a look at this small shell program called cdtest cat cdtest cd users steve bin pwd The program does a cd to users steve bin and then executes a pwd to verify that the change was made Let s run it pwd Get my bearings users steve cdtest users steve bin Now for the 64 000 question If you execute a pwd command now will you be in users steve or users steve bin pwd
411. used at the end of the line it signals line continuation to the shell The first echo gets executed if the grep succeeds the second if it fails These constructs are also often used in if commands if validsys sys amp amp timeok then sendmail user sys lt message fi If validsys returns an exit status of zero timeok is executed The exit status from this program is then tested for the if If it s zero then the sendmail program is executed If validsys returns a nonzero exit status timeok is not executed and this is used as the exit status that is tested by the if In that case sendmail won t be executed The use of the amp amp operator in the preceding case is like a logical AND both programs must return an exit status of zero for the sendmail program to be executed In fact you could have even written the preceding if as validsys sys amp amp timeok amp amp sendmail user sys lt message When the is used in an if the effect is like a logical OR if endofmonth specialrequest then sendreports fi If endofmonth returns a zero exit status sendreports is executed otherwise specialrequest is executed and if its exit status is zero sendreports is executed The net effect is that sendreports is executed if endofmonth or specialrequest return an exit status of zero In Chapter 9 Round and Round She Goes you ll learn about how to write loops in your programs However before proceed
412. user s phone book provided that you have read access to it This option should be added to the command line as a u option as well as to the menu For example rolo u pat Pizza would look up Pizza in pat s phone book no matter who is running rolo The program can find pat s phone book by looking at rolo in pat s home directory 6 What happens with rolo if the user adds an entry containing a or character 7 Add a s send option to rolo to mail a rolodex entry to a specified user So rolo s tom pizza should send the rolodex card entry for pizza to the user tom rs fi 4 FRevious NExt Chapter 15 Interactive and Nonstandard Shell Features IN THIS CHAPTER Getting the Right Shell The ENV File Command Line Editing Command History The vi Line Edit Mode The emacs Line Edit Mode Other Ways to Access Your History Functions Integer Arithmetic The alias Command Arrays Job Control The Restricted Shell rsh Miscellaneous Features Compatibility Summary Exercises In this chapter you ll learn about shell features that are either useful to interactive users or not part of the POSIX shell standard These features are available in Bash and the Korn shell the two most commonly available POSI X compliant shells The Korn shell was developed by David Korn of AT amp T Bell Laboratories It was designed to be upward compatible with the System V Bourne shell and the POSIX standard she
413. usion in if commands if i 100 then fi The i 100 returns an exit status of zero true if i equals 100 and one false otherwise and has the same effect as writing if i eq 100 then fi One advantage of using rather than test is the capability to perform arithmetic as part of the test if 1 10 0 then fi Here the comparison returns a true if i divided by 10 is not equal to zero while loops can also benefit from integer arithmetic For example x 0 while x lt 100 do commands done executes commands 100 times Note that some older versions of the Korn shell and Bash do not support the and operators Integer Types The Korn shell and Bash both support an integer data type You can declare variables to be integers by using the typeset command with the i option typeset i variables where variables are any valid shell variable names Initial values can be assigned to the variables at the time they are declared Arithmetic performed on integer variables with the construct is slightly faster than on noninteger ones because the shell internally stores the value of an integer variable as a binary number and not as a character string An integer variable cannot be assigned anything but an integer value or an integer expression If you attempt to assign a noninteger to it the message bad number is printed by the Korn shell typeset i i
414. ut saying that standard input for addi can be redirected as can standard output cat data 1234 7960 593 595 395 304 3234 999 394 493 addi lt data gt sums cat sums 9194 2 699 4233 887 The following program called number is a simplified version of the standard Unix nl command It takes one or more files given as arguments and displays them preceded by line numbers If no arguments are supplied it uses standard input instead cat number Number lines from files given as argument or from standard input if none supplied Lineno 1 cat while read line do echo lineno line Lineno lineno 1 done The variable Lineno the line number count is initially set to 1 Then the arguments typed to number are given to cat to be collectively written to standard output If no arguments are supplied will be null and cat will be passed no arguments This will cause it to read from standard input The output from cat is piped into the while loop For each line read by read the line is echoed at the terminal preceded by the value of Lineno whose value is then incremented by one number phonebook 1 Alice Chebba 973 555 2015 2 Barbara Swingle 201 555 9257 3 Billy Bach 201 555 7618 4 El Coyote 212 555 3232 5 Liz Stachiw 212 555 2298 6 Susan Goldberg 201 555 7776 7 Teri Zak 201 555 6000 8 Tony Iannino 973 555 1295 who number Try from standard input 1 root
