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Bash Guide for Beginners

1. u 1 eq gt lif i eq 5 if i 5 iif i 5 Sample condition test l fi fi fi endif End if statement ulimit ulimit ulimit limit Set resource limits pwd pwd pwd dirs Print working directory read read read lt Read from terminal trap 2 trap 2 trap 2 onintr Ignore interrupts unalias unalias unalias Remove aliases until until until Begin until loop while do __ while do while do while Begin while loop The Bourne Again SHell has many more features not listed here This table is just to give you an idea of how this shell incorporates all useful ideas from other shells there are no blanks in the column for bash More information on features found only in Bash can be retrieved from the Bash info pages in the Bash Features section More information You should at least read one manual being the manual of your shell The preferred choice would be info bash bash being the GNU shell and easiest for beginners Print it out and take it home study it whenever you have 5 minutes Appendix B GNU Free Documentation License Version 1 1 March 2000 Copyright C 2000 Free Software Foundation Inc 59 Temple Place Suite 330 Boston MA 02111 1307 USA Everyone is permitted to copy and distribute verbatim copies of this license document but changing it is not allowed B 1 Preamble The purpose of this License is to make a manual textbook or other written document free in the sense of freedom
2. A french translation is in the making and will be linked to as soon as it is finished 4 Revision History Revision History Revision 1 8 2006 03 15 Revised by MG clarified example in Chap4 corrected here doc in chap9 general checks and correction of typos added link to Chinese and Ukrainian translation note and stuff to know about awk in chap6 Revision 1 7 2005 09 05 Revised by MG Corrected typos in chapter 3 6 and 7 incorporated user remarks added a note in chap7 Revision 1 6 2005 03 01 Revised by MG Minor debugging added more keywords info about new Bash 3 0 took out blank image Revision 1 5 2004 12 06 Revised by MG Changes because of new domain minor corrections Revision 1 4 2004 10 18 Revised by MG Debugging added a couple of notes in chap9 replaced screenshots with screen sections Corrected some typos Revision 1 3 2004 07 09 Revised by MG Added tracer image 1x1 pixel http tille xalasys com images blank bash png added textobjects for all pictures fixed wrong links in index made signal list more clear Revision 1 2 2004 06 15 Revised by MG Added index more markup in screen sections Revision 1 1 2004 05 22 Revised by MG Last read through before going to press added more examples checked summaries exercises cleaned up introduction Revision 1 0 2004 04 27 Revised by TM Initial release for LDP more exercises more markup less errors and abuse added glossary Revision 1 0 bet
3. bob in echo STRING thisisaverylongname bob in echo STRING name thisisaverylong 10 3 3 3 Replacing parts of variable names This is done using the VAR PATTERN STRING or VAR PATTERN STRING syntax The first form replaces only the first match the second replaces all matches of PATTERN with STRING bob in echo STRING name string thisisaverylongstring More information can be found in the Bash info pages 10 4 Summary Normally a variable can hold any type of data unless variables are declared explicitly Constant variables are set using the readonly built in command An array holds a set of variables If a type of data is declared then all elements in the array will be set to hold only this type of data Bash features allow for substitution and transformation of variables on the fly Standard operations include calculating the length of a variable arithmetic on variables substituting variable content and substituting part of the content 10 5 Exercises Here are some brain crackers 1 Write a script that does the following file nhome edward cache fr 60kIkP bash beginners guide html 80 112 13 7 21 Bash Guide for Beginners Display the name ot the script being executed o Display the first third and tenth argument given to the script o Display the total number of arguments passed to the script o If there were more than three positional parameters use shift to move all the va
4. 6 3 4 User defined variables Apart from the built in variables you can define your own When awk encounters a reference to a variable which does not exist which is not predefined the variable is created and initialized to a null string For all subsequent references the value of the variable is whatever value was assigned last Variables can be a string or a numeric value Content of input fields can also be assigned to variables Values can be assigned directly using the operator or you can use the current value of the variable in combination with other operators kelly octarine gt cat revenues 20021009 20021013 consultancy BigComp 20021015 20021020 training EduComp 20021112 20021123 appdev SmartComp 20021204 20021215 training EduComp kelly octarine gt cat total awk total total 5 print Send bill for 5 dollar to 4 END print nTotal revenue kelly octarine gt awk f total awk test Send bill for 2500 dollar to BigComp Send bill for 2000 dollar to EduComp Send bill for 10000 dollar to SmartComp Send bill for 5000 dollar to EduComp file nhome edward cache fr 60kIkP bash beginners guide html 2500 2000 10000 5000 total 48 112 13 7 21 Bash Guide for Beginners total revenue 195098 kelly octarine gt C like shorthands like VAR value are also accepted 6 3 5 More examples The example from Section 5 3 2 becomes much easier w
5. 13 7 21 Bash Guide for Beginners Note the use ot the date command to generate all kinds of file and directory names See the man page tor more Use the system Wat The previous example is for the sake of demonstration Regular checks can easily be achieved using the system s cron facility Do not forget to redirect output and errors when using scripts that are executed from your crontab 9 2 2 3 Using keyboard input to control the while loop This script can be interrupted by the user when a CtrH C sequence is entered bin bash This script provides wisdom FORTUNE usr games fortune while true do echo On which topic do you want advice cat lt lt topics politics startrek kernelnewbies sports bofh excuses magic love literature drugs education topics echo echo n Make your choice read topic echo echo Free advice on the topic of topic echo FORTUNE topic echo done A here document is used to present the user with possible choices And again the true test repeats the commands from the CONSEQUENT COMMANDS list over and over again 9 2 2 4 Calculating an average This script calculates the average of user input which is tested before it is processed if input is not within range a message is printed If q is pressed the loop exits bin bash Calculate the average of a series of numbers SCORE 0 AVERAGE 0 SUM 0 NUM 0 while true do echo n Enter your score 0
6. There might be situations when you don t want users of your scripts to exit untimely using keyboard abort sequences for example because input has to be provided or cleanup has to be done The trap statement catches these sequences and can be programmed to execute a list of commands upon catching those signals The syntax for the trap statement is straightforward trap COMMANDS SIGNALS This instructs the trap command to catch the listed SIGNALS which may be signal names with or without the SIG prefix or signal numbers If a signal is 0 or EXIT the COMMANDS are executed when the shell exits If one of the signals is DEBUG the list of COMMANDS is executed after every simple command A signal may also be specified as ERR in that case COMMANDS are executed each time a simple command exits with a non zero status Note that these commands will not be executed when the non zero exit status comes from part of an if statement or from a while or until loop Neither will they be executed if a logical AND amp amp or OR result in a non zero exit code or when a command s return status is inverted using the operator The return status of the trap command itself is zero unless an invalid signal specification is encountered The trap command takes a couple of options which are documented in the Bash info pages Here is a very simple example catching Ctrl C from the user upon which a message is printed When you try to kill this program without
7. Table A 1 Common Shell Features Command Meaning gt Redirect output b gt Append to file lt Redirect input lt lt Here document redirect input Pipe output amp Run process in background Separate commands on same line i Match any character s in filename Match single character in filename Match any characters enclosed Execute in subshell Substitute output of enclosed command ae Partial quote allows variable and command expansion Full quote no expansion Quote following character var Use value for variable Process id 0 Command name n nth argument n from 0 to 9 All arguments as a simple word Begin comment bg Background execution break Break from loop statements cd Change directories continue Resume a program loop echo Display output eval Evaluate arguments exec Execute a new shell fg Foreground execution file home edward cache fr 6oklkP bash beginners guide html 87 112 Bash Guide for Beginners 13 7 21 jobs Show active jobs kill Terminate running jobs newgrp Change to a new group shift Shift positional parameters stop Suspend a background job suspend Suspend a foreground job time Time a comma
8. gt gt LOGFILE bzip2 TAR echo done gt gt LOGFILE file nhome edward cache fr 60kIkP bash beginners guide html images nRubbish removed gt gt LOGFILE 83 112 13 7 21 Bash Guide for Beginners cuUNU LUpysiy LU pOLNVEN e pLUUI LLL scp BZIP SERVER RDIR gt dev null 2 gt amp 1 echo done gt gt LOGFILE echo e Done backing up Bash course n FILES nRubbish removed gt gt LOGFILE rm BZIP DAY date w if DAY lt 2 then echo It is date A only backing up Bash course gt gt LOGFILE bupbash else buplinux bupbash fi echo e Remote backup date SUCCESS n gt gt LOGFILE This script runs from cron meaning without user interaction so we redirect standard error from the sep command to dev null It might be argued that all the separate steps can be combined in a command such as tar c dir_to_backup bzip2 ssh server cat gt backup tar bz2 However if you are interested in intermediate results which might be recovered upon failure of the script this is not what you want The expression command amp gt file is equivalent to command gt file 2 gt amp 1 11 3 Summary Functions provide an easy way of grouping commands that you need to execute repetitively When a function is running the positional parameters are changed to those of the function When it stops they are reset to those of the calling prog
9. gt Invoking Bash 1 2 2 2 Bash startup files Startup files are scripts that are read and executed by Bash when it starts The following subsections describe different ways to start the shell and the startup files that are read consequently 1 2 2 2 1 Invoked as an interactive login shell or with login Interactive means you can enter commands The shell is not running because a script has been activated A login shell means that you got the shell after authenticating to the system usually by giving your user name and password Files read e etc profile e bash_profile bash_login or profile first existing readable file is read e bash_logout upon logout Error messages are printed if configuration files exist but are not readable If a file does not exist bash searches for the next 1 2 2 2 2 Invoked as an interactive non login shell A non login shell means that you did not have to authenticate to the system For instance when you open a terminal using an icon or a menu item that is a non login shell Files read e bashrc This file is usually referred to in bash_profile if f bashrc then bashrc fi See Chapter 7 for more information on the if construct 1 2 2 2 3 Invoked non interactively All scripts use non interactive shells They are programmed to do certain tasks and cannot be instructed to do other jobs than those for which they are programmed Files read e defined by BASH_
10. html 75 112 13 7 21 Bash Guide for Beginners Lyuy a 7j ViInkcknvAn vea bob in declare p OTHERVAR declare OTHERVAR blah As soon as you restrict assignment of values to a variable it can only hold that type of data Possible restrictions are either integer constant or array See the Bash info pages for information on return status 10 1 3 Constants In Bash constants are created by making a variable read only The readonly built in marks each specified variable as unchangeable The syntax is readonly OPTION VARIABLE s The values of these variables can then no longer be changed by subsequent assignment If the f option is given each variable refers to a shell function see Chapter 11 If a is specified each variable refers to an array of variables If no arguments are given or if p is supplied a list of all read only variables is displayed Using the p option the output can be reused as input The return status is zero unless an invalid option was specified one of the variables or functions does not exist or f was supplied for a variable name instead of for a function name bob in readonly TUX penguinpower bob in TUX Mickeysoft bash TUX readonly variable 10 2 Array variables 10 2 1 Creating arrays An array is a variable containing multiple values Any variable may be used as an array There is no maximum limit to the size of an array nor any requirement that member variables be index
11. the most recently completed foreground process The following example shows a simple test anny gt if eq 0 J More input gt then echo That was a good job More input gt fi That was a good job anny gt The following example demonstrates that TEST COMMANDS might be any UNIX command that returns an exit status and that if again returns an exit status of zero anny gt if grep USER etc passwd More input gt then echo your user account is not managed locally fi your user account is not managed locally anny gt echo 0 anny gt The same result can be obtained as follows file nhome edward cache fr 60kIkP bash beginners guide html 52 112 13 7 21 Bash Guide for Beginners anny gt grep USEK etC passwa anny gt if ne then echo not a local account fi not a local account anny gt 7 1 2 2 Numeric comparisons The examples below use numerical comparisons anny gt num we l work txt anny gt echo num 201 anny gt if num gt 150 More input gt then echo echo you ve worked hard enough for today More input gt echo fi you ve worked hard enough for today anny gt This script is executed by cron every Sunday If the week number is even it reminds you to put out the garbage cans bin bash Calculate the week number using the date command WEEKOFFSET date V 2 Test if we have a remainder If not this is an even we
12. to assure everyone the effective freedom to copy and redistribute it with or without modifying it either commercially or noncommercially Secondarily this License preserves for the author and publisher a way to get credit for their work while not being considered responsible for modifications made by others This License is a kind of copyleft which means that derivative works of the document must themselves be free in the same sense It complements the GNU General Public License which is a copyleft license designed for free software We have designed this License in order to use it for manuals for free software because free software needs free documentation a free program should come with manuals providing the same freedoms that the software does But this License is not limited to software manuals it can be used for any textual work regardless of subject matter or whether it is published as a printed book We recommend this License principally for works whose purpose is instruction or reference B 2 Applicability and definitions This License applies to any manual or other work that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License The Document below refers to any such manual or work Any member of the public is a licensee and is addressed as you A Modified Version of the Document means any work containing the Document or a portion of it either copied verbatim or with
13. we can also divide them in categories according to the sort of content the variable contains In this respect variables come in 4 types e String variables e Integer variables Constant variables e Array variables We ll discuss these types in Chapter 10 For now we will work with integer and string values for our variables 3 2 2 Creating variables Variables are case sensitive and capitalized by default Giving local variables a lowercase name is a convention which is sometimes applied However you are free to use the names you want or to mix cases Variables can also contain digits but a name starting with a digit is not allowed prompt gt export lnumber 1 bash export 1number 1 not a valid identifier To set a variable in the shell use VARNAME value Putting spaces around the equal sign will cause errors It is a good habit to quote content strings when assigning values to variables this will reduce the chance that you make errors Some examples using upper and lower cases numbers and spaces franky franky 2 franky franky Franky franky franky Franky franky gt MYVAR1 2 7gt echo MYVAR1 gt first_name Franky gt echo first_name gt full_name Franky M Singh gt echo full_name M Singh gt MYVAR 2 2 bash MYVAR 2 2 command not found franky gt MYVAR1 2 bash MYVAR1 command not found franky gt MYVAR1 2 bash
14. 1 11064 franky gt echo 11064 franky gt echo 0 bash franky gt echo 0 franky gt ls doesnotexist ls doesnotexist No such file or directory franky gt echo 1 franky gt User franky starts entering the grep command which results in the assignment of the _ variable The process ID of his shell is 10662 After putting a job in the file home edward cache fr 60kIkP bash beginners guide html 28 112 13 7 21 Bash Guide for Beginners background the holds the process ID of the backgrounded job The shell running is bash When a mistake is made holds an exit code different from 0 zero 3 2 6 Script recycling with variables Apart from making the script more readable variables will also enable you to faster apply a script in another environment or for another purpose Consider the following example a very simple script that makes a backup of franky s home directory to a remote server bin bash This script makes a backup of my home directory cd home This creates the archive tar cf var tmp home_franky tar franky gt dev null 2 gt amp 1 First remove the old bzip2 file Redirect errors because this generates some if the archive does not exist Then create a new compressed file rm var tmp home_franky tar bz2 2 gt dev null bzip2 var tmp home_franky tar Copy the file to another host we have ssh keys for making this work without intervention scp var tmp home_fran
15. 100 q for quit read SCORE if SCORE lt 0 SCORE gt 100 then echo Be serious Common try again elif SCORE q then echo Average rating AVERAGE break else SUM SUM SCORE NUM NUM 1 AVERAGE SUM NUM fi done echo Exiting Note how the variables in the last lines are left unquoted in order to do arithmetic 9 3 The until loop 9 3 1 What is it The until loop is very similar to the while loop except that the loop executes until the TEST COMMAND executes successfully As long as this command fails the loop continues The syntax is the same as for the while loop file home edward cache fr 6oklkP bash beginners guide html 69 112 13 7 21 Bash Guide for Beginners until LES 1 COMMAND do CONSEQUEN T COMMANDS done The return status is the exit status of the last command executed in the CONSEQUENT COMMANDS list or zero if none was executed TEST COMMAND can again be any command that can exit with a success or failure status and CONSEQUENT COMMANDS can be any UNIX command script or shell construct As we already explained previously the may be replaced with one or more newlines wherever it appears 9 3 2 Example An improved picturesort sh script see Section 9 2 2 2 which tests for available disk space If not enough disk space is available remove pictures from the previous months bin bash This script copies files from my home
16. 11 22 06 CEST 2003 franky gt echo I d say Go for it I d say Go for it franky gt echo More input gt franky gt echo 3 3 5 ANSI C quoting Words in the form STRING are treated in a special way The word expands to a string with backslash escaped characters replaced as specified by the ANSI C standard Backslash escape sequences can be found in the Bash documentation 3 3 6 Locales A double quoted string preceded by a dollar sign will cause the string to be translated according to the current locale If the current locale is C or POSIX the dollar sign is ignored If the string is translated and replaced the replacement is double quoted 3 4 Shell expansion 3 4 1 General After the command has been split into tokens see Section 1 4 1 1 these tokens or words are expanded or resolved There are eight kinds of expansion performed which we will discuss in the next sections in the order that they are expanded After all expansions quote removal is performed 3 4 2 Brace expansion Brace expansion is a mechanism by which arbitrary strings may be generated Patterns to be brace expanded take the form of an optional PREAMBLE followed by a series of comma separated strings between a pair of braces followed by an optional POSTSCRIPT The preamble is prefixed to each string contained within the braces and the postscript is then appended to each resulting string expanding left to right
17. 12 5 8 12 5 Submit id hOELAlLtn020413 for anny Tue 14 Jan 2003 22 10 47 0100 Date Tue 14 Jan 2003 22 10 47 0100 From Anny lt anny octarine gt Message Id lt 200301142110 hOELALtn020413 octarine gt To anny octarine Subject disk report Tue Jan 14 22 10 47 CET 2003 Start thinking about cleaning out some stuff There s a partition that is 87 full anny testdir gt Of course you could have opened your mail program to check the results this is just to demonstrate that the script sends a decent mail with To Subject and From header lines file nome edward cache fr 60kIkP bash beginners guide html 57 112 13 7 21 Bash Guide for Beginners Many more examples using case statements can be found in your system s init script directory The startup scripts use start and stop cases to run or stop system processes A theoretical example can be found in the next section 7 3 2 Initscript example Initscripts often make use of case statements for starting stopping and querying system services This is an excerpt of the script that starts Anacron a daemon that runs commands periodically with a frequency specified in days case 1 in start start stop stop status status anacron restart stop start condrestart if test x pidof anacron x then stop start fi echo Usage 0 start stop restart condrestart status exit 1 esac The tasks to execute in each case such as stopp
18. A colon separated list of patterns used to decide which command lines should be saved in the history list HISTSIZE The maximum number of commands to remember on the history list default is 500 HOSTFILE Contains the name of a file in the same format as etc hosts that should be read when the shell needs to complete a hostname HOSTNAME The name of the current host HOSTTYPE A string describing the machine Bash is running on IGNOREEOF Controls the action of the shell on receipt of an EOF character as the sole input INPUTRC The name of the Readline initialization file overriding the default etc inputrc LANG Used to determine the locale category for any category not specifically selected with a variable starting with LC_ LC_ALL This variable overrides the value of LANG and any other LC_ variable specifying a locale category This variable determines the collation order used when sorting the results of file name expansion and determines the behavior of range PC COLLATE expressions equivalence classes and collating sequences within file name expansion and pattern matching LC CTYPE This variable determines the interpretation of characters and the behavior of character classes within file name expansion and pattern matching LC_MESSAGES This variable determines the locale used to translate double quoted strings preceded by a sign LC_NUMERIC This variable determines the
19. Brace expansions may be nested The results of each expanded string are not sorted left to right order is preserved franky gt echo spfel il al l spell spill spall file nhome edward cache fr 60kIkP bash beginners guide html 30 112 13 7 21 Bash Guide for Beginners Brace expansion is performed before any other expansions and any characters special to other expansions are preserved in the result It is strictly textual Bash does not apply any syntactic interpretation to the context of the expansion or the text between the braces To avoid conflicts with parameter expansion the string is not considered eligible for brace expansion A correctly formed brace expansion must contain unquoted opening and closing braces and at least one unquoted comma Any incorrectly formed brace expansion is left unchanged 3 4 3 Tilde expansion If a word begins with an unquoted tilde character all of the characters up to the first unquoted slash or all characters if there is no unquoted slash are considered a tilde prefix If none of the characters in the tilde prefix are quoted the characters in the tilde prefix following the tilde are treated as a possible login name If this login name is the null string the tilde is replaced with the value of the HOME shell variable If HOME is unset the home directory of the user executing the shell is substituted instead Otherwise the tilde prefix is replaced with the home directory associated wit
20. COMMAND LIST for the first match are executed The symbol is used for separating multiple patterns and the operator terminates a pattern list Each case plus its according commands are called a clause Each clause must be terminated with Each case statement is ended with the esac statement In the example we demonstrate use of cases for sending a more selective warning message with the disktest sh script anny testdir gt cat disktest sh bin bash This script does a very simple test for checking disk space space df h awk print 5 grep grep v Use sort n tail 1 cut d f1 case space in 1 6 Message AlLl is quiet 7 81 Message Start thinking about cleaning out some stuff There s a partition that is space full 9 1 8 Message Better hurry with that new disk One partition is space full 99 Message I m drowning here There s a partition at space Message I seem to be running with an nonexitent amount of disk space r esac echo Message mail s disk report date anny anny testdir gt You have new mail anny testdir gt tail 16 var spool mail anny From anny octarine Tue Jan 14 22 10 47 2003 Return Path lt anny octarine gt Received from octarine localhost 127 0 0 1 by octarine 8 12 5 8 12 5 with ESMTP id hOELAlLBG020414 for lt anny octarine gt Tue 14 Jan 2003 22 10 47 0100 Received from anny localhost by octarine 8