415. utable try it again nu This time it worked You can put any commands at all inside a file make the file executable and then execute its contents simply by typing its name to the shell It s that simple and that powerful The standard shell mechanisms such as I O redirection and pipes can be used on your own programs as well nu gt tally cat tally 8 Suppose that you re working on a proposal called sys caps and that the following command sequence is needed every time you want to print a new copy of the proposal tbl sys caps nroff mm Tlp lp Once again you can save yourself some typing by simply placing this command sequence into a file let s call it run making it executable and then just typing the name run whenever you want to get a new copy of the proposal cat run tbl sys caps nroff mm Tlp lp chmod x run run request id is laserl1 15 standard input The request id message is issued by the Lp command For the next example suppose that you want to write a shell program called stats that prints the date and time the number of users logged in and your current working directory You know that the three command sequences you need to use to get this information are date who wc l and pwd cat stats date who wc l pwd chmod x stats stats Try it out Wed Jul 10 11 55 50 EDT 2002 13 users steve documents proposals You can add some echo commands to stats t
416. ve If you wanted to see what terminals were currently being used you could cut out just the tty numbers field from the who command s output who cut c10 16 console tty02 tty08 tty10 How did you know that who displays the terminal identification in character positions 10 through 16 Simple You executed the who command at your terminal and counted out the appropriate character 2 positions 2 On some versions of the Unix system this field starts in character position 12 and not 10 You can use cut to extract as many different characters from a line as you want Here cut is used to display just the username and login time of all logged in users who cut cl 8 18 root Feb 24 08 54 steve Feb 24 12 55 george Feb 24 09 15 dawn Feb 24 15 55 The option c1 8 18 says extract characters 1 through 8 the username and also characters 18 through 3 the end of the line the login time 3 Again on some systems the login time field starts in column 25 The d and f Options The cut command as described previously is useful when you need to extract data from a file or command provided that file or command has a fixed format For example you could use cut on the who command because you know that the usernames are always displayed in character positions 1 8 the terminal in 10 16 and the login time in 18 29 Unfortunately not all your data will be so well organized For instance take a look at
417. verwrite the file or if the file doesn t already exist the file is added to the shell variable copylist The technique used here is the same used to accumulate the arguments inside filelist When the for loop is exited copylist contains a list of all the files to be copied This list can be null if each of the destination files exists and the user types no for each one So a test is made to ensure copyList is nonnull and if it is the copy is performed Take some time to review the logic of the final version of mycp it does a good job at illustrating many of the features you ve learned so far in this book Some exercises at the end of this chapter will help test your understanding of this program A Menu Driven Phone Program One nice thing about the read command is that it enables you to write menu driven shell programs As an example we ll return to our phone book programs add lu and rem and gather their execution together under one program which we ll call rolo for rolodex program rolo will display a list of choices to the user and then execute the appropriate program depending on the selection It will also prompt for the proper arguments to the program Here then is the program cat rolo rolo rolodex program to look up add and remove people from the phone book Display menu echo Would you like to 1 Look someone up 2 Add someone to the phone book 3 Remove someone from the phone book Pl
418. way to terminate execution of a shell program For example let s take another look at the rem program which removes an entry from the phonebook file cat rem Remove someone from the phone book grep v 1 phonebook gt tmp phonebook mv tmp phonebook phonebook This program has the potential to do unintended things to the phonebook file For example suppose that you type rem Susan Topple Here the shell will pass two arguments to rem The rem program will end up removing all Susan entries as specified by 1 It s always best to take precautions with a potentially destructive program like rem and to be certain as possible that the action intended by the user is consistent with the action that the program is taking One of the first checks that can be made in rem is for the correct number of arguments as was done before with the on program This time we ll use the exit command to terminate the program if the correct number of arguments isn t supplied cat rem Remove someone from the phone book version 2 if ne 1 then echo Incorrect number of arguments echo Usage rem name exit 1 fi grep v 1 phonebook gt tmp phonebook mv tmp phonebook phonebook rem Susan Goldberg Try it out Incorrect number of arguments Usage rem name The exit command returns an exit status of 1 to signal failure in case some other program wants to check it How could you have written