21. If there is not enough diskspace exit with an error message in the log file file home edward cache fr 60kIkP bash beginners guide html 58 112 13 7 21 Bash Guide for Beginners The script should clean up the compressed archive betore it exits Chapter 8 Writing interactive scripts In this chapter we will discuss how to interact with the users of our scripts e Printing user friendly messages and explanations e Catching user input e Prompting for user input e Using the file descriptors to read from and write to multiple files 8 1 Displaying user messages 8 1 1 Interactive or not Some scripts run without any interaction from the user at all Advantages of non interactive scripts include e The script runs in a predictable way every time e The script can run in the background Many scripts however require input from the user or give output to the user as the script is running The advantages of interactive scripts are among others More flexible scripts can be built e Users can customize the script as it runs or make it behave in different ways The script can report its progress as it runs When writing interactive scripts never hold back on comments A script that prints appropriate messages is much more user friendly and can be more easily debugged A script might do a perfect job but you will get a whole lot of support calls if it does not inform the user about what it is doing So include messages that tell th
22. LOCKFILE amp amp exit 0 Upon exit remove lockfile trap rm f LOCKFILE exit 255 EXIT touch LOCKFILE makewhatis u w file nhome edward cache fr 60kIkP bash beginners guide html 86 112 13 7 21 Bash Guide for Beginners CALL U 12 3 Summary Signals can be sent to your programs using the kill command or keyboard shortcuts These signals can be caught upon which action can be performed using the trap statement Some programs ignore signals The only signal that no program can ignore is the KILL signal 12 4 Exercises A couple of practical examples 1 Create a script that writes a boot image to a diskette using the dd utility If the user tries to interrupt the script using Ctrl C display a message that this action will make the diskette unusable 2 Write a script that automates the installation of a third party package of your choice The package must be downloaded from the Internet It must be decompressed unarchived and compiled if these actions are appropriate Only the actual installation of the package should be uninterruptable Appendix A Shell Features This document gives an overview of common shell features the same in every shell flavour and differing shell features shell specific features A 1 Common features The following features are standard in every shell Note that the stop suspend jobs bg and fg commands are only available on systems that support job control
23. So much erors all these erors are making me sick This is a line not containing any errors This is the last line sandy gt As you notice sed prints the entire file but the lines containing the search string are printed twice This is not what we want In order to only print those lines matching our pattern use the n option sandy gt sed n erors p example It is a text with erors Lots of erors So much erors all these erors are making me sick sandy gt 5 2 2 Deleting lines of input containing a pattern We use the same example text file Now we only want to see the lines not containing the search string sandy gt sed erors d example This is the first line of an example text This is a line not containing any errors This is the last line sandy gt The d command results in excluding lines from being displayed Matching lines starting with a given pattern and ending in a second pattern are showed like this sandy gt sed n This errors p example This is a line not containing any errors file home edward cache fr 6oklkP bash beginners guide html 41 112 13 7 21 Bash Guide for Beginners sandy gt 5 2 3 Ranges of lines This time we want to take out the lines containing the errors In the example these are lines 2 to 4 Specify this range to address together with the d command sandy gt sed 2 4d example This is the first line of an example text This is a line no
24. about a here document 1t 1s supposed to be a construct within the same script 1 nis 1s an example that installs a package automatically eventhough you should normally confirm bin bash This script installs packages automatically using yum if lt 1 then echo Usage 0 package exit 1 fi yum install 1 lt lt CONFIRM y CONFIRM And this is how the script runs When prompted with the Is this ok y N string the script answers y automatically root picon bin install sh tuxracer Gathering header information file s from server s Server Fedora Linux 2 i386 core Server Fedora Linux 2 i386 freshrpms Server JPackage 1 5 for Fedora Core 2 Server JPackage 1 5 generic Server Fedora Linux 2 i386 updates Finding updated packages Downloading needed headers Resolving dependencies Dependencies resolved I will do the following install tuxracer 0 61 26 i386 Is this ok y N EnterDownloading Packages Running test transaction Test transaction complete Success tuxracer 100 done 1 1 Installed tuxracer 0 61 26 1386 Transaction s Complete 8 3 Summary In this chapter we learned how to provide user comments and how to prompt for user input This is usually done using the echo read combination We also discussed how files can be used as input and output using file descriptors and redirection and how this can be combined with getting input from the user We stressed the importance
25. and output are accomplished by integer handles that track all open files for a given process These numeric values are known as file descriptors The best known file descriptors are stdin stdout and stderr with file descriptor numbers 0 and 2 respectively These numbers and respective devices are reserved Bash can take TCP or UDP ports on networked hosts as file descriptors as well The output below shows how the reserved file descriptors point to actual devices michel gt ls l dev std Lrwxrwxrwx 1 root root 17 Oct 2 07 46 dev stderr gt proc self fd 2 Lrwxrwxrwx 1 root root 17 Oct 2 07 46 dev stdin gt proc self fd 0 Lrwxrwxrwx 1 root root 17 Oct 2 07 46 dev stdout gt proc self fd 1 michel gt ls l proc self fd 0 2 Lrwx 1 michel michel 64 Jan 23 12 11 proc self fd 0 gt dev pts 6 Lrwx 1 michel michel 64 Jan 23 12 11 proc self fd 1 gt dev pts 6 Lrwx 1 michel michel 64 Jan 23 12 11 proc self fd 2 gt dev pts 6 You might want to check info MAKEDEYV and info proc for more information about proc subdirectories and the way your system handles standard file descriptors for each running process file nhome edward cache fr 60kIkP bash beginners guide html 62 112 13 7 21 Bash Guide for Beginners When you run a script trom the command line nothing much changes because the child shell process will use the same file descriptors as the parent When no such parent is available
26. and returns an exit status of 127 e Ifthe search was successful or if the command contains slashes the shell executes the command in a separate execution environment e Ifexecution fails because the file is not executable and not a directory it is assumed to be a shell script e Ifthe command was not begun asynchronously the shell waits for the command to complete and collects its exit status 1 4 1 8 Shell scripts When a file containing shell commands is used as the first non option argument when invoking Bash without c or s this will create a non interactive shell This shell first searches for the script file in the current directory then looks in PATH if the file cannot be found there 1 5 Developing good scripts file nhome edward cache fr 60kIkP bash beginners guide html 12 112 13 7 21 Bash Guide for Beginners 1 5 1 Properties of good scripts This guide is mainly about the last shell building block scripts Some general considerations before we continue 1 A script should run without errors 2 It should perform the task for which it is intended 3 Program logic is clearly defined and apparent 4 A script does not do unnecessary work 5 Scripts should be reusable 1 5 2 Structure The structure of a shell script is very flexible Even though in Bash a lot of freedom is granted you must ensure correct logic flow control and efficiency so that users executing the script can do so easily and correctly When
27. appears within double quotes word splitting and file name expansion are not performed on the results 3 4 6 Arithmetic expansion Arithmetic expansion allows the evaluation of an arithmetic expression and the substitution of the result The format for arithmetic expansion is EXPRESSION The expression is treated as if it were within double quotes but a double quote inside the parentheses is not treated specially All tokens in the expression undergo parameter expansion command substitution and quote removal Arithmetic substitutions may be nested Evaluation of arithmetic expressions is done in fixed width integers with no check for overflow although division by zero is trapped and recognized as an error The operators are roughly the same as in the C programming language In order of decreasing precedence the list looks like this Table 3 4 Arithmetic operators Operator Meaning VAR and VAR variable post increment and post decrement VAR and VAR variable pre increment and pre decrement and unary minus and plus and logical and bitwise negation ie exponentiation and multiplication division remainder and laddition subtraction lt lt and gt gt left and right bitwise shifts k gt lt and gt Ss comparison operators si and equality and inequality amp bitwise AND
28. as root 4 Edit the leaptest sh script from Section 7 2 4 so that it requires one argument the year Test that exactly one argument is supplied 5 Write a script called whichdaemon sh that checks if the httpd and init daemons are running on your system If an httpd is running the script should print a message like This machine is running a web server Use ps to check on processes 6 Write a script that makes a backup of your home directory on a remote machine using sep The script should report in a log file for instance log homebackup log If you don t have a second machine to copy the backup to use sep to test copying it to the localhost This requires SSH keys between the two hosts or else you have to supply a password The creation of SSH keys is explained in man ssh keygen The script should use tar cf for the creation of the backup and gzip or bzip2 for compressing the tar file Put all filenames in variables Put the name of the remote server and the remote directory in a variable This will make it easier to re use the script or to make changes to it in the future The script should check for the existence of a compressed archive If this exists remove it first in order to prevent output generation The script should also check for available diskspace Keep in mind that at any given moment you could have the data in your home directory the data in the tar file and the data in the compressed archive all together on your disk
29. be the only person reading your code A lot of users and system administrators run scripts that were written by other people If they want to see how you did it comments are useful to enlighten the reader file nhome edward cache fr 60kIkP bash beginners guide html 17 112 13 7 21 Bash Guide for Beginners Comments also make your own life easier Say that you had to read a lot of man pages in order to achieve a particular result with some command that you used in your script You won t remember how it worked if you need to change your script after a few weeks or months unless you have commented what you did how you did it and or why you did it Take the script1 sh example and copy it to commented script1 sh which we edit so that the comments reflect what the script does Everything the shell encounters after a hash mark on a line is ignored and only visible upon opening the shell script file bin bash This script clears the terminal displays a greeting and gives information about currently connected users The two example variables are set and displayed clear clear terminal window echo The script starts now echo Hi USER dollar sign is used to get content of variable echo echo I will now fetch you a list of connected users echo w show who is logged on and echo what they are doing echo I m setting two variables now COLOUR black set a local shell variable VALUE 9 set a local shell varia
30. command declare echo enable help let local logout printf read shopt type typeset ulimit and unalias Special built in commands When Bash is executing in POSIX mode the special built ins differ from other built in commands in three respects 1 Special built ins are found before shell functions during command lookup 2 Ifa special built in returns an error status a non interactive shell exits 3 Assignment statements preceding the command stay in effect in the shell environment after the command completes The POSIX special built ins are break continue eval exec exit export readonly return set shift trap and unset Most of these built ins will be discussed in the next chapters For those commands for which this is not the case we refer to the Info pages 1 3 3 Executing programs from a script When the program being executed is a shell script bash will create a new bash process using a fork This subshell reads the lines from the shell script one line at a time Commands on each line are read interpreted and executed as if they would have come directly from the keyboard While the subshell processes each line of the script the parent shell waits for its child process to finish When there are no more lines in the shell script to read the subshell terminates The parent shell awakes and displays a new prompt 1 4 Building blocks 1 4 1 Shell building blocks 1 4 1 1 Shell syntax If input is not commente
31. commands to execute file home edward cache fr 60kIkP bash beginners guide html 35 112 13 7 21 Bash Guide for Beginners 3 8 Exercises For this exercise you will need to read the useradd man pages because we are going to use the etc skel directory to hold default shell configuration files which are copied to the home directory of each newly added user First we will do some general exercises on setting and displaying variables 1 2 Create 3 variables VAR1 VAR2 and VAR3 initialize them to hold the values thirteen 13 and Happy Birthday respectively Display the values of all three variables Are these local or global variables Remove VAR3 Can you see the two remaining variables in a new terminal window Edit etc profite so that all users are greeted upon login test this For the root account set the prompt to something like Danger root is doing stuff in w preferably in a bright color such as red or pink or in reverse video mode Make sure that newly created users also get a nice personalized prompt which informs them on which system in which directory they are working Test your changes by adding a new user and logging in as that user Write a script in which you assign two integer values to two variables The script should calculate the surface of a rectangle which has these proportions It should be aired with comments and generate elegant output Don t forget to chmod yo
32. contains sample bashrc files or you might find a copy in usr share doc bash 2 05b startup files This is part of the bashrc that comes with the Bash documentation alias ll ls l alias dir ls ba alias c clear alias ls ls color alias mroe more alias pdw pwd alias sl ls color pskill local pid pid ps ax grep 1 grep v grep gawk print 1 echo n killing 1 process pid kill 9 pid echo slaughtered Apart from general aliases it contains useful aliases which make commands work even if you misspell them We will discuss aliases in Section 3 5 2 This file contains a function pskill functions will be studied in detail in Chapter 11 3 1 2 Individual user configuration files I don t have these files These files might not be in your home directory by default create them if needed 3 1 2 1 bash_profile This is the preferred configuration file for configuring user environments individually In this file users can add extra configuration options or change default settings franky gt cat bash_profile FATE EEE EE bash_profile file Executed from the bash shell when you log in Se Se HE file nhome edward cache fr 60kIkP bash beginners guide html 21 112 13 7 21 Bash Guide for Beginners FEE ETE TEE EE EE source bashrc source bash_ login case 0S in IRIX stty sane dec stty erase Sun0S stty erase ii k
33. declare is the following declare OPTION s VARIABLE value The following options are used to determine the type of data the variable can hold and to assign it attributes Table 10 1 Options to the declare built in Option Meaning a Variable is an array f Use function names only i The variable is to be treated as an integer arithmetic evaluation is performed when the variable is assigned a value see Section 3 4 6 p Display the attributes and values of each variable When p is used additional options are ignored r Make variables read only These variables cannot then be assigned values by subsequent assignment statements nor can they be unset t Give each variable the trace attribute X Mark each variable for export to subsequent commands via the environment Using instead of turns off the attribute instead When used in a function declare creates local variables The following example shows how assignment of a type to a variable influences the value bob in declare i VARIABLE 12 bob in VARIABLE string bob in echo VARIABLE 0 bob in declare p VARIABLE declare i VARIABLE 0 Note that Bash has an option to declare a numeric value but none for declaring string values This is because by default if no specifications are given a variable can hold any type of data file home edward cache fr 60kIkP bash beginners guide
34. etc init d amd script where Bash s optimazation features are used RETVAL 0 amp amp touch var lock subsys amd The commands after amp amp are only executed when the test proves to be true this is a shorter way to represent an if then fi structure The return code of the function is often used as exit code of the entire script You ll see a lot of initscripts ending in something like exit RETVAL 11 1 4 Displaying functions All functions known by the current shell can be displayed using the set built in without options Functions are retained after they are used unless they are unset after use The which command also displays functions lydia cointreau which zless zless is a function zless zcat PAGER lydia cointreau echo PAGER less This is the sort of function that is typically configured in the user s shell resource configuration files Functions are more flexible than aliases and provide a simple and easy way of adapting the user environment Here s one for DOS users dir ls F color auto lF color always less r 11 2 Examples of functions in scripts 11 2 1 Recycling There are plenty of scripts on your system that use functions as a structured way of handling series of commands On some Linux systems for instance you will find the etc rc d init d functions definition file which is sourced in all init scripts Using this method common tasks such as checking if a pr
35. holders but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections You may include a translation of this License provided that you also include the original English version of this License In case of a disagreement between the translation and the original English version of this License the original English version will prevail B 10 Termination You may not copy modify sublicense or distribute the Document except as expressly provided for under this License Any other attempt to copy modify sublicense or distribute the Document is void and will automatically terminate your rights under this License However parties who have received copies or rights from you under this License will not have their licenses terminated so long as such parties remain in full compliance file nhome edward cache fr 60kIkP bash beginners guide html 91 112 13 7 21 Bash Guide for Beginners B 11 Future revisions of this license The Free Software Foundation may publish new revised versions of the GNU Free Documentation License from time to time Such new versions will be similar in spirit to the present version but may differ in detail to address new problems or concerns See http Awww gnu org copyleft Each version of the License is given a distinguishing version number If the Document specifies that a particular numbered version of this License or any later version applie
36. starting on a new script ask yourself the following questions e Will I be needing any information from the user or from the user s environment e How will I store that information e Are there any files that need to be created Where and with which permissions and ownerships e What commands will I use When using the script on different systems do all these systems have these commands in the required versions e Does the user need any notifications When and why 1 5 3 Terminology The table below gives an overview of programming terms that you need to be familiar with Table 1 1 Overview of programming terms Term What is it Command control Testing exit status of a command in order to determine whether a portion of the program should be executed Conditional branch Logical point in the program when a condition determines what happens next Logic flow The overall design of the program Determines logical sequence of tasks so that the result is successful and controlled Loop Part of the program that is performed zero or more times User input Information provided by an external source while the program is running can be stored and recalled when needed 1 5 4 A word on order and logic In order to speed up the developing process the logical order of a program should be thought over in advance This is your first step when developing a script A number of methods can be us
37. the two characters inclusive using the locale s collating sequence and character set For example in the default C locale a d is equivalent to abcd Many locales sort characters in dictionary order and in these locales a d is typically not equivalent to abcd it might be equivalent to aBbCcDd for example To obtain the traditional interpretation of bracket expressions you can use the C locale by setting the LC_ALL environment variable to the value C Finally certain named classes of characters are predefined within bracket expressions See the grep man or info pages for more information about these predefined expressions cathy gt grep yf etc group sys X 3 root bin adm tty x 5 mail x 12 mail postfix ftp x 50 nobody x 99 floppy x 19 xfs x 43 nfsnobody x 65534 postfix x 89 cathy gt ls 1 9 xml appl xml chapl xml chap2 xml chap3 xml_ chap4 xml ee In the example all the lines containing either a y or f character are first displayed followed by an example of using a range with the Is command 4 2 2 3 Wildcards Use the for a single character match If you want to get a list of all five character English dictionary words starting with c and ending in h handy for solving crosswords cathy gt grep lt c h gt usr share dict words catch clash cloth coach couch cough crash crush If you want to display lines containing the literal dot character use the
38. to display lines starting with the string root cathy gt grep root etc passwd root x 0 0 root root bin bash If we want to see which accounts have no shell assigned whatsoever we search for lines ending in cathy gt grep etc passwd news x 9 13 news var spool news To check that PATH is exported in bashrc first select export lines and then search for lines starting with the string PATH so as not to display MANPATH and other possible paths cathy gt grep export bashrc grep lt PATH export PATH bin usr lib mh lib usr bin usr local bin usr ucb usr dbin PATH Similarly gt matches the end of a word If you want to find a string that is a separate word enclosed by spaces it is better use the w as in this example where we are displaying information for the root partition cathy gt grep w etc fstab LABEL ext3 defaults 11 If this option is not used all the lines from the file system table will be displayed 4 2 2 2 Character classes A bracket expression is a list of characters enclosed by and It matches any single character in that list if the first character of the list is the caret then it matches any character NOT in the list For example the regular expression 0123456789 matches any single digit Within a bracket expression a range expression consists of two characters separated by a hyphen It matches any single character that sorts between
39. to read input from the terminal Control is returned to the CtrHY i f shell the user can foreground background or kill the process Delayed suspend is only available on operating systems supporting this feature CtrHZ The suspend signal sends a SIGTSTP to a running program thus stopping it and returning control to the shell Terminal settings wa Check your stty settings Suspend and resume of output is usually disabled if you are using modern terminal emulations The standard xterm supports Ctrl S and CtrH Q by default 12 1 2 Usage of signals with kill Most modern shells Bash included have a built in kill function In Bash both signal names and numbers are accepted as options and arguments may be job or process IDs An exit status can be reported using the l option zero when at least one signal was successfully sent non zero if an error occurred Using the kill command from usr bin your system might enable extra options such as the ability to kill processes from other than your own user ID and specifying processes by name like with pgrep and pkill Both kill commands send the TERM signal if none is given This is a list of the most common signals Table 12 2 Common kill signals Signal name Signal value Effect SIGHUP 1 Hangup SIGINT 2 Interrupt from keyboard SIGKILL 9 Kill signal SIGTERM 15 Termination signal SIGSTOP 17 19 23 Sto
40. translation to a variety of formats suitable for input to text formatters A copy made in an otherwise Transparent file format whose markup has been designed to thwart or discourage subsequent modification by readers is not Transparent A copy that is not Transparent is called Opaque Examples of suitable formats for Transparent copies include plain ASCII without markup Texinfo input format LaTeX input format SGML or XML using a publicly available DTD and standard conforming simple HTML designed for human modification Opaque formats include PostScript PDF proprietary formats that can be read and edited only by proprietary word processors SGML or XML for which the DTD and or processing tools are not generally available and the machine generated HTML produced by some word processors for output purposes only file home edward cache fr 60kIkP bash beginners guide html 89 112 13 7 21 Bash Guide for Beginners ine 11e rage means Tor a printed ooox tne uue page nsei plus sucn Iouowing pages as are needed to noia legio1y me material mis License requires to appear in the title page For works in formats which do not have any title page as such Title Page means the text near the most prominent appearance of the work s title preceding the beginning of the body of the text B 3 Verbatim copying You may copy and distribute the Document in any medium either commercially or noncommercially provided that this License the copyrigh
41. uses regular expressions e Gawk in scripts e Gawk and variables To make it more fun As with sed entire books have been written about various versions of awk This introduction is far from complete and is only intended for understanding examples in the following chapters For more information best start with the documentation that comes with GNU awk GAWK Effective AWK Programming A User s Guide for GNU Awk 6 1 Getting started with gawk 6 1 1 What is gawk Gawk is the GNU version of the commonly available UNIX awk program another popular stream editor Since the awk program is often just a link to gawk we will refer to it as awk The basic function of awk is to search files for lines or other text units containing one or more patterns When a line matches one of the patterns special actions are performed on that line Programs in awk are different from programs in most other languages because awk programs are data driven you describe the data you want to work with and then what to do when you find it Most other languages are procedural You have to describe in great detail every step the program is to take When working with file home edward cache fr 6oklkP bash beginners guide html 44 112 13 7 21 Bash Guide for Beginners procedural languages it is usually much harder to clearly describe the data your program will process For this reason awk programs are often refreshingly easy to read and write R Wha
42. usr X11R6 bin added to their paths while root gets a couple of extra directories containing system commands After being used the function is unset so that it is not retained 11 2 3 Remote backups The following example is one that I use for making backups of the files for my books It uses SSH keys for enabling the remote connection Two functions are defined buplinux and bupbash that each make a tar file which is then compressed and sent to a remote server After that the local copy is cleaned up On Sunday only bupbash is executed bin bash LOGFILE nethome tille log backupscript log echo Starting backups for date gt gt LOGFILE buplinux DIR nethome tille xml db linux basics TAR Linux tar BZIP TAR bz2 SERVER rincewind RDIR var www intra tille html training cd DIR tar cf TAR src xml src images png src images eps echo Compressing TAR gt gt LOGFILE bzip2 TAR echo done gt gt LOGFILE echo Copying to SERVER gt gt LOGFILE scp BZIP SERVER RDIR gt dev null 2 gt amp 1 echo done gt gt LOGFILE echo e Done backing up Linux course nSource files PNG and EPS rm BZIP bupbash DIR nethome tille xml db TAR Bash tar BZIP TAR bz2 FILES bash programming SERVER rincewind RDIR var www intra tille html training cd DIR tar cf TAR FILES echo Compressing TAR
43. which therefore exports its variables menu and animal anny testdir gt cat feed sh bin bash This script acts upon the exit status given by penguin sh export menu 1 export animal 2 feed nethome anny testdir penguin sh feed menu animal file nhome edward cache fr 60kIkP bash beginners guide html 56 112 13 7 21 Bash Guide for Beginners case f n 1 echo Guard You d better give m a fish less they get violent st echo Guard It s because of people like you that they are leaving earth all the time a echo Guard Buy the food that the Zoo provides for the animals you how do you think we survive echo Guard Don t forget the guide esac anny testdir gt feed sh apple penguin Tux don t like that Tux wants fish Guard You d better give m a fish less they get violent As you can see exit status codes can be chosen freely Existing commands usually have a series of defined codes see the programmer s manual for each command for more information 7 3 Using case statements 7 3 1 Simplified conditions Nested if statements might be nice but as soon as you are confronted with a couple of different possible actions to take they tend to confuse For the more complex conditionals use the case syntax case EXPRESSION in CASE1 COMMAND LIST CASE2 COMMAND LIST CASEN COMMAND LIST esac Each case is an expression matching a pattern The commands in the
44. 0000000000000000000000000Nng line but i mean really 1l00000000000000000000000000000000 lt entry gt lt row gt Additionally if you know anything about XML write a BEGIN and END script to complete the table Or do it in HTML Chapter 7 Conditional statements In this chapter we will discuss the use of conditionals in Bash scripts This includes the following topics The if statement e Using the exit status of a command e Comparing and testing input and files e if then else constructs e if then elif else constructs e Using and testing the positional parameters e Nested if statements e Boolean expressions e Using case statements 7 1 Introduction to if 7 1 1 General At times you need to specify different courses of action to be taken in a shell script depending on the success or failure of a command The if construction allows you to specify such conditions The most compact syntax of the if command is if TEST COMMANDS then CONSEQUENT COMMANDS fi The TEST COMMAND list is executed and if its return status is zero the CONSEQUENT COMMANDS list is executed The return status is the exit status of the last command executed or zero if no condition tested true The TEST COMMAND often involves numerical or string comparison tests but it can also be any command that returns a status of zero when it succeeds and some other status when it fails Unary expressions are often used to examine the status of a file If the FILE a
45. 1 32 3 gt proc 21974 fd 8 2 3 2 Redirection of errors From the previous examples it is clear that you can provide input and output files for a script see Section 8 2 4 for more but some tend to forget about redirecting errors output which might be depended upon later on Also if you are lucky errors will be mailed to you and eventual causes of failure might get revealed If you are not as lucky errors will cause your script to fail and won t be caught or sent anywhere so that you can t start to do any worthwhile debugging When redirecting errors note that the order of precedence is significant For example this command issued in var spool ls lL 2 gt var tmp unaccessible in spool will redirect output of the Is command to the file unaccessible in spool in var tmp The command ls l gt var tmp spoollist 2 gt amp 1 will direct both standard input and standard error to the file spoollist The command ls Ll 2 gt amp 1 gt var tmp spoollist directs only the standard output to the destination file because the standard error is copied to standard output before the standard output is redirected For convenience errors are often redirected to dev nul1 if it is sure they will not be needed Hundreds of examples can be found in the startup scripts for your system Bash allows for both standard output and standard error to be redirected to the file whose name is the result of the expansion of FILE with this constr
46. 1 Bash Guide for Beginners willy gt set o allexport of f braceexpand on emacs on errexit of f hashall on histexpand on history on ignoreeof of f interactive comments on keyword of f monitor on noclobber of f noexec of f noglob off nolog off notify off nounset off onecmd off physical off posix off privileged off verbose off vi off xtrace off See the Bash Info pages section Shell Built in Commands gt The Set Built in for a description of each option A lot of options have one character shorthands the xtrace option for instance is equal to specifying set x 3 6 2 Changing options Shell options can either be set different from the default upon calling the shell or be set during shell operation They may also be included in the shell resource configuration files The following command executes a script in POSIX compatible mode willy scripts gt bash posix script sh For changing the current environment temporarily or for use in a script we would rather use set Use dash for enabling an option for disabling willy test gt set o noclobber willy test gt touch test willy test gt date gt test bash test cannot overwrite existing file willy test gt set o noclobber willy test gt date gt test The above example demonstrates the noclobber option which prevents existing files from being overwritten by redirection operations The same goes for one character options for in
47. 13 7 21 Bash Guide for Beginners Bash Guide for Beginners Machtelt Garrels Xalasys com lt tille wants no spam at _ xalasys dot com gt Version 1 8 Last updated 20060315 Edition Table of Contents Introduction 1 Why this guide 2 Who should read this book 3 New versions translations and availability 4 Revision History 5 Contributions 6 7 8 9 1 Feedback Copyright information What do you need Conventions used in this document 0 Organization of this document 1 Bash and Bash scripts 1 1 Common shell programs 1 2 Advantages of the Bourne Again SHell 1 3 Executing commands 1 4 Building blocks 1 5 Developing good scripts 1 6 Summary 1 7 Exercises 2 Writing and debugging scripts 2 1 Creating and running a script 2 2 Script basics 2 3 Debugging Bash scripts 2 4 Summary 2 5 Exercises 3 The Bash environment 3 1 Shell initialization files 3 2 Variables 3 3 Quoting characters 3 4 Shell expansion 3 5 Aliases 3 6 More Bash options 3 7 Summary 3 8 Exercises 4 Regular expressions 4 1 Regular expressions 4 2 Examples using grep 4 3 Pattern matching using Bash features 4 4 Summary 4 5 Exercises 5 The GNU sed stream editor 5 1 Introduction 5 2 Interactive editing 5 3 Non interactive editing 5 4 Summary 5 5 Exercises 6 The GNU awk programming language 6 1 Getting started with gawk
48. 2 alias2 gt gt CONFIG file nhome edward cache fr 60kIkP bash beginners guide html 64 112 13 7 21 Bash Guide for Beginners EXEL USQA SQA echo n Enter comment or ENTER for no comment read comment echo comment gt gt CONFIG rm TEMP michel testdir gt sysnotes sh Output will be saved in var tmp sysconfig out Saving root account info Enter comment or ENTER for no comment hint for password blue lagoon Saving hosts information Enter comment or ENTER for no comment in central DNS michel testdir gt cat var tmp sysconfig out Your root account info root x 0 0 root root bin bash hint for password blue lagoon Your local host configuration 127 0 0 1 localhost localdomain localhost 192 168 42 1 tintagel kingarthur com tintagel in central DNS 8 2 4 3 Closing file descriptors Since child processes inherit open file descriptors it is good practice to close a file descriptor when it is no longer needed This is done using the exec fd lt amp syntax In the above example file descriptor 7 which has been assigned to standard input is closed each time the user needs to have access to the actual standard input device usually the keyboard The following is a simple example redirecting only standard error to a pipe michel gt cat listdirs sh bin bash This script prints standard output unchanged while standard error is redirected for processing by awk INP
49. 2 command not found franky gt unset MYVAR1 first_name full_name franky gt echo MYVAR1 first_name full_name lt no output gt franky 3 2 3 gt Exporting variables A variable created like the ones in the example above is only available to the current shell It is a local variable child processes of the current shell will not be aware of this variable In order to pass variables to a subshell we need to export them using the export built in command Variables that are exported are referred to as environment variables Setting and exporting is usually done in one step export VARNAME value A subshell can change variables it inherited from the parent but the changes made by the child don t affect the parent This is demonstrated in the example franky gt full_name Franky M Singh file home edward cache fr 6oklkP bash beginners guide html 25 112 13 7 21 Bash Guide for Beginners franky gt bash franky gt echo full_name franky gt exit franky gt export full_name franky gt bash franky gt echo full_name Franky M Singh franky gt export full_name Charles the Great franky gt echo full_name Charles the Great franky gt exit franky gt echo full_name Franky M Singh franky gt When first trying to read the value of full_name in a subshell it is not there echo shows a null string The subshell quits and full_name is exported in
50. 6 2 The print program 6 3 Gawk variables 6 4 Summary file nhome edward cache fr 60kIkP bash beginners guide html 1 112 13 7 21 6 5 Exercises 7 Conditional statements 7 1 Introduction to if 7 2 More advanced if usage 7 3 Using case statements 7 4 Summary 7 5 Exercises 8 Writing interactive scripts 8 1 Displaying user messages 8 2 Catching user input 8 3 Summary 8 4 Exercises 9 Repetitive tasks 9 1 The for loop 9 2 The while loop 9 3 The until loop 9 4 1 0 redirection and loops 9 5 Break and continue 9 6 Making menus with the select built in 9 7 The shift built in 9 8 Summary 9 9 Exercises 10 More on variables 10 1 Types of variables 10 2 Array variables 10 3 Operations on variables 10 4 Summary 10 5 Exercises 11 Functions 11 1 Introduction 11 2 Examples of functions in scripts 11 3 Summary 11 4 Exercises 12 Catching signals 12 1 Signals 12 2 Traps 12 3 Summary 12 4 Exercises A Shell Features A 1 Common features A 2 Differing features B GNU Free Documentation License B 1 Preamble B 2 Applicability and definitions B 3 Verbatim copying B 4 Copying in quantity B 5 Modifications B 6 Combining documents B 7 Collections of documents B 8 Aggregation with independent works B 9 Translation B 10 Termination B 11 Future revisions of this license B 12 How to use this License for yo
51. CAL_MACHINE octarine LOGNAME f ranky LS COLORS no 00 fi 00 di 01 34 Ln 01 36 pi 40 33 s0 01 35 bd 40 33 01 cd 40 33 01 o0r 01 05 37 41 mi 01 05 37 41 ex 01 32 cmd 01 32 MACHINES octarine MAILCHECK 60 MAIL var mail franky MANPATH usr man usr share man usr local man usr X11R6 man MEAN_MACHINES octarine MOZ DIST BIN usr lib mozilla MOZILLA_FIVE_HOME usr lib mozilla MOZ_PROGRAM usr lib mozilla mozilla bin MTOOLS FAT _COMPATIBILITY 1 MYMALLOC 0 NNTPPORT 119 NNTPSERVER news NPX_ PLUGIN PATH plugin ns4plugin usr lib netscape plugins OLDPWD nethome f ranky 0S Linux PAGER Less PATH nethome franky bin Linux nethome franky bin usr local bin usr local sbin usr X11R6 bin usr bin usr sbin bin sbin PS1 033 1 44m franky is in w 033 0m PS2 More input gt PwD nethome f ranky SESSION MANAGER local octarine hq xalasys com tmp ICE unix 22106 SHELL bin bash SHELL_LOGIN login SHLVL 2 SSH_AGENT PID 22161 SSH_ASKPASS usr lLibexec openssh gnome ssh askpass SSH_AUTH SOCK tmp ssh XxXmhQ4fC agent 22106 START_WM twm TERM xterm TYPE type USERNAME f ranky USER f ranky _ usr bin printenv VISUAL vi WINDOWID 20971661 XAPPLRESDIR nethome f ranky app defaults XAUTHORITY nethome franky Xauthority XENVIRONMENT nethome franky Xdefaults XFILESEARCHPATH usr X11R6 lib X11 L T N C S USr X11R6 Lib X11 1L T N C S UST X11R6 Lib X11 T N C S uSr X11R6 1Lib X11 L T XKEYS
52. ENV file nhome edward cache fr 60kIkP bash beginners guide html 7 112 13 7 21 Bash Guide for Beginners PATH is not used to search for this file so if you want to use it best refer to it by giving the full path and file name 1 2 2 2 4 Invoked with the sh command Bash tries to behave as the historical Bourne sh program while conforming to the POSIX standard as well Files read e etc profile e profile When invoked interactively the ENV variable can point to extra startup information 1 2 2 2 5 POSIX mode This option is enabled either using the set built in set o posix or by calling the bash program with the posix option Bash will then try to behave as compliant as possible to the POSIX standard for shells Setting the POSIXLY_ CORRECT variable does the same Files read e defined by ENV variable 1 2 2 2 6 Invoked remotely Files read when invoked by rshd e bashre Bw Avoid use of r tools Be aware of the dangers when using tools such as rlogin telnet rsh and rep They are intrinsically insecure because confidential data is sent over the network unencrypted If you need tools for remote execution file transfer and so on use an implementation of Secure SHell generally known as SSH freely available from http Avww openssh org Different client programs are available for non UNIX systems as well see your local software mirror 1 2 2 2 7 Invoked when UID is not equal to EUID No startu
53. F option to grep file home edward cache fr 60kIkP bash beginners guide html 38 112 13 7 21 Bash Guide for Beginners For matching multiple characters use the asterisk This example selects all words starting with c and ending in h from the system s dictionary cathy gt grep lt c h gt usr share dict words caliph cash catch cheesecloth cheetah output omitted If you want to find the literal asterisk character in a file or output use grep F cathy gt grep etc profile cathy gt grep F etc profile for i in etc profile d sh do 4 3 Pattern matching using Bash features 4 3 1 Character ranges Apart from grep and regular expressions there s a good deal of pattern matching that you can do directly in the shell without having to use an external program As you already know the asterisk and the question mark match any string or any single character respectively Quote these special characters to match them literally cathy gt touch cathy gt ls But you can also use the square braces to match any enclosed character or range of characters if pairs of characters are separated by a hyphen An example cathy gt ls ld a cx z drwxr xr x 2 cathy cathy 4096 Jul 20 2002 app defaults drwxrwxr x 4 cathy cathy 4096 May 25 2002 arabic drwxrwxr x 2 cathy cathy 4096 Mar 4 18 30 bin drwxr xr x 7 cathy cathy 4096 Sep 2 2001 crossover drwxrwxr x 3 cathy ca
54. M LOGIN IDLE JCPU PCPU WHAT root tty2 Sat 2pm 5 36m 0 24s 0 05s bash willy 0 Sat 2pm 0 00s willy pts 3 Sat 2pm 43 13 36 82s 36 82s BitchX willy ir willy pts 2 Sat 2pm 43 13 0 13s 0 06s usr bin screen echo echo I m setting two variables now I m setting two variables now COLOUR black VALUE 9 echo This is a string This is a string echo And this is a number And this is a number echo echo I m giving you back your prompt now I m giving you back your prompt now echo la Future Bash Features There is now a full fledged debugger for Bash available at SourceForge Right now however you need a patched version of bash 2 05 The new debugging features might become available in bash 3 0 2 3 2 Debugging on part s of the script file nhome edward cache fr 60kIkP bash beginners guide html 18 112 13 7 21 Bash Guide for Beginners Using the set Bash built in you can run in normal mode those portions of the script of which you are sure they are without fault and display debugging information only for troublesome zones Say we are not sure what the w command will do in the example commented script1 sh then we could enclose it in the script like this set x activate debugging from here w set x stop debugging from here Output then looks like this willy scripts gt script1 sh The script starts now Hi willy I will now fetch you a list of connected
55. NT COMMANDS command or zero if none was executed 9 2 2 Examples 9 2 2 1 Simple example using while Here is an example for the impatient bin bash This script opens 4 terminal windows i while i lt 4 do xterm amp i i 1 done 9 2 2 2 Nested while loops The example below was written to copy pictures that are made with a webcam to a web directory Every five minutes a picture is taken Every hour a new directory is created holding the images for that hour Every day a new directory is created containing 24 subdirectories The script runs in the background bin bash This script copies files from my homedirectory into the webserver directory use scp and SSH keys for a remote directory A new directory is created every hour PICSDIR home carol pics WEBDIR var www carol webcam while true do DATE date Y m d HOUR date H mkdir WEBDIR DATE while HOUR ne 00 do DESTDIR WEBDIR DATE HOUR mkdir DESTDIR mv PICDIR jpg DESTDIR sleep 3600 HOUR date H done done Note the use of the true statement This means continue execution until we are forcibly interrupted with kill or Ctrl C This small script can be used for simulation testing it generates files bin bash This generates a file every 5 minutes while true do touch pic date s jpg sleep 300 done file nome edward cache fr 60kIkP bash beginners guide html 68 112
56. UTDIR 1 exec 6 gt amp 1 ls INPUTDIR 2 gt amp 1 gt amp 6 6 gt amp Closes fd 6 for awk but not for ls awk BEGIN FS print YOU HAVE NO ACCESS TO 2 6 gt amp exec 6 gt amp 8 2 4 4 Here documents Frequently your script might call on another program or script that requires input The here document provides a way of instructing the shell to read input from the current source until a line containing only the search string is found no trailing blanks All of the lines read up to that point are then used as the standard input for a command The result is that you don t need to call on separate files you can use shell special characters and it looks nicer than a bunch of echo s michel gt cat startsurf sh bin bash This script provides an easy way for users to choose between browsers echo These are the web browsers on this system Start here document cat lt lt BROWSERS mozilla links lynx konqueror opera netscape BROWSERS End here document echo n Which is your favorite read browser echo Starting browser please wait browser amp michel gt startsurf sh These are the web browsers on this system mozilla links lynx konqueror opera netscape Which is your favorite opera Starting opera please wait file nhome edward cache fr 60kIkP bash beginners guide html 65 112 13 7 21 Bash Guide for Beginners Although we talk
57. YMDB usr X11R6 1ib X11 XKeysymDB XMODIFIERS im none XTERMID XWINHOME usr X11R6 X X11R6 YACC bison y 3 2 1 2 Local variables Local variables are only available in the current shell Using the set built in command without any options will display a list of all variables including environment variables and functions The output will be sorted according to the current locale and displayed in a reusable format Below is a diff file made by comparing printenv and set output after leaving out the functions which are also displayed by the set command franky gt diff set sorted printenv sorted grep lt awk print 2 BASE nethome franky Shell hq xalasys com octarine aliases BASH bin bash BASH_VERSINFO 0 2 BASH_VERSION 2 05b 0 1 release COLUMNS 80 DIRSTACK DO_FORTUNE EUID 504 GROUPS HERE home f ranky HISTFILE nethome franky bash_ history HOSTTYPE i686 IFS LINES 24 MACHTYPE i686 pc linux gnu OPTERR 1 OPTIND 1 OSTYPE Linux gnu PIPESTATUS 0 0 PPID 10099 file nome edward cache fr 60kIkP bash beginners guide html 24 112 13 7 21 PS4 PWD_REAL pwd SHELLOPTS braceexpand emacs hashall histexpand history interactive comments monitor THERE home f ranky UID 504 was 3 2 1 3 Awk the GNU Awk programming language is explained in Chapter 6 Variables by content Bash Guide for Beginners Apart from dividing variables in local and global variables
58. a 2003 04 20 Revised by MG Pre release 5 Contributions Thanks to all the friends who helped or tried to and to my husband your encouraging words made this work possible Thanks to all the people who submitted bug reports examples and remarks among many many others e Hans Bol one of the groupies e Mike Sim remarks on style Dan Richter for array examples Gerg Ferguson for ideas on the title e Mendel Leo Cooper for making room e linux be for keeping my feet on the ground e Frank Wang for his detailed remarks on all the things I did wrong Special thanks to Tabatha Marshall who volunteered to do a complete review and spell and grammar check We make a great team she works when I sleep And vice versa 6 Feedback Missing information missing links missing characters remarks Mail it to file nhome edward cache fr 60kIkP bash beginners guide html 4 112 13 7 21 lt tille wants no spam Bash Guide for Beginners at__xalasys dot com gt the maintainer of this document 7 Copyright information Copyright 2003 2005 Machtelt Garrels Permission is granted to copy distribute and or modify this document under the terms of the GNU Free Documentation License Version 1 1 or any later version published by the Free Software Foundation with the Invariant Sections being New versions of this document Contributions Feedback and Copyright information with no Front Cover Texts a
59. a backslash 6 2 3 The print command and regular expressions A regular expression can be used as a pattern by enclosing it in slashes The regular expression is then tested against the entire text of each record The syntax is as follows awk EXPRESSION PROGRAM file s The following example displays only local disk device information networked file systems are not shown kelly is in gt df h awk dev hd print 6 t 5 46 boot 10 opt 84 usr 97 var 73 voll 8 kelly is in gt Slashes need to be escaped because they have a special meaning to the awk program Below another example where we search the etc directory for files ending in conf and starting with either kelly is in etc gt ls l awk lt a x conf print 9 amd conf antivir conf xcdroast conf xinetd conf kelly is in etc gt x using extended regular expressions This example illustrates the special meaning of the dot in regular expressions the first one indicates that we want to search for any character after the first search string the second is escaped because it is part of a string to find the end of the file name 6 2 4 Special patterns In order to precede output with comments use the BEGIN statement kelly is in etc gt ls l awk BEGIN print Files found n lt a x conf print 9 Files found amd conf antivir conf xcdroast conf xine