419. who have to support a few different directory hierarchies you can create programs like db to execute whenever you have to work on one of your directories In that program you can also include definitions for other variables for example you might want to change your prompt in PS1 to something like DB to let you know that your database variables have been set up You may also want to change your PATH to include a directory that has programs related to the database and your CDPATH variable so that directories in the database will be easily accessible with the cd command You can even change HOME so that a cd without any arguments returns you directly to your database directory If you make these sorts of changes you ll probably want to execute db in a subshell and not in the current shell because doing the latter leaves all the modified variables around after you ve finished your work on the database The trick to doing it right is to start up a new shell from inside the subshell with all the modified variables exported to it Then when you re finished working with the database you can log off the new shell by pressing Ctri d Let s take a look at how this works Here is a new version of db cat db Set up and export variables related to the data base HOME usr2 data BIN HOME bin RPTS HOME rpts DATA HOME rawdata PATH PATH BIN CDPATH HOME RPTS PS1 DB export HOME BIN RPTS DATA PATH CDPATH PS1 Star
420. word from the previous command and enter input mode the last word is the default Attempt filename generation on the current word if a match is found replace the current word with the match and enter input mode List files that begin with current word Complete pathname of current word if current word is a directory append a if current word is a file append a space Undo the last text modification command Restore the current line to its original state Soft function key if an alias of the name __ letter is defined its value will be executed Execute vi editor on line count if count is omitted the current line is used Linefeed and print current line L Reprint the current line Ctrl j Execute the current line Ctri m Execute the current line Enter Execute the current line Insert a at the beginning of the line and enter the line into the command history same as I Enter fe revious next gt u Cn Quoting Four different types of quoting mechanisms are recognized These are summarized in Table A 5 Table A 5 Summary of Quotes Quote Description Pasar Removes special meaning of all enclosed characters meat Removes special meaning of all enclosed characters except and XC Removes special meaning of character c that follows inside double quotes removes special meaning of newline and that follows but is otherwise not interpreted used for line continuation if appears as last ch
421. worked fine In the second case however both Susan entries were removed because they both matched the pattern You can use the add program to add them back to the phone book add Susan Goldberg 201 555 7776 add Susan Topple 212 555 4932 In Chapter 8 Decisions Decisions you ll learn how to determine whether more than one matching entry is found and take some other action if that s the case For example you might want to alert the user that more than one match has been found and further qualification of the name is required This can be very helpful because most implementations of grep will match everything if an empty string is passed as the pattern Incidentally before leaving this program note that sed could have also been used to delete the matching entry In such a case the grep could be replaced with sed 1 d phonebook gt tmp phonebook to achieve the same result The double quotes are needed around the sed command to ensure that the value of 1 is substituted while at the same time ensuring that the shell doesn t see a command line like sed Stromboli Pizza d phonebook gt tmp phonebook and pass three arguments to sed rather than two rs i N If you supply more than nine arguments to a program you cannot access the tenth and greater arguments with 10 11 and so on If you try to access the tenth argument by writing 10 the shell actually substitutes the value of 1 followed by a 0 Ins