60. a variable as in VARNAME option Option to a command as in the a option to the Is command argument Argument to a command as in read man Ls command options Command synopsis or general usage on a separated line arguments filename Name of a file or directory for example Change to the usr bin directory Key Keys to hit on the keyboard such as type Q to quit Button Graphical button to click like the OK button Menu gt Choice Choice to select from a graphical menu for instance Select Help gt About Mozilla in your browser Terminology Important term or concept The Linux kernel is the heart of the system The backslash in a terminal view or command synopsis indicates an unfinished line In other words if you see a long command that is cut into multiple lines means Don t press Enter yet See Chapter 1 link to related subject within this guide The author Clickable link to an external web resource 10 Organization of this document This guide discusses concepts useful in the daily life of the serious Bash user While a basic knowledge of the usage of the shell is required we start with a discussion of the basic shell components and practices in the first three chapters Chapters four to six are discussions of basic tools that are commonly used in shell scripts Chapters eight to twelve discuss the most common constru
61. abled by default History is saved in the file pointed to by HISTFILE when the shell exits By default HISTFILE points to bash_history e Alias expansion is enabled e In the absence of traps the SIGTERM signal is ignored e In the absence of traps SIGINT is caught and handled Thus typing Ctrl C for example will not quit your interactive shell e Sending SIGHUP signals to all jobs on exit is configured with the huponexit option e Commands are executed upon read e Bash checks for mail periodically e Bash can be configured to exit when it encounters unreferenced variables In interactive mode this behavior is disabled e When shell built in commands encounter redirection errors this will not cause the shell to exit e Special built ins returning errors when used in POSIX mode don t cause the shell to exit The built in commands are listed in Section 1 3 Failure of exec will not exit the shell e Parser syntax errors don t cause the shell to exit e Simple spell check for the arguments to the ed built in is enabled by default e Automatic exit after the length of time specified in the TMOUT variable has passed is enabled More information e Section 3 2 e Section 3 6 e See Chapter 12 for more about signals e Section 3 4 discusses the various expansions performed upon entering a command 1 2 2 4 Conditionals Conditional expressions are used by the compound command and by the test and built in commands Expressions may be u
62. aced by the text of the alias The alias name and the replacement text may contain any valid shell input including shell metacharacters with the exception that the alias name may not contain The first word of the replacement text is tested for aliases but a word that is identical to an alias being expanded is not expanded a second time This means that one may alias Is to Is F for instance and Bash will not try to recursively expand the replacement text If the last character of the alias value is a space or tab character then the next command word following the alias is also checked for alias expansion Aliases are not expanded when the shell is not interactive unless the expand_aliases option is set using the shopt shell built in 3 5 2 Creating and removing aliases Aliases are created using the alias shell built in For permanent use enter the alias in one of your shell initialization files if you just enter the alias on the command line it is only recognized within the current shell franky gt alias dh df h franky gt dh Filesystem Size Used Avail Use Mounted on dev hda7 1 3G 272M 1018M 22 dev hdal 121M 9 4M 105M 9 boot dev hda2 13G 8 7G 3 7G 70 home dev hda3 13G 5 3G 7 1G 43 opt none 243M 0 243M 0 dev shm dev hda6 3 9G 3 2G 572M 85 usr dev hda5 5 2G 4 3G 725M 86 var franky gt unalias dh franky gt dh bash dh command not found franky gt Bash always reads at least one comple
63. alse cathy gt grep c false etc passwd 7 cathy gt grep i ps bash grep v history home cathy bashrc PS1 033 1 44m USER is in w 033 Om With the first command user cathy displays the lines from etc passwd containing the string root Then she displays the line numbers containing this search string With the third command she checks which users are not using bash but accounts with the nologin shell are not displayed Then she counts the number of accounts that have bin false as the shell The last command displays the lines from all the files in her home directory starting with bash excluding matches containing the string history so as to exclude matches from bash_history which might contain the same string in upper or lower cases Note that the search is for the string ps and not for the command ps Now let s see what else we can do with grep using regular expressions file nhome edward cache fr 60kIkP bash beginners guide html 37 112 13 7 21 Bash Guide for Beginners 4 2 2 Grep and regular expressions Ifyou are not on Linux We use GNU grep in these examples which supports extended regular expressions GNU grep is the default on Linux systems If you are working on proprietary systems check with the V option which version you are using GNU grep can be downloaded from http gnu org directory 4 2 2 1 Line and word anchors From the previous example we now exclusively want
64. ap years anny testdir gt cat testleap sh file nhome edward cache fr 60kIkP bash beginners guide html 55 112 13 7 21 Bash Guide for Beginners VDinN vVast This script will test if we re in a leap year or not year date Y if year 400 eq 0 then echo This is a leap year February has 29 days elif year 4 eq 0 J then if year 100 ne then echo This is a leap year February has 29 days else echo This is not a leap year February has 28 days apa else echo This is not a leap year February has 28 days fi anny testdir gt date Tue Jan 14 20 37 55 CET 2003 anny testdir gt testleap sh This is not a leap year 7 2 4 Boolean operations The above script can be shortened using the Boolean operators AND amp amp and OR Figure 7 2 Example using Boolean operators wa We use the double brackets for testing an arithmetic expression see Section 3 4 6 This is equivalent to the let statement You will get stuck using angular brackets here if you try something like year 400 because here the angular brackets don t represent an actual command by themselves Among other editors gvim is one of those supporting colour schemes according to the file format such editors are useful for detecting errors in your code 7 2 5 Using the exit statement and if We already briefly met the exit statement in Section 7 2 1 3 It terminates execution of t
65. asic courses are available though they should be This is a practical guide which while not always being too serious tries to give real life instead of theoretical examples I partly wrote it because I don t get excited with stripped down and over simplified examples written by people who know what they are talking about showing some really cool Bash feature so much out of its context that you cannot ever use it in practical circumstances You can read that sort of stuff after finishing this book which contains exercises and examples that will help you survive in the real world From my experience as UNIX Linux user system administrator and trainer I know that people can have years of daily interaction with their systems without having the slightest knowledge of task automation Thus they often think that UNIX is not userfriendly and even worse they get the impression that it is slow and old fashioned This problem is another one that can be remedied by this guide 2 Who should read this book Everybody working on a UNIX or UNIX like system who wants to make life easier on themselves power users and sysadmins alike can benefit from reading this book Readers who already have a grasp of working the system using the command line will learn the ins and outs of shell scripting that ease execution of daily tasks System administration relies a great deal on shell scripting common tasks are often automated using simple scripts This document is ful
66. ble echo This is a string COLOUR display content of variable echo And this is a number VALUE display content of variable echo echo I m giving you back your prompt now echo In a decent script the first lines are usually comment about what to expect Then each big chunk of commands will be commented as needed for clarity s sake Linux init scripts as an example in your system s init d directory are usually well commented since they have to be readable and editable by everyone running Linux 2 3 Debugging Bash scripts 2 3 1 Debugging on the entire script When things don t go according to plan you need to determine what exactly causes the script to fail Bash provides extensive debugging features The most common is to start up the subshell with the x option which will run the entire script in debug mode Traces of each command plus its arguments are printed to standard output after the commands have been expanded but before they are executed This is the commented script1 sh script ran in debug mode Note again that the added comments are not visible in the output of the script willy scripts gt bash x scriptl sh clear echo The script starts now The script starts now echo Hi willy Hi willy echo echo I will now fetch you a list of connected users I will now fetch you a list of connected users echo Ww 4 50pm up 18 days 6 49 4 users load average 0 58 0 62 0 40 USER TTY FRO
67. ble with the Bourne shell and incorporating many useful features from other shells When the shell is started it reads its configuration files The most important are e etc profile e bash profile e bashrc Bash behaves different when in interactive mode and also has a POSIX compliant and a restricted mode Shell commands can be split up in three groups the shell functions shell built ins and existing commands in a directory on your system Bash supports additional built ins not found in the plain Bourne shell Shell scripts consist of these commands arranged as shell syntax dictates Scripts are read and executed line per line and should have a logical structure 1 7 Exercises These are some exercises to warm you up for the next chapter 1 Where is the bash program located on your system 2 Use the version option to find out which version you are running 3 Which shell configuration files are read when you login to your system using the graphical user interface and then opening a terminal window 4 Are the following shells interactive shells Are they login shells o A shell opened by clicking on the background of your graphical desktop selecting Terminal or such from a menu o A shell that you get after issuing the command ssh localhost o A shell that you get when logging in to the console in text mode o A shell obtained by the command xterm amp o A shell opened by the mysystem sh script o A shell that you get on a re
68. braces which are optional but serve to protect the variable to be expanded from characters immediately following it which could be interpreted as part of the name When braces are used the matching ending brace is the first not escaped by a backslash or within a quoted string and not within an embedded arithmetic expansion command substitution or parameter expansion The basic form of parameter expansion is PARAMETER The value of PARAMETER is substituted The braces are required when PARAMETER is a positional parameter with more than one digit or when PARAMETER is followed by a character that is not to be interpreted as part of its name If the first character of PARAMETER is an exclamation point Bash uses the value of the variable formed from the rest of PARAMETER as the name of the variable this variable is then expanded and that value is used in the rest of the substitution rather than the value of PARAMETER itself This is known as indirect expansion You are certainly familiar with straight parameter expansion since it happens all the time even in the simplest of cases such as the one above or the following franky gt echo SHELL bin bash The following is an example of indirect expansion franky gt echo N NNTPPORT NNTPSERVER NPX_PLUGIN PATH Note that this is not the same as echo N The following construct allows for creation of the named variable if it does not yet exist VAR value Exa
69. cache fr 60kIkP bash beginners guide html 109 112 13 7 21 Bash Guide for Beginners OCCU 1 2 2 1U read Section 8 2 1 readonly Section 10 1 3 regular expression operators Section 4 1 2 Section 5 2 Section 6 2 4 regular expressions Section 4 1 remote invocation Section 1 2 2 2 6 removing aliases Section 3 5 2 reserved variables Section 3 2 4 return Section 11 1 3 sed Section 5 1 sed editing commands Section 5 1 2 sed options Section 5 1 2 sed script Section 5 3 2 select Section 9 6 set Section 3 2 1 2 Section 3 6 1 Section 11 1 4 shift Section 9 7 signals Section 12 1 1 single quotes Section 3 3 3 source Section 2 1 3 special parameters Section 3 2 5 special variables file nhome edward cache fr 60kIkP bash beginners guide html 110 112 13 7 21 oeuuun 9 2 9 standard error Section 8 2 3 1 standard input Section 8 2 3 1 standard output Section 8 2 3 1 string comparisons Section 7 1 2 3 stty Section 12 1 1 submenu Section 9 6 2 subshell Section 2 2 1 substitution Section 10 3 3 1 Section 10 3 3 3 substring Section 10 3 3 2 syntax Section 1 4 1 1 T tesh Section 1 1 2 terminology Section 1 5 3 then Section 7 1 1 2 tilde expansion Section 3 4 4 transformation of variables Section 10 3 3 traps Section 12 2 1 true Section 9 2 2 2 U unalias Section 3 5 1 Section 3 5 2 unset Bash Guide for Beginners file n
70. camels Done feeding Guard Buy the food that the Zoo provides at the entry you Guard You want to poison them do you Leaving Thanks for visiting the Zoo hope to see you again soon michel test gt feed sh apple Usage of the feed script feed sh food on menu animal name More about escape characters can be found in Section 3 3 2 The following table gives an overview of sequences recognized by the echo command Table 8 1 Escape sequences used by the echo command Sequence Meaning a Alert bell b Backspace c Suppress trailing newline e Escape f Form feed n Newline r Carriage return t Horizontal tab v Vertical tab Backslash ONNN The eight bit character whose value is the octal value NNN zero to three octal digits NNN The eight bit character whose value is the octal value NNN one to three octal digits xHH The eight bit character whose value is the hexadecimal value one or two hexadecimal digits For more information about the printf command and the way it allows you to format output see the Bash info pages file home edward cache fr 60kIkP bash beginners guide html 60 112 13 7 21 Bash Guide for Beginners 8 2 Catching user input 8 2 1 Using the read built in command The read built in command is the counterpart of the echo and printf commands The s
71. can do syntax highlighting for other languages such as C Bash HTML etc 6 3 Gawk variables As awk is processing the input file it uses several variables Some are editable some are read only 6 3 1 The input field separator The field separator which is either a single character or a regular expression controls the way awk splits up an input record into fields The input record is scanned for character sequences that match the separator definition the fields themselves are the text between the matches The field separator is represented by the built in variable FS Note that this is something different from the IFS variable used by POSIX compliant shells The value of the field separator variable can be changed in the awk program with the assignment operator Often the right time to do this is at the beginning of execution before any input has been processed so that the very first record is read with the proper separator To do this use the special BEGIN pattern In the example below we build a command that displays all the users on your system with a description kelly is in gt awk BEGIN FS print 1 t 5 etc passwd output omitted kelly Kelly Smith franky Franky B eddy Eddy White willy William Black cathy Catherine the Great sandy Sandy Li Wong kelly is in gt In an awk script it would look like this kelly is in gt cat printnames awk BEGIN FS print 1 t 5 k
72. chine tom gt cat n mysystem sh bin bash clear echo This is information provided by mysystem sh Program starts now 2 3 4 5 echo Hello USER 6 echo 7 8 9 echo Today s date is date this is week date V echo 10 11 echo These users are currently connected 12 w cut d f 1 grep v USER sort u 13 echo 14 15 echo This is uname s running on a uname m processor 16 echo 17 18 echo This is the uptime information 19 uptime 20 echo 21 22 echo That s all folks A script always starts with the same two characters After that the shell that will execute the commands following the first line is defined This script starts with clearing the screen on line 2 Line 3 makes it print a message informing the user about what is going to happen Line 5 greets the user Lines 6 9 13 16 and 20 are only there for orderly output display purposes Line 8 prints the current date and the number of the week Line 11 is again an informative message like lines 3 18 and 22 Line 12 formats the output of the w line 15 shows operating system and CPU information Line 19 gives the uptime and load information Both echo and printf are Bash built in commands The first always exits with a 0 status and simply prints arguments followed by an end of line character on the standard output while the latter allows for definition of a formatting string and gives a non zero exit status code upon
73. compliant with a certain scheme to PHP files The conversion is done by taking out the first 25 and the last 21 lines replacing these with two PHP tags that provide header and footer lines carol octarine html cat html2php sh bin bash specific conversion script for my html files to php LIST ls html file nhome edward cache fr 60kIkP bash beginners guide html 67 112 13 7 21 Bash Guide for Beginners Tor 1 1N LLd1 Go NEWNAME 1ls i sed e s html php cat beginfile gt NEWNAME cat i sed e 1 25d tac sed e 1 21d tac gt gt NEWNAME cat endfile gt gt NEWNAME done Since we don t do a line count here there is no way of knowing the line number from which to start deleting lines until reaching the end The problem is solved using tac which reverses the lines in a file 9 2 The while loop 9 2 1 What is it The while construct allows for repetitive execution of a list of commands as long as the command controlling the while loop executes successfully exit status of zero The syntax is while CONTROL COMMAND do CONSEQUENT COMMANDS done CONTROL COMMAND can be any command s that can exit with a success or failure status The CONSEQUENT COMMANDS can be any program script or shell construct As soon as the CONTROL COMMAND fails the loop exits In a script the command following the done statement is executed The return status is the exit status of the last CONSEQUE
74. config Update or query run level information for system services chmod Change file access permissions chown Change file owner and group compress Compress files cp Copy files and directories crontab Maintain crontab files csh Open a C shell file nome edward cache fr 60kIkP bash beginners guide html 93 112 13 7 21 Bash Guide for Beginners cul Remove sections from each line of file s D date Print or set system date and time dd Convert and copy a file disk dump df Report file system disk usage dheped DHCP client daemon diff Find differences between two files dig Send domain name query packets to name servers dmesg Print or control the kernel ring buffer du Estimate file space usage E echo Display a line of text ediff Diff to English translator egrep Extended grep eject Unmount and eject removable media emacs Start the Emacs editor exec Invoke subprocess es exit Exit current shell export Add function s to the shell environment F fax2ps Convert a TIFF facsimile to PostScript file nome edward cache fr 60kIkP bash beginners guide html 94 112 13 7 21 Bash Guide for Beginners iuuat Format floppy disk fdisk Partition table manipulator for Linux fetchmail Fetch mail from a POP IMAP ETRN or ODMR capable server fg Bring a job in the foreground file Determine file type find Find files formail Mail re f
75. ct on the other hand execution file home edward cache fr 6oklkP bash beginners guide html 71 112 13 7 21 Bash Guide for Beginners resumes with PKS 1 CUOMMAND at the top of the loop 9 5 3 Examples In the following example file names are converted to lower case If no conversion needs to be done a continue statement restarts execution of the loop These commands don t eat much system resources and most likely similar problems can be solved using sed and awk However it is useful to know about this kind of construction when executing heavy jobs that might not even be necessary when tests are inserted at the correct locations in a script sparing system resources carol octarine test cat tolower sh bin bash This script converts all file names containing upper case characters into file names containing only lower cases LIST 1s for name in LIST do if name upper then continue fi ORIG name NEW echo name tr A Z a z mv ORIG NEW echo new name for ORIG is NEW done This script has at least one disadvantage it overwrites existing files The noclobber option to Bash is only useful when redirection occurs The b option to the mv command provides more security but is only safe in case of one accidental overwrite as is demonstrated in this test carol octarine test rm carol octarine test touch test Test TEST carol octarine test bash x tolowe
76. cts in shell scripts All chapters come with exercises that will test your preparedness for the next chapter e Chapter 1 Bash basics why Bash is so good building blocks first guidelines on developing good scripts e Chapter 2 Script basics writing and debugging Chapter 3 The Bash Environment initialization files variables quoting characters shell expansion order aliases options file home edward cache fr 60kIkP bash beginners guide html 5 112 13 7 21 Bash Guide for Beginners e Chapter 4 Regular expressions an introduction e Chapter 5 Sed an introduction to the sed line editor e Chapter 6 Awk introduction to the awk programming language e Chapter 7 Conditional statements constructs used in Bash to test conditions e Chapter 8 Interactive scripts making scripts user friendly catching user input e Chapter 9 Executing commands repetitively constructs used in Bash to automate command execution e Chapter 10 Advanced variables specifying variable types introduction to arrays of variables operations on variables e Chapter 11 Functions an introduction e Chapter 12 Catching signals introduction to process signalling trapping user sent signals Chapter 1 Bash and Bash scripts In this introduction module we Describe some common shells e Point out GNU Bash advantages and features Describe the shell s building blocks e Discuss Bash initialization files See how the shell executes co
77. d the shell reads it and divides it into words and operators employing quoting rules to define the meaning of each character of input Then these words and operators are translated into commands and other constructs which return an exit status available for inspection or processing The above fork and exec scheme is only applied after the shell has analyzed input in the following way e The shell reads its input from a file from a string or from the user s terminal e Input is broken up into words and operators obeying the quoting rules see Chapter 3 These tokens are separated by metacharacters Alias expansion is performed e The shell parses analyzes and substitutes the tokens into simple and compound commands e Bash performs various shell expansions breaking the expanded tokens into lists of filenames and commands and arguments e Redirection is performed if necessary redirection operators and their operands are removed from the argument list e Commands are executed e Optionally the shell waits for the command to complete and collects its exit status 1 4 1 2 Shell commands A simple shell command such as touch file1 file2 file3 consists of the command itself followed by arguments separated by spaces More complex shell commands are composed of simple commands arranged together in a variety of ways in a pipeline in which the output of one command becomes the input of a second in a loop or conditional construct or in some othe
78. dard input which s the output ot the filter command and last the tooter txt tile he special meaning of the hyphen as a command line argument to refer to the standard input or standard output is a misconception that has crept into many programs There might also be problems when specifying hyphen as the first argument since it might be interpreted as an option to the preceding command Using dev fd allows for uniformity and prevents confusion michel gt filter body txt cat header txt dev fd 0 footer txt lp In this clean example all output is additionally piped through Ip to send it to the default printer 8 2 4 2 Read and exec 8 2 4 2 1 Assigning file descriptors to files Another way of looking at file descriptors is thinking of them as a way to assign a numeric value to a file Instead of using the file name you can use the file descriptor number The exec built in command is used to assign a file descriptor to a file Use exec fdN gt file for assigning file descriptor N to file for output and exec fdN lt file for assigning file descriptor N to file for input After a file descriptor has been assigned to a file it can be used with the shell redirection operators as is demonstrated in the following example michel gt exec 4 gt result txt michel gt filter body txt cat header txt dev fd 0 footer txt gt amp 4 michel gt cat result txt This text is printed at the beginning of each print job and thanks the