422. xecuted If command returns an exit status of nonzero command gets skipped For example if you write sort bigdata gt tmp sortout amp amp mv tmp sortout bigdata then the mv command will be executed only if the sort is successful Note that this is equivalent to writing if sort bigdata gt tmp sortout then mv tmp sortout bigdata fi The command z EDITOR amp amp EDITOR bin ed tests the value of the variable EDITOR If it s null bin ed is assigned to it The construct works similarly except that the second command gets executed only if the exit status of the first is nonzero So if you write grep name phonebook echo Couldn t find name the echo command will get executed only if the grep fails that is if it can t find name in phonebook or if it can t open the file phonebook In this case the equivalent if command would look like if grep name phonebook then else echo Couldn t find name fi You can write a pipeline on either the left or right hand sides of these constructs On the left the exit status tested is that of the last command in the pipeline thus who grep name gt dev null echo name s not logged on causes execution of the echo if the grep fails The amp amp and can also be combined on the same command line who grep name gt dev null amp amp echo name s not logged on echo name is logged on Recall that when is
423. xport CDPATH can be rewritten as export PATH PATH HOME bin CDPATH HOME usr spool uucppublic The output of export with a p argument is a list of the exported variables and their values in the form export variable value or export variable if variable has been exported but not yet set The false Command General Format false The false command simply returns a nonzero exit status The fc Command General Format fc e editor lnr first last fc s old new first The fc command is used to edit commands in the command history A range of commands is specified from first to last where first and last can be either command numbers or strings a negative number is taken as an offset from the current command number a string specifies the most recently entered command beginning with that string The commands are read into the editor and executed upon exit from the editor If no editor is specified the value of the shell variable FCEDIT is used if FCEDIT is not set ed is used The l option lists the commands from first to last that is an editor is not invoked If the n option is also selected these commands are not preceded by command numbers The r option to fc reverses the order of the commands If last is not specified it defaults to first If first is also not specified it defaults to the previous command for editing and to 16 for listing The s option causes the selected command to be executed wit
424. y not on the directory itself as the previous version of mycp did The basename command gives the base filename of its argument for example basename usr bin troff gives troff basename troff gives troff This ensures that the copy is made to the correct place For example if mycp tmp data bin is typed where bin is a directory you want to copy tmp data into bin data and not into bin tmp data Here s some sample output Note the effect of the c escape characters ls Check out current directory bin progl ls bin Look inside bin lu nu prog1 mycp progl prog2 Simple case mycp progl bin Copy into directory bin progl already exists overwrite yes no yes A Final Version of mycp The last modification to mycp makes the program virtually equivalent to the standard Unix cp command by allowing a variable number of arguments Recall that any number of files can precede the name of a directory as in cp progl prog2 greetings bin To modify mycp to accept any number of files you can use this approach 1 Get each argument but the last from the command line and store it in the shell variable filelist 2 Store the last argument in the variable to 3 If to is not a directory there must be exactly two arguments 4 For each file in filelist check whether the file already exists If it does ask the user whether the file should be overwritten If the answer is yes or if the file doesn t already exist add the f
425. y default and then standard output to foo You recall that you can also dynamically redirect standard input or output in a program using the exec command exec lt datafile redirects standard input from the file datafile Subsequent commands executed that read from standard input will read from datafile instead The command exec gt tmp output does the same thing with standard output All commands that subsequently write to standard output will write to tmp output unless explicitly redirected elsewhere Naturally standard error can be reassigned this way as well exec 2 gt tmp errors Here all output to standard error will go to tmp errors lt amp and gt amp The characters gt amp have the effect of closing standard output If preceded by a file descriptor the associated file is closed instead So writing the impractical ls gt amp causes the output from ls to go nowhere because standard output is closed by the shell before Ls is executed The same thing applies for input using lt amp we lt amp 0 0 0 Inline Input Redirection If the lt lt characters follow a command in the format command lt lt word the shell uses the lines that follow as the standard input for command until a line that contains just word is found Here s a small example at the terminal wc l lt lt ENDOFDATA Use lines up to ENDOFDATA as standard input gt here s a line gt and another Vv