79. detines how many characters to Keep starting trom the frst character atter the oftset point 1t LENG H 1s omitted the remainder ot the variable content is taken bob in export STRING thisisaverylongname bob in echo STRING 4 isaverylongname bob in echo STRING 6 5 avery VAR WORD and VAR H WORD These constructs are used for deleting the pattern matching the expansion of WORD in VAR WORD is expanded to produce a pattern just as in file name expansion If the pattern matches the beginning of the expanded value of VAR then the result of the expansion is the expanded value of VAR with the shortest matching pattern or the longest matching pattern indicated with If VAR is or the pattern removal operation is applied to each positional parameter in turn and the expansion is the resultant list If VAR is an array variable subscribed with or the pattern removal operation is applied to each member of the array in turn and the expansion is the resultant list This is shown in the examples below bob in echo ARRAY one two one three one four bob in echo ARRAY one two three four bob in echo ARRAY t one wo one hree one four bob in echo ARRAY t one wo one hree one four bob in echo ARRAY t one one one four The opposite effect is obtained using and as in this example below WORD should match a trailing portion of string
80. directory into the webserver directory A new directory is created every hour If the pics are taking up too much space the oldest are removed while true do DISKFUL df h WEBDIR grep v File awk print 5 cut d f1 until DISKFUL ge 90 do DATE date Y m d HOUR date H mkdir WEBDIR DATE while HOUR ne 00 do DESTDIR WEBDIR DATE HOUR mkdir DESTDIR mv PICDIR jpg DESTDIR sleep 3600 HOUR date H done DISKFULL df h WEBDIR grep v File awk print 5 cut d f1 done TOREMOVE find WEBDIR type d a mtime 30 for i in TOREMOVE do rm rf i done done Note the initialization of the HOUR and DISKFULL variables and the use of options with Is and date in order to obtain a correct listing for TOREMOVE 9 4 I 0 redirection and loops 9 4 1 Input redirection Instead of controlling a loop by testing the result of a command or by user input you can specify a file from which to read input that controls the loop In such cases read is often the controlling command As long as input lines are fed into the loop execution of the loop commands continues As soon as all the input lines are read the loop exits Since the loop construct is considered to be one command structure such as while TEST COMMAND do CONSEQUENT COMMANDS done the redirection should occur after the done statement so that it complies with the form command l
81. dirs F N Access directory stack var value VAR value var value a Variable assignment export var jexport VAR value ike ies nv var lSet environment variable nnnn s nn More than 9 arguments can be referenced a a se All arguments as separate words s argv Number of arguments s status Exit status of the most recently executed command s PID of most recently backgrounded process s d Current options file source file or file fite source fite Read commands in file alias x y falias x y alias x y Name x stands for command y case case case es bas Choose alternatives done done done end End a loop statement esac esac esac endsw End case or switch exit n exit n lexit n exit expr Exit with a status for do for do for do foreach Loop through variables a i dotdto nocaseglob noglob noglob Ignore substitution characters for filename generation hash hash falias t hashstat Display hashed commands tracked aliases hash cmds hash cmds falias t cmds rehash Remember command locations hash r hash r unhash Forget command locations history history history List previous commands ArrowUp Enter or fr Redo previous command Istr r str Istr Redo last command that starts with str Yemdis iyi raspend temd sixlyl cae kk aa y in most recent command starting with cmd 88 112 13 7 21 Bash Guide for Beginners
82. ds are executed because LIST does not expand to any items the return status is zero NAME can be any variable name although i is used very often LIST can be any list of words strings or numbers which can be literal or generated by any command The COMMANDS to execute can also be any operating system commands script program or shell statement The first time through the loop NAME is set to the first item in LIST The second time its value is set to the second item in the list and so on The loop terminates when NAME has taken on each of the values from LIST and no items are left in LIST 9 1 2 Examples 9 1 2 1 Using command substitution for specifying LIST items The first is a command line example demonstrating the use of a for loop that makes a backup copy of each xml file After issuing the command it is safe to start working on your sources carol octarine articles ls xml filel xml file2 xml file3 xml carol octarine articles ls xml gt list carol octarine articles for i in cat list do cp i i bak done carol octarine articles ls xml filel xml filel xml bak file2 xml file2 xml bak file3 xml file3 xml bak This one lists the files in sbin that are just plain text files and possibly scripts for i in ls sbin do file sbin i grep ASCII done 9 1 2 2 Using the content of a variable to specify LIST items The following is a specific application script for converting HTML files
83. e home edward cache fr 60kIkP bash beginners guide html 32 112 13 7 21 Bash Guide for Beginners However this will only calculate the result of EXPRESSION and do no tests franky gt echo 365 24 8760 See Section 7 1 2 2 among others for practical examples in scripts 3 4 7 Process substitution Process substitution is supported on systems that support named pipes FIFOs or the dev fd method of naming open files It takes the form of lt LIST or gt LIST The process LIST is run with its input or output connected to a FIFO or some file in dev fd The name of this file is passed as an argument to the current command as the result of the expansion If the gt LIST form is used writing to the file will provide input for LIST If the lt LIST form is used the file passed as an argument should be read to obtain the output of LIST Note that no space may appear between the lt or gt signs and the left parenthesis otherwise the construct would be interpreted as a redirection When available process substitution is performed simultaneously with parameter and variable expansion command substitution and arithmetic expansion More information in Section 8 2 3 3 4 8 Word splitting The shell scans the results of parameter expansion command substitution and arithmetic expansion that did not occur within double quotes for word splitting The shell treats each character of IFS as a delimiter and splits the
84. e If there is no section entitled History in the Document create one stating the title year authors and publisher of the Document as given on its Title Page then add an item describing the Modified Version as stated in the previous sentence J Preserve the network location if any given in the Document for public access to a Transparent copy of the Document and likewise the network locations given in the Document for previous versions it was based on These may be placed in the History section You may omit a network location for a work that was published at least four years before the Document itself or if the original publisher of the version it refers to gives permission K In any section entitled Acknowledgements or Dedications preserve the section s title and preserve in the section all the substance and tone of each of the contributor acknowledgements and or dedications given therein L Preserve all the Invariant Sections of the Document unaltered in their text and in their titles Section numbers or the equivalent are not considered part of the section titles file nome edward cache fr 60kIkP bash beginners guide html 90 112 13 7 21 Bash Guide for Beginners m Delete any secuon enuuea Endorsements Sucn a section may not be included 1N the Moaited version N Do not retitle any existing section as Endorsements or to conflict in title with any Invariant Section If the Modified Version includes new front ma
85. e Bourne Again SHell 1 2 1 Bash is the GNU shell The GNU project GNU s Not UNIX provides tools for UNIX like system administration which are free software and comply to UNIX standards Bash is an sh compatible shell that incorporates useful features from the Korn shell ksh and C shell csh It is intended to conform to the IEEE POSIX P1003 2 ISO 9945 2 Shell and Tools standard It offers functional improvements over sh for both programming and interactive use these include command line editing unlimited size command history job control shell functions and aliases indexed arrays of unlimited size and integer arithmetic in any base from two to sixty four Bash can run most sh scripts without modification Like the other GNU projects the bash initiative was started to preserve protect and promote the freedom to use study copy modify and redistribute software It is generally known that such conditions stimulate creativity This was also the case with the bash program which has a lot of extra features that other shells can t offer 1 2 2 Features only found in bash 1 2 2 1 Invocation In addition to the single character shell command line options which can generally be configured using the set shell built in command there are several multi character options that you can use We will come across a couple of the more popular options in this and the following chapters the complete list can be found in the Bash info pages Bash features
86. e user to wait for output because a calculation is being done If possible try to give an indication of how long the user will have to wait If the waiting should regularly take a long time when executing a certain task you might want to consider integrating some processing indication in the output of your script When prompting the user for input it is also better to give too much than too little information about the kind of data to be entered This applies to the checking of arguments and the accompanying usage message as well Bash has the echo and printf commands to provide comments for users and although you should be familiar with at least the use of echo by now we will discuss some more examples in the next sections 8 1 2 Using the echo built in command The echo built in command outputs its arguments separated by spaces and terminated with a newline character The return status is always zero echo takes a couple of options e e interprets backslash escaped characters e n suppresses the trailing newline As an example of adding comments we will make the feed sh and penguin sh from Section 7 2 1 2 a bit better michel test gt cat penguin sh bin bash This script lets you present different menus to Tux He will only be happy when given a fish To make it more fun we added a couple more animals if menu fish then if animal penguin then echo e Hmmmmmm fish Tux happy n elif a
87. ed one of the most common is working with lists Itemizing the list of tasks involved in a program allows you to describe each process Individual tasks can be referenced by their item number Using your own spoken language to pin down the tasks to be executed by your program will help you to create an understandable form of your program Later you can replace the everyday language statements with shell language words and constructs The example below shows such a logic flow design It describes the rotation of log files This example shows a possible repetitive loop controlled by the number of base log files you want to rotate 1 Do you want to rotate logs a Ifyes i Enter directory name containing the logs to be rotated ii Enter base name of the log file iii Enter number of days logs should be kept iv Make settings permanent in user s crontab file b Ifno go to step 3 2 Do you want to rotate another set of logs a If yes repeat step 1 file home edward cache fr 60kIkP bash beginners guide html 13 112 13 7 21 Bash Guide for Beginners b Ifno go to step 3 3 Exit The user should provide information for the program to do something Input from the user must be obtained and stored The user should be notified that his crontab will change 1 5 5 An example Bash script mysystem sh The mysystem sh script below executes some well known commands date w uname uptime to display information about you and your ma
88. ed expressions 4 2 Examples using grep 4 2 1 What is grep grep searches the input files for lines containing a match to a given pattern list When it finds a match in a line it copies the line to standard output by default or whatever other sort of output you have requested with options Though grep expects to do the matching on text it has no limits on input line length other than available memory and it can match arbitrary characters within a line If the final byte of an input file is not a newline grep silently supplies one Since newline is also a separator for the list of patterns there is no way to match newline characters in a text Some examples cathy gt grep root etc passwd root x 0 0 root root bin bash operator x 11 0 operator root sbin nologin cathy gt grep n root etc passwd 1 root x 0 0 root root bin bash 12 operator x 11 0 operator root sbin nologin cathy gt grep v bash etc passwd grep v nologin sync x 5 0 sync sbin bin sync shutdown x 6 0 shutdown sbin sbin shutdown halt x 7 0 halt sbin sbin halt news x 9 13 news var spool news mailnull x 47 47 var spool mqueue dev null xfs x 43 43 X Font Server etc X11 fs bin false rpc x 32 32 Portmapper RPC user bin false nscd x 28 28 NSCD Daemon bin false named x 25 25 Named var named bin false squid x 23 23 var spool squid dev null ldap x 55 55 LDAP User var lib ldap bin false apache x 48 48 Apache var www bin f
89. ed or assigned contiguously Arrays are zero based the first element is indexed with the number 0 Indirect declaration is done using the following syntax to declare a variable ARRAY INDEXNR value The INDEXNR is treated as an arithmetic expression that must evaluate to a positive number Explicit declaration of an array is done using the declare built in declare a ARRAYNAME A declaration with an index number will also be accepted but the index number will be ignored Attributes to the array may be specified using the declare and readonly built ins Attributes apply to all variables in the array you can t have mixed arrays Array variables may also be created using compound assignments in this format ARRAY valuel value2 valueN Each value is then in the form of indexnumber string The index number is optional If it is supplied that index is assigned to it otherwise the index of the element assigned is the number of the last index that was assigned plus one This format is accepted by declare as well If no index numbers are supplied indexing starts at zero Adding missing or extra members in an array is done using the syntax ARRAYNAME indexnumber value Remember that the read built in provides the a option which allows for reading and assigning values for member variables of an array 10 2 2 Dereferencing the variables in an array In order to refer to the content of an item in an array use curly braces This is
90. eginners guide html 84 112 13 7 21 Bash Guide for Beginners y our system contamns a Man page unsung au we avauaoie signals DUL Gepenumyg on your operaung system 1t mignot De Opened m a alerent way UN most Linux systems this will be man 7 signal When in doubt locate the exact man page and section using commands like man k signal grep list or apropos signal grep list Signal names can be found using kill l 12 1 1 2 Signals to your Bash shell In the absence of any traps an interactive Bash shell ignores SIGTERM and SIGQUIT SIGINT is caught and handled and if job control is active SIGTTIN SIGTTOU and SIGTSTP are also ignored Commands that are run as the result of a command substitution also ignore these signals when keyboard generated SIGHUP by default exits a shell An interactive shell will send a SIGHUP to all jobs running or stopped see the documentation on the disown built in if you want to disable this default behavior for a particular process Use the huponexit option for killing all jobs upon receiving a SIGHUP signal using the shopt built in 12 1 1 3 Sending signals using the shell The following signals can be sent using the Bash shell Table 12 1 Control signals in Bash Standard key combination Meaning Ctrl C The interrupt signal sends SIGINT to the job running in the foreground The delayed suspend character Causes a running process to be stopped when it attempts
91. ek so send a message Else do nothing if WEEKOFFSET eq 0 then echo Sunday evening put out the garbage cans mail s Garbage cans out your your_ domain org 7 1 2 3 String comparisons An example of comparing strings for testing the user ID if whoami root then echo You have no permission to run 0 as non root user exit 1 fi With Bash you can shorten this type of construct The compact equivalent of the above test is as follows whoami root amp amp echo you are using a non privileged account exit 1 Similar to the amp amp expression which indicates what to do if the test proves true specifies what to do if the test is false Regular expressions may also be used in comparisons anny gt gender female anny gt if gender f More input gt then echo Pleasure to meet you Madame fi Pleasure to meet you Madame anny gt Real Programmers Most programmers will prefer to use the test built in command which is equivalent to using square brackets for comparison like this test whoami root amp amp echo you are using a non privileged account exit 1 See the info pages for Bash for more information on pattern matching with the EXPRESSION and EXPRESSION constructs 7 2 More advanced if usage 7 2 1 if then else constructs 7 2 1 1 Dummy example This is the construct to use to take one course of action if the i
92. elly is in gt awk f printnames awk etc passwd output omitted Choose input field separators carefully to prevent problems An example to illustrate this say you get input in the form of lines that look like this Sandy L Wong 64 Zoo St Antwerp 2000X You write a command line or a script which prints out the name of the person in that record awk BEGIN FS print 1 2 3 inputfile But a person might have a PhD and it might be written like this Sandy L Wong PhD 64 Zoo St Antwerp 2000X Your awk will give the wrong output for this line If needed use an extra awk or sed to uniform data input formats The default input field separator is one or more whitespaces or tabs 6 3 2 The output separators file nhome edward cache fr 60kIkP bash beginners guide html 47 112 13 7 21 Bash Guide for Beginners 6 3 2 1 The output field separator Fields are normally separated by spaces in the output This becomes apparent when you use the correct syntax for the print command where arguments are separated by commas kelly octarine test gt cat test recordl datal record2 data2 kelly octarine test gt awk print 1 2 test recordidatal record2data2 kelly octarine test gt awk print 1 2 test recordl datal record2 data2 kelly octarine test gt If you don t put in the commas print will treat the items to output as one argument thus omitting the use of the default outpu
93. ers 1 2 N for this purpose refers to the number of command line arguments 0 refers to the name of the script The following is a simple example Figure 7 1 Testing of a command line argument with if Wa Here s another example using two arguments anny gt cat weight sh bin bash This script prints a message about your weight if you give it your weight in kilos and hight in centimeters weight 1 height 2 idealweight height 110 if weight le idealweight then echo You should eat a bit more fat else echo You should eat a bit more fruit fi anny gt bash x weight sh 55 169 weight 55 height 169 idealweight 59 55 le 59 echo You should eat a bit more fat You should eat a bit more fat 7 2 1 3 Testing the number of arguments The following example shows how to change the previous script so that it prints a message if more or less than 2 arguments are given anny gt cat weight sh bin bash This script prints a message about your weight if you give it your weight in kilos and hight in centimeters if 2 then echo Usage 0 weight_in_kilos length _in_ centimeters exit fi weight 1 height 2 idealweight height 110 if weight le idealweight then echo You should eat a bit more fat else echo You should eat a bit more fruit fi anny gt weight sh 70 150 You should eat a bit more fruit f
94. eting this as a separate command While your command line input can be of virtually unlimited length your monitor is not and printed paper certainly isn t Using the backslash also allows for copying and pasting of the above lines into a terminal window The h option to Is is used for supplying humanly readable size formats for bigger files The output of a long listing displaying the total amount of blocks in the directory is given when a directory is the argument This line is useless to us so we add an asterisk We also add the d option for the same reason in case asterisk expands to a directory file nhome edward cache fr 60kIkP bash beginners guide html 45 112 13 7 21 Bash Guide for Beginners The backslash in this example marks the continuation of a line See Section 3 3 2 You can take out any number of columns and even reverse the order In the example below this is demonstrated for showing the most critical partitions kelly octarine gt df h sort rnk 5 head 3 awk print Partition 6 t 5 full Partition var 86 full Partition usr 85 full Partition home 70 full kelly octarine gt The table below gives an overview of special formatting characters Table 6 1 Formatting characters for gawk Sequence Meaning a Bell character n Newline character t Tab Quotes dollar signs and other meta characters should be escaped with
95. eturn to the subject of notifying users in Chapter 8 Figure 2 1 scriptl sh Pa Write this script for yourself as well It might be a good idea to create a directory scripts to hold your scripts Add the directory to the contents of the PATH variable export PATH PATH scripts If you are just getting started with Bash use a text editor that uses different colours for different shell constructs Syntax highlighting is supported by vim gvim x emacs kwrite and many other editors check the documentation of your favorite editor Different prompts The prompts throughout this course vary depending on the author s mood This resembles much more real life situations than the standard educational prompt The only convention we stick to is that the root prompt ends in a hash mark 2 1 3 Executing the script The script should have execute permissions for the correct owners in order to be runnable When setting permissions check that you really obtained the permissions that you want When this is done the script can run like any other command willy scripts gt chmod u x script1 sh willy scripts gt ls l scriptl sh rWXrw r 1 willy willy 456 Dec 24 17 11 scriptl sh willy gt scriptl sh The script starts now Hi willy file nhome edward cache fr 60kIkP bash beginners guide html 16 112 13 7 21 Bash Guide for Beginners I will now fetch you a list of connected users 3 38pm up 18 days 5 37 4 user
96. f aliases known to the current shell franky gt alias alias cd alias cd alias cd alias PAGER less r alias Txterm export TERM xterm alias XARGS xargs r alias cdrecord cdrecord dev 0 0 0 speed 8 alias e vi alias egrep grep E file nhome edward cache fr 60kIkP bash beginners guide html 33 112 13 7 21 Bash Guide for Beginners alias ewformat fdformat n dev fd0Qul743 ewfsck alias fgrep grep F alias ftp ncftp d15 alias h history 10 alias fformat fdformat dev fd0H1440 alias j jobs l alias ksane setterm reset alias ls ls F color auto alias m less alias md mkdir alias od od Ax ta txC alias p pstree p alias ping ping vcl alias sb ssh blubber alias sl ls alias ss ssh octarine alias sss ssh C serverl us xalasys com alias sssu ssh C l root serverl us xalasys com alias tar gtar alias tmp cd tmp alias unaliasall unalias a alias vi eval resize vi alias vt100 export TERM vt100 alias which type alias xt xterm bg black fg white amp franky gt Aliases are useful for specifying the default version of a command that exists in several versions on your system or to specify default options to a command Another use for aliases is for correcting incorrect spelling The first word of each simple command if unquoted is checked to see if it has an alias If so that word is repl
97. f commands test true and another if it tests false An example freddy scripts gt gender male freddy scripts gt if gender f file nhome edward cache fr 60kIkP bash beginners guide html 53 112 13 7 21 Bash Guide for Beginners More input gt then ecno Pleasure to meet you Madame More input gt else echo How come the lady hasn t got a drink yet More input gt fi How come the lady hasn t got a drink yet freddy scripts gt Like the CONSEQUENT COMMANDS list following the then statement the ALTERNATE CONSEQUENT COMMANDS list following the else statement can hold any UNIX style command that returns an exit status Another example extending the one from Section 7 1 2 1 anny gt su Password root elegance root if grep USER etc passwd 1 gt dev null gt then echo your user account is not managed locally gt else echo your account is managed from the local etc passwd file gt fi your account is managed from the local etc passwd file root elegance root We switch to the root account to demonstrate the effect of the else statement your root is usually a local account while your own user account might be managed by a central system such as an LDAP server 7 2 1 2 Checking command line arguments Instead of setting a variable and then executing a script it is frequently more elegant to put the values for the variables on the command line We use the positional paramet