426. y to specify a path to a particular file Another notational convention is the single period which always refers to the current directory Now it s time to examine commands designed for working with directories Displaying Your Working Directory The pwd Command The pwd command is used to help you get your bearings by telling you the name of your current working directory Recall the directory structure from Figure 2 3 The directory that you are placed in after you log in to the system is called your home directory You can assume from Figure 2 3 that the home directory for the user steve is users steve Therefore whenever steve logs in to the system he will automatically be placed inside this directory To verify that this is the case the pwd print working directory command can be issued pwd users steve The output from the command verifies that steve s current working directory is users steve Changing Directories The cd Command You can change your current working directory by using the cd command This command takes as its argument the name of the directory you want to change to Let s assume that you just logged in to the system and were placed inside your home directory users steve This is depicted by the arrow in Figure 2 4 Figure 2 4 Current working directory is steve users pat steve ruth documents programs a memos proposals letters wb collect mon plan dact sysA newhire no JSK AMG
427. ype will be appended to the line mary had a little lamb_ da mary had a little lambda Two other commands useful in moving the cursor are the Esc f and Esc b commands The Esc f command moves the cursor forward to the end of the current word where a word is a String of letters numbers and underscores delimited by blanks or punctuation The Esc b command moves the cursor backward to the beginning of the previous word mary had a little lambda Esc b mary had a little lambda mary had a little lambda Esc b mary had a little lambda mary had a little lambda Esc b mary had a little lambda mary had a little lambda Esc f mary had a little lambda Fr Fir fF fF HF Fr Fr fF FR FA mary had a little lambda Esc f mary had a little lambda At any time you can press the Enter key and the current line will be executed as a command mary had a little lambda Hit Enter enter command ksh mary not found Accessing Commands from Your History So far you ve learned how to edit the current line As we said before the shell keeps a history of recently entered commands To access these commands you can use the emacs commands Ctrl p and Ctrl n The Ctrl p command replaces the current line on your terminal with the previously entered command putting the cursor at the end of the line Let s assume that these commands have just been entered pwd users pat cd tmp echo this is a test this is a test Now use Ctrl p to access the
428. ypes of I O redirection that the shell supports Table A 7 1 O Redirection Construct Meaning lt file Redirect standard input from file gt file Redirect standard output to file file is created if it doesn t exist and zeroed if it does gt file Redirect standard output to file file is created if it doesn t exist and zeroed if it does the noclobber C option to set is ignored gt gt file Like gt only output is appended to file if it already exists lt lt word Redirect standard input from lines that follow up until a line containing just word parameter substitution occurs on the lines and back quoted commands are executed and the backslash character interpreted if any character in word is quoted none of this processing occurs and the lines are passed through unaltered if word is preceded by a leading tabs on the lines are removed lt amp digit Standard input is redirected from the file associated with file descriptor digit gt amp digit Standard output is redirected to the file associated with file descriptor digit lt amp Standard input is closed gt amp Standard output is closed lt gt file Open file for both reading and writing Note that filename substitution is not performed on file Any of the constructs listed in the first column of the table may be preceded by a file descriptor number to have the same effect on the file associated with that file descriptor The file descriptor 0 i
429. yway i 1 expr i 1 2 This is the older method for performing arithmetic on shell variables Do the same type of thing as shown previously only use the command substitution mechanism to assign the output from expr back to the variable i 1 1 expr i 1 Add 1 to i echo i 2 In legacy shell programs you re more likely to see expr used with back quotes i expr i 1 Add 1 to i echo i 3 Note that like the shell s built in integer arithmetic expr only evaluates integer arithmetic expressions You can use awk or bc if you need to do floating point calculations Also note that expr has other operators One of the most frequently used ones is the operator which is used to match characters in the first operand against a regular expression given as the second operand By default it returns the number of characters matched The expr command expr file returns the number of characters stored in the variable file because the regular expression matches all the characters in the string For more details on expr consult your Unix User s Manual Table A 5 in Appendix A summarizes the way quotes are handled by the shell fe previous next gt Exercises 1 Given the following assignments x y z one gt two gt three now date symbol gt and these files in your current directory echo names testl u vv zebra What will the ou

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