98. f the Bourne shell a set of add ons and plug ins This means that the Bourne Again shell is compatible with the Bourne shell commands that work in sh also work in bash However the reverse is not always the case All examples and exercises in this book use bash e cesh or C shell the syntax of this shell resembles that of the C programming language Sometimes asked for by programmers tesh or Turbo C shell a superset of the common C shell enhancing user friendliness and speed e ksh or the Korn shell sometimes appreciated by people with a UNIX background A superset of the Bourne shell with standard configuration a nightmare for beginning users The file etc shells gives an overview of known shells on a Linux system mia gt cat etc shells bin bash bin sh bin tcsh bin csh Your default shell is set in the etc passwd file like this line for user mia mia L2NOfqdlPrHwE 504 504 Mia Maya home mia bin bash file nhome edward cache fr 60kIkP bash beginners guide html 6 112 13 7 21 Bash Guide for Beginners To switch from one shell to another just enter the name of the new shell in the active terminal The system finds the directory where the name occurs using the PATH settings and since a shell is an executable file program the current shell activates it and it gets executed A new prompt is usually shown because each shell has its typical appearance mia gt tcsh mia post21 1 2 Advantages of th
99. failure This is the same script using the printf built in tom gt cat mysystem sh bin bash clear printf This is information provided by mysystem sh Program starts now printf Hello USER n n printf Today s date is date this is week date V n n printf These users are currently connected n w cut d f 1 grep v USER sort u printf n printf This is uname s running on a uname m processor n n printf This is the uptime information n uptime printf n printf That s all folks n Creating user friendly scripts by means of inserting messages is treated in Chapter 8 Standard location of the Bourne Again shell This implies that the bash program is installed in bin FA If stdout is not available If you execute a script from cron supply full path names and redirect output and errors Since the shell runs in non interactive mode any errors will cause the script to exit prematurely if you don t think about this The following chapters will discuss the details of the above scripts 1 5 6 Example init script An init script starts system services on UNIX and Linux machines The system log daemon the power management daemon the name and mail daemons are common examples These scripts also known as startup scripts are stored in a specific location on your system such as etc rc d init d or etc init d Init the initial process reads its configuration
100. file descriptor FD is open and refers to a terminal u FILE True if FILE exists and its SUID set user ID bit is set w FILE True if FILE True if FILE exists and is writable x FILE True if FILE exists and is executable 0 FILE True if FILE exists and is owned by the effective user ID GFILE True if FILE exists and is owned by the effective group ID L FILE True if FILE exists and is a symbolic link N FILE True if FILE exists and has been modified since it was last read S FILE True if FILE exists and is a socket ee one True if FILE1 has been changed more recently than FILE2 or if FILE1 exists and FILE2 does not ee ane True if FILE1 is older than FILE2 or is FILE2 exists and FILE1 does not see er True if FILE1 and FILE2 refer to the same device and inode numbers o OPTIONNAME True if shell option OPTIONNAME is enabled z STRING True of the length of STRING is zero nS True of the length of STRING is non zero Shor True if the strings are equal may be used instead of for strict POSIX compliance Sooke 7 True if the strings are not equal Shore gt True if STRINGI sorts before STRING2 lexicographically in the current locale Sor 7 True if STRING1 sorts after STRING2 lexicographically in the current locale ARG1 OP OP is one of eq ne 1t le gt or ge These arithmetic binary operators return true if ARG1 is equal to not equal to less
101. files and decides which services to start or stop in each run level A run level is a configuration of processes each system has a single user run level for instance for performing administrative tasks for which the system has to be in an unused state as much as possible such as recovering a critical file file nhome edward cache fr 60kIkP bash beginners guide html 14 112 13 7 21 Bash Guide for Beginners system from a backup Reboot and shutdown run levels are usually also configured The tasks to be executed upon starting a service or stopping it are listed in the startup scripts It is one of the system administrator s tasks to configure init so that services are started and stopped at the correct moment When confronted with this task you need a good understanding of the startup and shutdown procedures on your system We therefore advise that you read the man pages for init and inittab before starting on your own initialization scripts Here is a very simple example that will play a sound upon starting and stopping your machine bin bash This script is for etc rc d init d Link in rc3 d S99audio greeting and rcQ d KOlaudio greeting case 1 in start cat usr share audio at_your_service au gt dev audio stop cat usr share audio oh_no_not_again au gt dev audio esac exit 0 The case statement often used in this kind of script is described in Section 7 2 5 1 6 Summary Bash is the GNU shell compati
102. find that most practical examples for which arrays could be used are already implemented on your system using arrays however on a lower level in the C programming language in which most UNIX commands are written A good example is the Bash history built in command Those readers who are interested might check the built ins directory in the Bash source tree and take a look at fc def which is processed when compiling the built ins Another reason good examples are hard to find is that not all shells support arrays so they break compatibility After long days of searching I finally found this example operating at an Internet provider It distributes Apache web server configuration files onto hosts in a web farm bin bash Id chap10 xml v 1 8 2005 09 05 12 39 22 tille Exp Log chapl0 xml v Revision 1 8 2005 09 05 12 39 22 tille sorry made a mistake Revision 1 6 2005 03 01 19 39 20 tille removed blank tracer images added info on future debugging features more keywords minor corrections Revision 1 5 2004 12 06 12 27 09 tille changes for new domainname minor corrections Revision 1 6 2004 10 18 18 58 06 tille debugging typos removed replaced screenshots in chap9 with screen sections Revision 1 5 2004 06 24 14 02 48 tille dded tracer image Revision 1 4 2004 06 15 08 47 12 tille more markup index Revision 1 3 2004 05 22 13 34 18 tille review for fultus Revision 1 2 2004 04 26 13 24 41 tille updates by tabatha Re
103. for instance when you run a script using the cron facility the standard file descriptors are pipes or other temporary files unless some form of redirection is used This is demonstrated in the example below which shows output from a simple at script michel gt date Fri Jan 24 11 05 50 CET 2003 michel gt at 1107 warning commands will be executed using in order a SHELL b login shell c bin sh at gt ls l proc self fd gt var tmp fdtest at at gt lt EOT gt job 10 at 2003 01 24 11 07 michel gt cat var tmp fdtest at total 0 Lr x 1 michel michel 64 Jan 24 11 07 gt var spool at 0000c010959eb deleted L wx 1 michel michel 64 Jan 24 11 07 1 gt var tmp fdtest at L wx 1 michel michel 64 Jan 24 11 07 2 gt var spool at spool a0000c010959eb Ur x 1 michel michel 64 Jan 24 11 07 3 gt proc 21949 fd And one with cron michel gt crontab l DO NOT EDIT THIS FILE edit the master and reinstall tmp crontab 21968 installed on Fri Jan 24 11 30 41 2003 Cron version Id chap8 xml v 1 8 2005 09 05 12 39 22 tille Exp 32 11 ls l proc self fd gt var tmp fdtest cron michel gt cat var tmp fdtest cron total 0 Lr x 1 michel michel 64 Jan 24 11 32 0 gt pipe 124440 L wx 1 michel michel 64 Jan 24 11 32 1 gt var tmp fdtest cron L wx 1 michel michel 64 Jan 24 11 32 2 gt pipe 124441 Ur x 1 michel michel 64 Jan 24 1
104. g the search string and count the number of occurrences of the string 5 Display a list of usr share doc subdirectories containing information about shells 6 How many README files do these subdirectories contain Don t count anything in the form of README a string 7 Make a list of files in your home directory that were changed less that 10 hours ago using grep but leave out directories 8 Put these commands in a shell script that will generate comprehensible output 9 Can you find an alternative for we 1 using grep 10 Using the file system table etc fstab for instance list local disk devices 11 Make a script that checks whether a user exists in etc passwd For now you can specify the user name in the script you don t have to work with arguments and conditionals at this stage 12 Display configuration files in etc that contain numbers in their names Chapter 5 The GNU sed stream editor At the end of this chapter you will know about the following topics e What is sed e Interactive use of sed Regular expressions and stream editing e Using sed commands in scripts This is an introduction These explanations are far from complete and certainly not meant to be used as the definite user manual for sed This chapter is only included in order to show some more interesting topics in the next chapters and because every power user should have a basic knowledge of things that can be done with this editor For detailed
105. ginners invocation Section 1 2 2 1 file nhome edward cache fr 60kIkP bash beginners guide html 107 112 13 7 21 Bash Guide for Beginners kill Section 12 1 2 killall Section 12 1 2 ksh Korn shell Section 1 1 2 L length of a variable Section 10 3 2 line anchors Section 4 2 2 1 locale Section 3 3 6 locate Section 2 1 1 logic flow Section 1 5 4 login shell Section 1 2 2 2 1 M menu Section 9 6 metacharacters Section 4 1 2 N nested if statements Section 7 2 3 noglob Section 2 3 2 non interactive editing Section 5 3 non interactive shell Section 1 2 2 2 3 non login shell Section 1 2 2 2 2 numeric comparisons Section 7 1 2 2 file nhome edward cache fr 60kIkP bash beginners guide html 108 112 13 7 21 O options Section 3 6 1 output field separator Section 6 3 2 1 output record separator Section 6 3 2 2 P parameter expansion PATH Section 2 1 2 pattern matching Section 4 3 positionalparams Section 3 2 5 Section 11 1 3 POSIX Section 1 2 1 POSIX mode Section 1 2 2 2 5 primary expressions Section 7 1 1 1 printenv Section 3 2 1 1 printf Section 1 5 5 Section 6 3 6 process substitution Section 3 4 8 profile Section 3 1 2 3 prompt Section 3 1 3 Q quoting characters Section 3 3 R redirection Section 1 4 1 7 Section 3 6 2 Section 8 2 3 Section 9 4 rbash Bash Guide for Beginners file nhome edward
106. gnored franky gt date 20021226 file nhome edward cache fr 60kIkP bash beginners guide html 29 112 13 7 21 Bash Guide for Beginners franky gt echo date 20021226 franky gt echo date date In this example the variable date is created and set to hold a value The first echo displays the value of the variable but for the second the dollar sign is escaped 3 3 3 Single quotes Single quotes are used to preserve the literal value of each character enclosed within the quotes A single quote may not occur between single quotes even when preceded by a backslash We continue with the previous example franky gt echo date date 3 3 4 Double quotes Using double quotes the literal value of all characters enclosed is preserved except for the dollar sign the backticks backward single quotes and the backslash The dollar sign and the backticks retain their special meaning within the double quotes The backslash retains its meaning only when followed by dollar backtick double quote backslash or newline Within double quotes the backslashes are removed from the input stream when followed by one of these characters Backslashes preceding characters that don t have a special meaning are left unmodified for processing by the shell interpreter A double quote may be quoted within double quotes by preceding it with a backslash franky gt echo date 20021226 franky gt echo date Sun Apr 20
107. gt ls commented scripts sh scriptl sh willy scripts gt rm willy scripts gt ls commented scripts sh scriptl sh Alternatively these modes can be specified in the script itself by adding the desired options to the first line shell declaration Options can be combined as is usually the case with UNIX commands bin bash xv Once you found the buggy part of your script you can add echo statements before each command of which you are unsure so that you will see exactly where and why things don t work In the example commented script1 sh script it could be done like this still assuming that the displaying of users gives us problems echo debug message now attempting to start w command w In more advanced scripts the echo can be inserted to display the content of variables at different stages in the script so that flaws can be detected echo Variable VARNAME is now set to VARNAME file nhome edward cache fr 60kIkP bash beginners guide html 19 112 13 7 21 Bash Guide for Beginners 2 4 Summary A shell script is a reusable series of commands put in an executable text file Any text editor can be used to write scripts Scripts start with followed by the path to the shell executing the commands from the script Comments are added to a script for your own future reference and also to make it understandable for other users It is better to have too many explanations than not enough Debugging a scri
108. guage handling simple data reformatting jobs with just a few lines of code It is the free version of the general UNIX awk command This tools reads lines of input data and can easily recognize columned output The print program is the most common for filtering and formatting defined fields On the fly variable declaration is straightforward and allows for simple calculation of sums statistics and other operations on the processed input stream Variables and commands can be put in awk scripts for background processing Other things you should know about awk The language remains well known on UNIX and alikes but for executing similar tasks Perl is now more commonly used However awk has a much steeper learning curve In other words Perl is more difficult to learn Both Perl and awk share the reputation of being incomprehensible even to the actual authors of the programs that use these languages So document your code 6 5 Exercises These are some practical examples where awk can be useful 1 For the first exercise your input is lines in the following form Username Firstname Lastname Telephone number Make an awk script that will convert such a line to an LDAP record in this format dn uid Username dc example dc com cn Firstname Lastname sn Lastname telephoneNumber Telephone number Create a file containing a couple of test records and check 2 Create a Bash script using awk and standard UNIX commands that will sh
109. h In my company we have demos on our web site and every week someone has to test all of them So I have a cron job that fills an array with the possible candidates uses date W to find the week of the year and does a modulo operation to find the correct index The lucky person gets notified by e mail And this was his way of solving it bin bash This is get tester address sh First we test whether bash supports arrays Support for arrays was only added recently whotest 0 test echo Failure arrays not supported in this version of bash amp amp exit 2 Our list of candidates Feel free to add or remove candidates wholist Bob Smith lt bob example com gt Jane L Williams lt jane example com gt Eric S Raymond lt esr example com gt Larry Wall lt wall example com gt Linus Torvalds lt linus example com gt Count the number of possible testers Loop until we find an empty string count 0 while x wholist count x do count count 1 done Now we calculate whose turn it is week date W The week of the year 0 53 week week 0 Remove possible leading zero let index week count week modulo count the lucky person email wholist index Get the lucky person s e mail address echo email Output the person s e mail address This script is then used in other scripts such as
110. h the prompt even more fun See the section Controlling the Prompt in the Bash info pages 1 2 2 10 The restricted shell When invoked as rbash or with the restricted or r option the following happens e The cd built in is disabled e Setting or unsetting SHELL PATH ENV or BASH_ENV is not possible e Command names can no longer contain slashes e Filenames containing a slash are not allowed with the source built in command e The hash built in does not accept slashes with the p option e Import of functions at startup is disabled SHELLOPTS is ignored at startup Output redirection using gt gt gt lt gt amp amp gt and gt gt is disabled e The exec built in is disabled e The f and d options are disabled for the enable built in e A default PATH cannot be specified with the command built in e Turning off restricted mode is not possible When a command that is found to be a shell script is executed rbash turns off any restrictions in the shell spawned to execute the script More information e Section 3 2 e Section 3 6 e Info Bash gt Basic Shell Features gt Redirections e Section 8 2 3 advanced redirection 1 3 Executing commands 1 3 1 General Bash determines the type of program that is to be executed Normal programs are system commands that exist in compiled form on your system When such a program is executed a new process is created because Bash makes an exact copy of itself This child proce
111. h the specified login name If the tilde prefix is the value of the shell variable PWD replaces the tilde prefix If the tilde prefix is the value of the shell variable OLDPWD if it is set is substituted If the characters following the tilde in the tilde prefix consist of a number N optionally prefixed by a or a the tilde prefix is replaced with the corresponding element from the directory stack as it would be displayed by the dirs built in invoked with the characters following tilde in the tilde prefix as an argument If the tilde prefix without the tilde consists of a number without a leading or is assumed If the login name is invalid or the tilde expansion fails the word is left unchanged man Each variable assignment is checked for unquoted tilde prefixes immediately following a or In these cases tilde expansion is also performed Consequently one may use file names with tildes in assignments to PATH MAILPATH and CDPATH and the shell assigns the expanded value Example franky gt export PATH PATH testdir testdir will be expanded to HOME testdir so if HOME is var home f ranky the directory var home franky testdir will be added to the content of the PATH variable 3 4 4 Shell parameter and variable expansion The character introduces parameter expansion command substitution or arithmetic expansion The parameter name or symbol to be expanded may be enclosed in
112. hat no output is omitted here The script only stores information about the people Michel is interested in but it will always say you are added to the list unless you are already in it Other people can now start executing the script anny octarine tmp friends sh Hello anny This script will register you in Michel s friends database Enter your name and press ENTER anny Enter your gender and press ENTER f How old are you 22 Which colour of hair do you have black You are added to Michel s friends list After a while the f riends list begins to look like this tille 24 black anny 22 black katya 22 blonde maria 21 black output omitted Of course this situation is not ideal since everybody can edit but not delete Michel s files You can solve this problem using special access modes on the script file see SUID and SGID in the Introduction to Linux guide 8 2 3 Redirection and file descriptors 8 2 3 1 General As you know from basic shell usage input and output of a command may be redirected before it is executed using a special notation the redirection operators interpreted by the shell Redirection may also be used to open and close files for the current shell execution environment Redirection can also occur in a script so that it can receive input from a file for instance or send output to a file Later the user can review the output file or it may be used by another script as input File input
113. he entire script It is most often used if the input requested from the user is incorrect if a statement did not run successfully or if some other error occurred The exit statement takes an optional argument This argument is the integer exit status code which is passed back to the parent and stored in the variable A zero argument means that the script ran successfully Any other value may be used by programmers to pass back different messages to the parent so that different actions can be taken according to failure or success of the child process If no argument is given to the exit command the parent shell uses the current value of the variable Below is an example with a slightly adapted penguin sh script which sends its exit status back to the parent feed sh anny testdir gt cat penguin sh bin bash This script lets you present different menus to Tux He will only be happy when given a fish We ve also added a dolphin and presumably a camel if menu fish then if animal penguin then echo Hmmmmmm fish Tux happy elif animal dolphin then echo Pweetpeettreetppeterdepweet else echo prrrrrrrt fi else if animal penguin then echo Tux don t like that Tux wants fish exit 1 elif animal dolphin then echo Pweepwishpeeterdepweet exit 2 else echo Will you read this sign exit 3 fi fi This script is called upon in the next one
114. hen we use an awk script kelly octarine html gt cat make html from text awk BEGIN print lt html gt n lt head gt lt title gt Awk generated HTML lt title gt lt head gt n lt body bgcolor ffffff gt n lt pre gt print 0 END print lt pre gt n lt body gt n lt html gt And the command to execute is also much more straightforward when using awk instead of sed kelly octarine html gt awk f make html from text awk testfile gt file html Awk examples on your system We refer again to the directory containing the initscripts on your system Enter a command similar to the following to see more practical examples of the widely spread usage of the awk command grep awk etc init d 6 3 6 The printf program For more precise control over the output format than what is normally provided by print use printf The printf command can be used to specify the field width to use for each item as well as various formatting choices for numbers such as what output base to use whether to print an exponent whether to print a sign and how many digits to print after the decimal point This is done by supplying a string called the format string that controls how and where to print the other arguments The syntax is the same as for the C language printf statement see your C introduction guide The gawk info pages contain full explanations 6 4 Summary The gawk utility interprets a special purpose programming lan
115. her host we have ssh keys for making this work without intervention scp BZIPFILE SERVER REMOTEDIR gt dev null 2 gt amp 1 Create a timestamp in a logfile date gt LOGFILE echo backup succeeded gt LOGFILE Large directories and low bandwidth The above is purely an example that everybody can understand using a small directory and a host on the same subnet Depending on your bandwidth the size of the directory and the location of the remote server it can take an awful lot of time to make backups using this mechanism For larger directories and lower bandwidth use rsync to keep the directories at both ends synchronized 3 3 Quoting characters 3 3 1 Why A lot of keys have special meanings in some context or other Quoting is used to remove the special meaning of characters or words quotes can disable special treatment for special characters they can prevent reserved words from being recognized as such and they can disable parameter expansion 3 3 2 Escape characters Escape characters are used to remove the special meaning from a single character A non quoted backslash is used as an escape character in Bash It preserves the literal value of the next character that follows with the exception of newline If a newline character appears immediately after the backslash it marks the continuation of a line when it is longer that the width of the terminal the backslash is removed from the input stream and effectively i
116. home edward cache fr 60kIkP bash beginners guide html 111 112 13 7 21 OCCUULE J 4 4 OCUUUL 1U 4 J OCCUUL 11 1 4 until Section 9 3 user input Section 8 2 1 Section 8 2 2 user messages Section 8 1 1 Vv variables Section 3 2 Section 10 1 variable expansion Section 3 4 5 verbose Section 2 3 2 vi m Section 2 1 1 W wait Section 12 2 2 whereis Section 2 1 1 which Section 2 1 1 while Section 9 2 wildcards Section 4 2 2 3 word anchors Section 4 2 2 1 word splitting Section 3 4 9 X xtrace Section 2 3 1 Section 2 3 2 Y Z Bash Guide for Beginners file home edward cache fr 6oklkP bash beginners guide html 112 112
117. ic License to permit their use in free software Glossary This section contains an alphabetical overview of common UNIX commands More information about the usage can be found in the man or info pages A a2ps Format files for printing on a PostScript printer acroread PDF viewer adduser Create a new user or update default new user information alias Create a shell alias for a command anacron Execute commands periodically does not assume continuously running machine apropos Search the whatis database for strings apt get APT package handling utility aspell Spell checker at atq atrm Queue examine or delete jobs for later execution aumix Adjust audio mixer g awk Pattern scanning and processing language file nhome edward cache fr 60kIkP bash beginners guide html 92 112 13 7 21 Bash Guide for Beginners B bash Bourne Again SHell batch Queue examine or delete jobs for later execution bg Run a job in the background bitmap Bitmap editor and converter utilities for the X window System bzip2 A block sorting file compressor C cat Concatenate files and print to standard output cd Change directory cdp cdplay An interactive text mode program for controlling and playing audio CD Roms under Linux cdparanoia An audio CD reading utility which includes extra data verification features cdrecord Record a CD R chattr Change file attributes chgrp Change group ownership chk
118. if bitwise exclusive OR bitwise OR amp amp lo gical AND logical OR expr expr expr conditional evaluation N lt lt gt gt amp and assignments 5 separator between expressions Shell variables are allowed as operands parameter expansion is performed before the expression is evaluated Within an expression shell variables may also be referenced by name without using the parameter expansion syntax The value of a variable is evaluated as an arithmetic expression when it is referenced A shell variable need not have its integer attribute turned on to be used in an expression Constants with a leading 0 zero are interpreted as octal numbers A leading 0x or 0X denotes hexadecimal Otherwise numbers take the form BASE JN where BASE is a decimal number between 2 and 64 representing the arithmetic base and N is a number in that base If BASE is omitted then base 10 is used The digits greater than 9 are represented by the lowercase letters the uppercase letters and _ in that order If BASE is less than or equal to 36 lowercase and uppercase letters may be used interchangably to represent numbers between 10 and 35 Operators are evaluated in order of precedence Sub expressions in parentheses are evaluated first and may override the precedence rules above Wherever possible Bash users should try to use the syntax with angular brackets EXPRESSION fil
119. ile nhome edward cache fr 60kIkP bash beginners guide html 54 112 13 7 21 Bash Guide for Beginners anny gt We1lgnt sn U 4DU 355 Usage weight sh weight_in_kilos length _in_ centimeters The first argument is referred to as 1 the second as 2 and so on The total number of arguments is stored in Check out Section 7 2 5 for a more elegant way to print usage messages 7 2 1 4 Testing that a file exists This test is done in a lot of scripts because there s no use in starting a lot of programs if you know they re not going to work bin bash This script gives information about a file FILENAME 1 echo Properties for FILENAME if f FILENAME then echo Size is ls lh FILENAME awk print 5 echo Type is file FILENAME cut d f2 echo Inode number is ls i FILENAME cut d f1 echo df h FILENAME grep v Mounted awk print On 1 which is mounted as the 6 partition else echo File does not exist fi Note that the file is referred to using a variable in this case it is the first argument to the script Alternatively when no arguments are given file locations are usually stored in variables at the beginning of a script and their content is referred to using these variables Thus when you want to change a file name in a script you only need to do it once 7 2 2 if then elif else constructs 7 2 2 1 General This is the full form of
120. in QUIT echo Exiting break yo echo You picked FILENAME REPLY chmod go rwx FILENAME esac 7 done rm QUIT 9 6 2 Submenus Any statement within a select construct can be another select loop enabling a submenu s within a menu By default the PS3 variable is not changed when entering a nested select loop If you want a different prompt in the submenu be sure to set it at the appropriate time s 9 7 The shift built in 9 7 1 What does it do The shift command is one of the Bourne shell built ins that comes with Bash This command takes one argument a number The positional parameters are shifted to the left by this number N The positional parameters from N 1 to are renamed to variable names from 1 to N 1 Say you have a command that takes 10 arguments and N is 4 then 4 becomes 1 5 becomes 2 and so on 10 becomes 7 and the original 1 2 and 3 are thrown away If N is zero or greater than the total number of arguments see Section 7 2 1 2 If N is not present it is assumed to be 1 The return status is zero unless N is greater than or less than zero otherwise it is non zero 9 7 2 Examples A shift statement is typically used when the number of arguments to a command is not known in advance for instance when users can give as many arguments as they like In such cases the arguments are usually processed in a while loop with a test condition of This condition is true as l
121. information refer to the sed info and man pages 5 1 Introduction 5 1 1 What is sed A Stream EDitor is used to perform basic transformations on text read from a file or a pipe The result is sent to standard output The syntax for the sed command has no output file specification but results can be saved to a file using output redirection The editor does not modify the original input What distinguishes sed from other editors such as vi and ed is its ability to filter text that it gets from a pipeline feed You do not need to interact with the editor while it is running that is why sed is sometimes called a batch editor This feature allows use of editing commands in scripts greatly easing repetitive editing tasks When facing replacement of text in a large number of files sed is a great help 5 1 2 sed commands The sed program can perform text pattern substitutions and deletions using regular expressions like the ones used with the grep command see Section 4 2 The editing commands are similar to the ones used in the vi editor Table 5 1 Sed editing commands Command Result a Append text below current line c Change text in the current line with new text d Delete text file home edward cache fr 60kIkP bash beginners guide html 40 112 13 7 21 Bash Guide for Beginners A Insert text above current line p Print text r Read a file s Search and
122. ing and starting the daemon are defined in functions which are partially sourced from the etc rc d init d functions file See Chapter 11 for more explanation 7 4 Summary In this chapter we learned how to build conditions into our scripts so that different actions can be undertaken upon success or failure of a command The actions can be determined using the if statement This allows you to perform arithmetic and string comparisons and testing of exit code input and files needed by the script A simple if then fi test often preceeds commands in a shell script in order to prevent output generation so that the script can easily be run in the background or through the cron facility More complex definitions of conditions are usually put in a case statement Upon successful condition testing the script can explicitly inform the parent using the exit 0 status Upon failure any other number may be returned Based on the return code the parent program can take appropriate action 7 5 Exercises Here are some ideas to get you started using if in scripts 1 Use an if then elif else construct that prints information about the current month The script should print the number of days in this month and give information about leap years if the current month is February 2 Do the same using a case statement and an alternative use of the date command 3 Modify etc profile so that you get a special greeting message when you connect to your system
123. ion files 3 1 1 1 etc profile When invoked interactively with the Login option or when invoked as sh Bash reads the etc profile instructions These usually set the shell variables PATH USER MAIL HOSTNAME and HISTSIZE On some systems the umask value is configured in etc profile on other systems this file holds pointers to other configuration files such as e etc inputrc the system wide Readline initialization file where you can configure the command line bell style e the etc profile d directory which contains files configuring system wide behavior of specific programs All settings that you want to apply to all your users environments should be in this file It might look like this etc profile System wide environment and startup programs for login setup PATH PATH usr X11R6 bin No core files by default ulimit S c 0 gt dev null 2 gt amp 1 USER id un LOGNAME USER MAIL var spool mail USER HOSTNAME bin hostname file nhome edward cache fr 60kIkP bash beginners guide html 20 112 13 7 21 Bash Guide for Beginners HISTSIZE 1000 Keyboard bell display style the readline config file if z INPUTRC a f HOME inputrc then INPUTRC etc inputrc fi PS1 u h W export PATH USER LOGNAME MAIL HOSTNAME HISTSIZE INPUTRC PS1 Source initialization files for specific programs ls vim less for i in etc profile d sh do if r i then i fi do
124. is is the first line of an example text It is a text with errors Lots of errors So much errors all these errors are making me sick This is a line not containing any errors This is the last line sandy gt To insert a string at the beginning of each line of a file for instance for quoting sandy gt sed s gt example This is the first line of an example text It is a text with erors Lots of erors So much erors all these erors are making me sick This is a line not containing any errors This is the last line VVVVVV sandy gt Insert some string at the end of each line sandy gt sed s EOL example This is the first line of an example text EOL It is a text with erors E0OL Lots of erors EOL So much erors all these erors are making me sick EOL This is a line not containing any errors EOL This is the last line EOL sandy gt Multiple find and replace commands are separated with individual e options sandy gt sed e s erors errors g e s last final g example This is the first line of an example text file nhome edward cache fr 60kIkP bash beginners guide html 42 112 13 7 21 Bash Guide for Beginners lt s a text with errors Lots of errors So much errors all these errors are making me sick This is a line not containing any errors This is the final line sandy gt Keep in mind that by default sed prints its results to the standard output most likely your
125. ky tar bz2 bordeaux opt backup franky gt dev null 2 gt amp 1 Create a timestamp in a logfile date gt home franky log home_backup log echo backup succeeded gt home franky log home_backup log First of all you are more likely to make errors if you name files and directories manually each time you need them Secondly suppose franky wants to give this script to carol then carol will have to do quite some editing before she can use the script to back up her home directory The same is true if franky wants to use this script for backing up other directories For easy recycling make all files directories usernames servernames etcetera variable Thus you only need to edit a value once without having to go through the entire script to check where a parameter occurs This is an example bin bash This script makes a backup of my home directory Change the values of the variables to make the script work for you BACKUPDIR home BACKUPFILES f ranky TARFILE var tmp home_franky tar BZIPFILE var tmp home_franky tar bz2 SERVER bordeaux REMOTEDIR opt backup f ranky LOGFILE home f ranky log home_backup log cd BACKUPDIR This creates the archive tar cf TARFILE BACKUPFILES gt dev null 2 gt amp 1 First remove the old bzip2 file Redirect errors because this generates some if the archive does not exist Then create a new compressed file rm BZIPFILE 2 gt dev null bzip2 TARFILE Copy the file to anot
126. l of examples that will encourage you to write your own and that will inspire you to improve on existing scripts Prerequisites not in this course e You should be an experienced UNIX or Linux user familiar with basic commands man pages and documentation e Being able to use a text editor e Understand system boot and shutdown processes init and initscripts e Create users and groups set passwords e Permissions special modes e Understand naming conventions for devices partitioning mounting unmounting file systems e Adding removing software on your system See Introduction to Linux or your local TLDP mirror if you haven t mastered one or more of these topics Additional information can be found in your system documentation man and info pages or at the Linux Documentation Project 3 New versions translations and availability file nhome edward cache fr 60kIkP bash beginners guide html 3 112 13 7 21 Bash Guide for Beginners The most recent edition can be found at http tille xalasys com training bash You should find the same version at http tldp org LDP Bash Beginners Guide html index html This guide is available in print from Fultus com Figure 1 Bash Guide for Beginners front cover Z This guide has been translated e Chinese translation at http xiaowang net bgb cn by Wang Wei e Ukrainian translation at http docs linux org ua index php LDP Bash_beginners_guide by Yaroslav Fedevych and his team
127. left unchanged If the nullglob option is set and no matches are found the word is removed If the shell option nocasegLob is enabled the match is performed without regard to the case of alphabetic characters nu When a pattern is used for file name generation the character at the start of a file name or immediately following a slash must be matched explicitly unless the shell option dotglob is set When matching a file name the slash character must always be matched explicitly In other cases the character is not treated specially The GLOBIGNORE shell variable may be used to restrict the set of file names matching a pattern If GLOBIGNORE is set each matching file name that also matches one of the patterns in GLOBIGNORE is removed from the list of matches The file names and are always ignored even when GLOBIGNORE is set However setting GLOBIGNORE has the effect of enabling the dotglob shell option so all other file names beginning with a will match To get the old behavior of ignoring file names beginning with a make one of the patterns in GLOBIGNORE The dotglob option is disabled when GLOBIGNORE is unset 3 5 Aliases 3 5 1 What are aliases An alias allows a string to be substituted for a word when it is used as the first word of a simple command The shell maintains a list of aliases that may be set and unset with the alias and unalias built in commands Issue the alias without options to display a list o
128. line COMPREPLY An array variable from which Bash reads the possible completions generated by a shell function invoked by the programmable completion facility file home edward cache fr 60kIkP bash beginners guide html 26 112 13 7 21 Bash Guide for Beginners IDIRSTACK An array variable containing the current contents of the directory stack EUID The numeric effective user ID of the current user FCEDIT The editor used as a default by the e option to the fe built in command FIGNORE A colon separated list of suffixes to ignore when performing file name completion IFUNCNAME _ The name of any currently executing shell function GLOBIGNORE A colon separated list of patterns defining the set of file names to be ignored by file name expansion GROUPS An array variable containing the list of groups of which the current user is a member histchars Up to three characters which control history expansion quick substitution and tokenization HISTCMD The history number or index in the history list of the current command HISTCONTROL Defines whether a command is added to the history file HISTFILE The name of the file to which the command history is saved The default value is bash_history HISTFILESIZE The maximum number of lines contained in the history file defaults to 500 HISTIGNORE
129. ll discuss these expansion types in detail in Section 3 4 1 4 1 6 Redirections Before a command is executed its input and output may be redirected using a special notation interpreted by the shell Redirection may also be used to open and close files for the current shell execution environment 1 4 1 7 Executing commands When executing a command the words that the parser has marked as variable assignments preceding the command name and redirections are saved for later reference Words that are not variable assignments or redirections are expanded the first remaining word after expansion is taken to be the name of the command and the rest are arguments to that command Then redirections are performed then strings assigned to variables are expanded If no command name results variables will affect the current shell environment An important part of the tasks of the shell is to search for commands Bash does this as follows e Check whether the command contains slashes If not first check with the function list to see if it contains a command by the name we are looking for e Ifcommand is not a function check for it in the built in list e Ifcommand is neither a function nor a built in look for it analyzing the directories listed in PATH Bash uses a hash table data storage area in memory to remember the full path names of executables so extensive PATH searches can be avoided e Ifthe search is unsuccessful bash prints an error message
130. locale category used for number formatting ILINENO __ The line number in the script or shell function currently executing LINES Used by the select built in to determine the column length for printing selection lists IMACHTYPE A string that fully describes the system type on which Bash is executing in the standard GNU CPU COMPANY SYSTEM format IMAILCHECK How often in seconds that the shell should check for mail in the files specified in the MAILPATH or MAIL variables OLDPWD The previous working directory as set by the ed built in OPTERR If set to the value 1 Bash displays error messages generated by the getopts built in OSTYPE A string describing the operating system Bash is running on PIPESTATUS An array variable containing a list of exit status values from the processes in the most recently executed foreground pipeline which may contain only a single command POSIXLY CORRECT f this variable is in the environment when bash starts the shell enters POSIX mode PPID The process ID of the shell s parent process PROMPT COMMANDI f set the value is interpreted as a command to execute before the printing of each primary prompt PS1 PS3 The value of this variable is used as the prompt for the select command Defaults to PS4 The value is the prompt printed before the command line is echoed when the x option is set defaults to PWD The current
131. lues 3 places to the left o Print all the values of the remaining arguments o Print the number of arguments Test with zero one three and over ten arguments 2 Write a script that implements a simple web browser in text mode using wget and links dump to display HTML pages to the user The user has 3 choices enter a URL enter b for back and q to quit The last 10 URLs entered by the user are stored in an array from which the user can restore the URL by using the back functionality Chapter 11 Functions In this chapter we will discuss e What functions are e Creation and displaying of functions from the command line e Functions in scripts e Passing arguments to functions e When to use functions 11 1 Introduction 11 1 1 What are functions Shell functions are a way to group commands for later execution using a single name for this group or routine The name of the routine must be unique within the shell or script All the commands that make up a function are executed like regular commands When calling on a function as a simple command name the list of commands associated with that function name is executed A function is executed within the shell in which it has been declared no new process is created to interpret the commands Special built in commands are found before shell functions during command lookup The special built ins are break continue eval exec exit export readonly return set shift trap and u
132. m uptime and average load userdel Delete a user account and related files Vv vi m Start the vi improved editor vimtutor The Vim tutor vmstat Report virtual memory statistics W Show who is logged on and what they are doing wall Send a message to everybody s terminal Print the number of bytes words and lines in files which Shows the full path of shell commands who Show who is logged on who am i Print effective user ID whois Query a whois or nicname database write Send a message to another user X xauth X authority file utility xcdroast Graphical front end to cdrecord xclock Analog digital clock for X file home edward cache fr 6oklkP bash beginners guide html 102 112 13 7 21 Bash Guide for Beginners ACULISUIE Monitor system console messages with X xdm X Display Manager with support for XDMCP host chooser xdvi DVI viewer xfs X font server xhost Server access control program for X xinetd The extended Internet services daemon xload System load average display for X xlsfonts Server font list displayer for X xmms Audio player for X xpdf PDF viewer xterm Terminal emulator for X Z zeat Compress or expand files zgrep Search possibly compressed files for a regular expression zmore Filter for viewing compressed text Index A aliases Section 3 5 1 ANSI C quoting Section 3 3 5 arguments Section 7 2 1 2 arithmetic expansi
133. make a backup of his or her home directory Implement the following features o Test for the number of arguments The script should run without arguments If any arguments are present exit after printing a usage message o Determine whether the backups directory has enough free space to hold the backup o Ask the user whether a full or an incremental backup is wanted If the user does not have a full backup file yet print a message that a full backup will be taken In case of an incremental backup only do this if the full backup is not older than a week o Compress the backup using any compression tool Inform the user that the script is doing this because it might take some time during which the user might start worrying if no output appears on the screen o Print a message informing the user about the size of the compressed backup See info tar or Introduction to Linux chapter 9 Preparing your data for background information 4 Write a script called simple useradd sh that adds a local user to the system This script should o Take only one argument or else exit after printing a usage message o Check etc passwd and decide on the first free user ID Print a message containing this ID file nhome edward cache fr 60kIkP bash beginners guide html 66 112 13 7 21 Bash Guide for Beginners o Create a private group for this user checking the etc group file Print a message containing the group ID o Gather information from the ope
134. mmands e Look into some simple script examples 1 1 Common shell programs 1 1 1 General shell functions The UNIX shell program interprets user commands which are either directly entered by the user or which can be read from a file called the shell script or shell program Shell scripts are interpreted not compiled The shell reads commands from the script line per line and searches for those commands on the system see Section 1 2 while a compiler converts a program into machine readable form an executable file which may then be used in a shell script Apart from passing commands to the kernel the main task of a shell is providing a user environment which can be configured individually using shell resource configuration files 1 1 2 Shell types Just like people know different languages and dialects your UNIX system will usually offer a variety of shell types e sh or Bourne Shell the original shell still used on UNIX systems and in UNIX related environments This is the basic shell a small program with few features While this is not the standard shell it is still available on every Linux system for compatibility with UNIX programs e bash or Bourne Again shell the standard GNU shell intuitive and flexible Probably most advisable for beginning users while being at the same time a powerful tool for the advanced and professional user On Linux bash is the standard shell for common users This shell is a so called superset o
135. modifications and or translated into another language A Secondary Section is a named appendix or a front matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document s overall subject or to related matters and contains nothing that could fall directly within that overall subject For example if the Document is in part a textbook of mathematics a Secondary Section may not explain any mathematics The relationship could be a matter of historical connection with the subject or with related matters or of legal commercial philosophical ethical or political position regarding them The Invariant Sections are certain Secondary Sections whose titles are designated as being those of Invariant Sections in the notice that says that the Document is released under this License The Cover Texts are certain short passages of text that are listed as Front Cover Texts or Back Cover Texts in the notice that says that the Document is released under this License A Transparent copy of the Document means a machine readable copy represented in a format whose specification is available to the general public whose contents can be viewed and edited directly and straightforwardly with generic text editors or for images composed of pixels generic paint programs or for drawings some widely available drawing editor and that is suitable for input to text formatters or for automatic
136. mote host for which you didn t have to give the login and or password because you use SSH and maybe SSH keys 5 Can you explain why bash does not exit when you type Ctrl C on the command line 6 Display directory stack content 7 If it is not yet the case set your prompt so that it displays your location in the file system hierarchy for instance add this line to bashrc export PS1 u h w gt 8 Display hashed commands for your current shell session 9 How many processes are currently running on your system Use ps and we the first line of output of ps is not a process 10 How to display the system hostname Only the name nothing more file nhome edward cache fr 60kIkP bash beginners guide html 15 112 13 7 21 Bash Guide for Beginners Chapter 2 Writing and debugging scripts After going through this chapter you will be able to e Write a simple script Define the shell type that should execute the script e Put comments in a script e Change permissions on a script e Execute and debug a script 2 1 Creating and running a script 2 1 1 Writing and naming A shell script is a sequence of commands for which you have a repeated use This sequence is typically executed by entering the name of the script on the command line Alternatively you can use scripts to automate tasks using the cron facility Another use for scripts is in the UNIX boot and shutdown procedure where operation of daemons and services are defined i
137. mple franky gt echo FRANKY franky gt echo FRANKY Franky Franky Special parameters among others the positional parameters may not be assigned this way however We will further discuss the use of the curly braces for treatment of variables in Chapter 10 More information can also be found in the Bash info pages 3 4 5 Command substitution file nhome edward cache fr 60kIkP bash beginners guide html 31 112 13 7 21 Bash Guide for Beginners Command substitution allows the output of a command to replace the command itself Command substitution occurs when a command is enclosed like this command or like this using backticks command Bash performs the expansion by executing COMMAND and replacing the command substitution with the standard output of the command with any trailing newlines deleted Embedded newlines are not deleted but they may be removed during word splitting franky gt echo date Thu Feb 6 10 06 20 CET 2003 When the old style backquoted form of substitution is used backslash retains its literal meaning except when followed by or The first backticks not preceded by a backslash terminates the command substitution When using the COMMAND form all characters between the parentheses make up the command none are treated specially Command substitutions may be nested To nest when using the backquoted form escape the inner backticks with backslashes If the substitution
138. n a list or the ending point of a range in a list file home edward cache fr 60kIkP bash beginners guide html 36 112 13 7 21 Bash Guide for Beginners Matches the empty string at the beginning of a line also represents the characters not in the range of a list Matches the empty string at the end of a line b Matches the empty string at the edge of a word B Matches the empty string provided it s not at the edge of a word lt Match the empty string at the beginning of word gt Match the empty string at the end of word Two regular expressions may be concatenated the resulting regular expression matches any string formed by concatenating two substrings that respectively match the concatenated subexpressions Two regular expressions may be joined by the infix operator the resulting regular expression matches any string matching either subexpression Repetition takes precedence over concatenation which in turn takes precedence over alternation A whole subexpression may be enclosed in parentheses to override these precedence rules 4 1 3 Basic versus extended regular expressions In basic regular expressions the metacharacters and lose their special meaning instead use the backslashed versions A AC and Check in your system documentation whether commands using regular expressions support extend
139. n a standard shell shutdown Bring the system down sleep Wait for a given period slocate Security Enhanced version of the GNU Locate sirnn text mode Usenet client snort Network intrusion detection tool sort Sort lines of text files ssh Secure shell ssh keygen Authentication key generation stty Change and print terminal line settings su Switch user file home edward cache fr 6oklkP bash beginners guide html 100 112 13 7 21 Bash Guide for Beginners r tac Concatenate and print files in reverse tail Output the last part of files talk Talk to a user tar Archiving utility tesh Open a Turbo C shell telnet User interface to the TELNET protocol insecure tex Text formatting and typesetting time Time a simple command or give resource usage tin News reading program top Display top CPU processes touch Change file timestamps traceroute Print the route packets take to network host tripwire A file integrity checker for UNIX systems troff Format documents Tab Window Manager for the X Window System U ulimit Controll resources umask Set user file creation mask umount Unmount a file system uncompress Decompress compressed files file nome edward cache fr 60kIkP bash beginners guide html 101 112 13 7 21 Bash Guide for Beginners uuy Remove duplicate lines from a sorted file update Kernel daemon to flush dirty buffers back to disk uptime Display syste
140. n init scripts To create a shell script open a new empty file in your editor Any text editor will do vim emacs gedit dtpad et cetera are all valid You might want to chose a more advanced editor like vim or emacs however because these can be configured to recognize shell and Bash syntax and can be a great help in preventing those errors that beginners frequently make such as forgetting brackets and semi colons Put UNIX commands in the new empty file like you would enter them on the command line As discussed in the previous chapter see Section 1 3 commands can be shell functions shell built ins UNIX commands and other scripts Give your script a sensible name that gives a hint about what the script does Make sure that your script name does not conflict with existing commands In order to ensure that no confusion can rise script names often end in sh even so there might be other scripts on your system with the same name as the one you chose Check using which whereis and other commands for finding information about programs and files which a script_name whereis script_name locate script_name 2 1 2 scriptl sh In this example we use the echo Bash built in to inform the user about what is going to happen before the task that will create the output is executed It is strongly advised to inform users about what a script is doing in order to prevent them from becoming nervous because the script is not doing anything We will r
141. n is used for the EOF sequence it is more user friendly to have a break command as one of the items Any other value of the read line will set WORD to be a null string The read line is saved in the REPLY variable The RESPECTIVE COMMANDS are executed after each selection until the number representing the break is read This exits the loop file nhome edward cache fr 60kIkP bash beginners guide html 72 112 13 7 21 Bash Guide for Beginners 9 6 1 2 Examples This is a very simple example but as you can see it is not very user friendly carol octarine testdir cat private sh bin bash echo This script can make any of the files in this directory private echo Enter the number of the file you want to protect select FILENAME in do echo You picked FILENAME REPLY it is now only accessible to you chmod go rwx FILENAME done carol octarine testdir private sh This script can make any of the files in this directory private Enter the number of the file you want to protect 1 archive 20030129 2 bash 3 private sh 1 You picked archive 20030129 1 Setting the PS3 prompt and adding a possibility to quit makes it better bin bash echo This script can make any of the files in this directory private echo Enter the number of the file you want to protect PS3 Your choice QUIT QUIT THIS PROGRAM I feel safe now touch QUIT select FILENAME in do case FILENAME
142. name of the mail file The positional parameters are the words following the name of a shell script They are put into the variables 1 2 3 and so on As long as needed variables are added to an internal array holds the total number of parameters as is demonstrated with this simple script bin bash positional sh This script reads 3 positional parameters and prints them out POSPAR1 1 POSPAR2 2 POSPAR3 3 echo 1 is the first positional parameter 1 echo 2 is the second positional parameter 2 echo 3 is the third positional parameter 3 echo echo The total number of positional parameters is Upon execution one could give any numbers of arguments franky gt positional sh one two three four five one is the first positional parameter 1 two is the second positional parameter 2 three is the third positional parameter 3 The total number of positional parameters is 5 franky gt positional sh one two one is the first positional parameter 1 two is the second positional parameter 2 is the third positional parameter 3 The total number of positional parameters is 2 More on evaluating these parameters is in Chapter 7 and Section 9 7 Some examples on the other special parameters franky gt grep dictionary usr share dict words dictionary franky gt echo _ usr share dict words franky gt echo 10662 franky gt mozilla amp
143. nary or binary Unary expressions are often used to examine the status of a file You only need one object for instance a file to do the operation on There are string operators and numeric comparison operators as well these are binary operators requiring two objects to do the operation on If the FILE argument to one of the primaries is in the form dev fd N then file descriptor N is checked If the FILE argument to one of the primaries is one of dev stdin dev stdout or dev stderr then file descriptor 0 1 or 2 respectively is checked Conditionals are discussed in detail in Chapter 7 More information about the file descriptors in Section 8 2 3 1 2 2 5 Shell arithmetic The shell allows arithmetic expressions to be evaluated as one of the shell expansions or by the let built in Evaluation is done in fixed width integers with no check for overflow though division by 0 is trapped and flagged as an error The operators and their precedence and associativity are the same as in the C language see Chapter 3 1 2 2 6 Aliases Aliases allow a string to be substituted for a word when it is used as the first word of a simple command The shell maintains a list of aliases that may be set and unset with the alias and unalias commands Bash always reads at least one complete line of input before executing any of the commands on that line Aliases are expanded when a command is read not when it is executed Therefore an alias definition a
144. nd umask Set or list file permissions unset Erase variable or function definitions wait Wait for a background job to finish A 2 Differing features The table below shows major differences between the standard shell sh Bourne Again SHell bash Korn shell ksh and the C shell cesh ps Shell compatibility lea Since the Bourne Again SHell is a superset of sh all sh commands will also work in bash but not vice versa bash has many more features of its own and as the table below demonstrates many features incorporated from other shells Since the Turbo C shell is a superset of cesh all esh commands will work in tesh but not the other way round Table A 2 Differing Shell Features file home edward cache fr 60kIkP bash beginners guide html sh bash ksh cesh Meaning Action s Default user prompt gt Pi gt Force redirection gt file 5 f 5 2 gt amp 1 amp gt file or gt file 2 gt amp 1 gt file 2 gt amp 1 gt amp file Redirect stdout and stderr to file G B Expand elements in list command command or command S commana command Substitute output of enclosed command HOME HOME SHOME home Home directory F Home directory symbol
145. nd no Back Cover Texts A copy of the license is included in Appendix B entitled GNU Free Documentation License The author and publisher have made every effort in the preparation of this book to ensure the accuracy of the information However the information contained in this book is offered without warranty either express or implied Neither the author nor the publisher nor any dealer or distributor will be held liable for any damages caused or alleged to be caused either directly or indirectly by this book The logos trademarks and symbols used in this book are the properties of their respective owners 8 What do you need bash available from http Avww gnu org directory GNU The Bash shell is available on nearly every Linux system and can these days be found on a wide variety of UNIX systems Compiles easily if you need to make your own tested on a wide variety of UNIX Linux MS Windows and other systems 9 Conventions used in this document The following typographic and usage conventions occur in this text Table 1 Typographic and usage conventions Text type Meaning Quoted text Quotes from people quoted computer output terminal view Literal computer input and output captured from the terminal usually rendered with a light grey background command Name of a command that can be entered on the command line VARIABLE Name of a variable or pointer to content of
146. ne Settings for program initialization source etc java conf export NPX_PLUGIN_PATH JRE_HOME plugin ns4plugin usr lib netscape plugins PAGER usr bin less unset i This configuration file sets some basic shell environment variables as well as some variables required by users running Java and or Java applications in their web browser See Section 3 2 See Chapter 7 for more on the conditional if used in this file Chapter 9 discusses loops such as the for construct The Bash source contains sample profile files for general or individual use These and the one in the example above need changes in order for them to work in your environment 3 1 1 2 etc bashre On systems offering multiple types of shells it might be better to put Bash specific configurations in this file since etc profite is also read by other shells such as the Bourne shell Errors generated by shells that don t understand the Bash syntax are prevented by splitting the configuration files for the different types of shells In such cases the user s bashrc might point to etc bashrc in order to include it in the shell initialization process upon login You might also find that etc profile on your system only holds shell environment and program startup settings while etc bashrc contains system wide definitions for shell functions and aliases The etc bashrc file might be referred to in etc profite or in individual user shell initialization files The source
147. ne rather than newline e readline is used to obtain the line weHane read returns after reading NCHARS characters rather than waiting for a complete line of input oner Display PROMPT without a trailing newline before attempting to read any input The prompt is displayed only if input is coming from a terminal 7 If this option is given backslash does not act as an escape character The backslash is considered to be part of the line In particular a backslash newline pair may not be used as a line continuation s Silent mode If input is coming from a terminal characters are not echoed t Cause read to time out and return failure if a complete line of input is not read within TIMEOUT seconds This option has no effect if read is not reading input TIMEOUT from the terminal or from a pipe u FD Read input from file descriptor FD This is a straightforward example improving on the Leaptest sh script from the previous chapter michel test gt cat leaptest sh bin bash This script will test if you have given a leap year or not echo Type the year that you want to check 4 digits followed by ENTER read year if year 400 0 year 4 O amp amp year 100 then echo year is a leap year else echo This is not a leap year fi michel test gt leaptest sh Type the year that you want to check 4 digits followed by ENTER 2000 2000 is a leap year 8 2 2 P
148. necessary as you can see from the following example to bypass the shell interpretation of expansion operators If the index number is or all members of an array are referenced t bob in ARRAY one two three bob in echo ARRAY one two three bob in echo ARRAY one bob in echo ARRAY 2 three bob in ARRAY 3 four file nhome edward cache fr 60kIkP bash beginners guide html 76 112 13 7 21 Bash Guide for Beginners DOD Ln j ECNO PS AKKATLT IF one two three four Referring to the content of a member variable of an array without providing an index number is the same as referring to the content of the first element the one referenced with index number zero 10 2 3 Deleting array variables The unset built in is used to destroy arrays or member variables of an array bob in unset ARRAY 1 bob in echo ARRAY one three four bob in unset ARRAY bob in echo ARRAY lt no output gt 10 2 4 Examples of arrays Practical examples of the usage of arrays are hard to find You will find plenty of scripts that don t really do anything on your system but that do use arrays to calculate mathematical series for instance And that would be one of the more interesting examples most scripts just show what you can do with an array in an oversimplified and theoretical way The reason for this dullness is that arrays are rather complex structures You will
149. ners file nhome edward cache fr 60kIkP bash beginners guide html 105 112 13 7 21 peuuuL 0 1 4 ete bashre Section 3 1 1 2 ete passwd Section 1 1 2 ete profile Section 3 1 1 ete shells Section 1 1 2 exec Section 1 3 1 Section 8 2 4 2 execute permissions Section 2 1 3 execution Section 2 1 3 exit Section 7 2 5 exit status Section 7 1 2 1 expansion Section 1 4 1 5 Section 3 4 export Section 3 2 3 extended regular expressions Section 4 1 3 F file descriptors Section 8 2 3 Section 8 2 4 1 file name expansion Section 3 4 9 find and replace Section 5 2 4 for Section 9 1 fork Section 1 3 1 functions Section 11 1 1 G gawk file nhome edward cache fr 60kIkP bash beginners guide html Bash Guide for Beginners 106 112 13 7 21 Secuon 0 1 1 gawk commands Section 6 1 2 gawk fields Section 6 2 1 gawk formatting Section 6 2 2 gawk scripts Section 6 2 5 gawk variables Section 6 3 gedit Section 2 1 1 global variables Section 3 2 1 1 globbing Section 2 3 2 grep Section 4 2 1 H here document Section 8 2 4 4 if Section 7 1 1 init Section 1 3 1 Section 1 5 6 initialization files Section 3 1 input field separator Section 3 2 4 1 Section 3 2 5 Section 6 3 interactive editing Section 5 2 interactive scripts Section 8 1 interactive shell Section 1 2 2 2 1 Section 1 2 2 2 2 Section 1 2 2 3 3 Bash Guide for Be
150. nfsstat Print statistics about networked file systems nice Run a program with modified scheduling priority nmap Network exploration tool and security scanner ntsysv Simple interface for configuring run levels file home edward cache fr 60kIkP bash beginners guide html 98 112 13 7 21 Bash Guide for Beginners P passwd Change password pdf2ps Ghostscript PDF to PostScript translator perl Practical Extraction and Report Language P8 Page through text output ping Send echo request to a host pr Convert text files for printing printenv Print all or part of environment procmail Autonomous mail processor ps Report process status pstree Display a tree of processes pwd Print present working directory Q quota Display disk usage and limits R rcp Remote copy unsafe rdesktop Remote Desktop Protocol client reboot Stop and restart the system renice Alter priority of a running process rlogin Remote login telnet insecure rm Remove a file rmdir file home edward cache fr 6oklkP bash beginners guide html 99 112 13 7 21 Bash Guide for Beginners KemOve d GIreclory roff A survey of the roff typesetting system rpm RPM Package Manager rsh Remote shell insecure sep Secure remote copy screen Screen manager with VT 100 emulation set Display set or change variable setterm Set terminal attributes sftp Secure encrypted ftp sh Ope
151. ng of each of the documents in all other respects You may extract a single document from such a collection and distribute it individually under this License provided you insert a copy of this License into the extracted document and follow this License in all other respects regarding verbatim copying of that document B 8 Aggregation with independent works A compilation of the Document or its derivatives with other separate and independent documents or works in or on a volume of a storage or distribution medium does not as a whole count as a Modified Version of the Document provided no compilation copyright is claimed for the compilation Such a compilation is called an aggregate and this License does not apply to the other self contained works thus compiled with the Document on account of their being thus compiled if they are not themselves derivative works of the Document If the Cover Text requirement of section 3 is applicable to these copies of the Document then if the Document is less than one quarter of the entire aggregate the Document s Cover Texts may be placed on covers that surround only the Document within the aggregate Otherwise they must appear on covers around the whole aggregate B 9 Translation Translation is considered a kind of modification so you may distribute translations of the Document under the terms of section 4 Replacing Invariant Sections with translations requires special permission from their copyright
152. nimal dolphin then echo e a a aPweetpeettreetppeterdepweet a a a n else echo e prrrrrrrt n fi else if animal penguin then echo e Tux don t like that Tux wants fish n exit 1 elif animal dolphin then echo e a a a a a aPweepwishpeeterdepweet a a a exit 2 else echo e Will you read this sign Don t feed the animal s n exit 3 fi file home edward cache fr 6oklkP bash beginners guide html 59 112 13 7 21 Bash Guide for Beginners Tl michel test gt cat feed sh bin bash This script acts upon the exit status given by penguin sh if 2 then echo e Usage of the feed script t 0 food on menu animal name n exit 1 else export menu 1 export animal 2 echo e Feeding menu to animal n feed nethome anny testdir penguin sh feed menu animal result echo e Done feeding n case result in 1 echo e Guard You d better give m a fish less they get violent n ri echo e Guard No wonder they flee our planet n a echo e Guard Buy the food that the Zoo provides at the entry you n echo e Guard You want to poison them do you n a echo e Guard Don t forget the guide n esac fi echo Leaving echo e a a aThanks for visiting the Zoo hope to see you again soon n michel test gt feed sh apple camel Feeding apple to camel Will you read this sign Don t feed the
153. ns which will leave a clean window after closing them franky gt cat bash_logout EAE AEE AE EAE AE AE EEE AEE EE EEA EEE EEE EE EE UHRR HAAA HARRH HAREE RRHHHHE Bash_logout file commands to perform from the bash shell at logout time EAE EAE AE EAE AE AEE AEE AEE EE HEA AEE EE EA U HANU UHRR HARAR HARRH HAREE RRHHHHE clear franky gt 3 1 3 Changing shell configuration files When making changes to any of the above files users have to either reconnect to the system or source the altered file for the changes to take effect By interpreting the script this way changes are applied to the current shell session Figure 3 1 Different prompts for different users wa Most shell scripts execute in a private environment variables are not inherited by child processes unless they are exported by the parent shell Sourcing a file containing shell commands is a way of applying changes to your own environment and setting variables in the current shell This example also demonstrates the use of different prompt settings by different users In this case red means danger When you have a green prompt don t worry too much Note that source resourcefile is the same as resourcefile Should you get lost in all these configuration files and find yourself confronted with settings of which the origin is not clear use echo statements just like for debugging scripts see Section 2 3 2 You might add lines like this ech
154. nset 11 1 2 Function syntax Functions either use the syntax function FUNCTION COMMANDS or FUNCTION COMMANDS Both define a shell function FUNCTION The use of the built in command function is optional however if it is not used parentheses are needed The commands listed between curly braces make up the body of the function These commands are executed whenever FUNCTION is specified as the name of a command The exit status is the exit status of the last command executed in the body Common mistakes wi The curly braces must be separated from the body by spaces otherwise they are interpreted in the wrong way The body of a function should end in a semicolon or a newline 11 1 3 Positional parameters in functions Functions are like mini scripts they can accept parameters they can use variables only known within the function using the local shell built in and they can return values to the calling shell A function also has a system for interpreting positional parameters However the positional parameters passed to a function are not the same as the ones passed to a command or script file nhome edward cache fr 60kIkP bash beginners guide html 81 112 13 7 21 Bash Guide for Beginners When a tuncuon 1s executed the arguments to the Iunction become the positional parameters during its execution 1 ne special parameter that expands to the number or positional parameters is updated to reflect the change Po
155. o Linux boot loader links Text mode WWW browser In Make links between files loadkeys Load keyboard translation tables locate Find files logout Close current shell Ip Send requests to the LP print service Ipe Line printer control program Ipq Print spool queue examination program Ipr Offline print Iprm Remove print requests Is List directory content lynx Text mode WWW browser M mail Send and receive mail man Read man pages mcopy Copy MSDOS files to from Unix mndir Display an MSDOS directory memusage file nome edward cache fr 60kIkP bash beginners guide html 97 112 13 7 21 Bash Guide for Beginners visplay memory usage memusagestat Display memory usage statistics mesg Control write access to your terminal mformat Add an MSDOS file system to a low level formatted floppy disk mkbootdisk Creates a stand alone boot floppy for the running system mkdir Create directory mkisofs Create a hybrid IS09660 filesystem more Filter for displaying text one screen at the time mount Mount a file system or display information about mounted file systems mozilla Web browser mt Control magnetic tape drive operation mtr Network diagnostic tool mv Rename files N named Internet domain name server neftp Browser program for ftp services insecure netstat Print network connections routing tables interface statistics masquerade connections and multi cast memberships
156. o Now executing bash_profile or like this echo Now setting PS1 in bashrc export PS1 some value echo PS1 is now set to PS1 3 2 Variables 3 2 1 Types of variables As seen in the examples above shell variables are in uppercase characters by convention Bash keeps a list of two types of variables 3 2 1 1 Global variables Global variables or environment variables are available in all shells The env or printenv commands can be used to display environment variables These programs come with the sh utils package Below is a typical output franky gt printenv CC gcc CDPATH usr local usr CFLAGS 02 fomit frame pointer COLORTERM gnome terminal CXXFLAGS 02 fomit frame pointer DISPLAY 0 DOMAIN hq xalasys com e TOR vi FCEDIT vi FIGNORE 0 G_BROKEN_FILENAMES 1 GDK_USE_XFT 1 file nhome edward cache fr 60kIkP bash beginners guide html 23 112 13 7 21 Bash Guide for Beginners GDMSESSION Default GNOME _DESKTOP_SESSION ID Default GTK_RC_FILES etc gtk gtkrc nethome franky gtkrc 1 2 gnome2 GWMCOLOR darkgreen GWMTERM xterm HISTFILESIZE 5000 history control ignoredups HISTSIZE 2000 HOME nethome f ranky HOSTNAME octarine hq xalasys com INPUTRC etc inputrc IRCNAME f ranky JAVA_HOME usr java j2sdk1 4 0 LANG en_US LDFLAGS s LD_LIBRARY_PATH usr lib mozilla usr lib mozilla plugins LESSCHARSET Latin1l LESS edfMQ LESSOPEN usr bin lesspipe sh s LEX flex LO
157. o echo echo echo read echo case esac done On which topic do you want advice 1 politics 2 startrek 3 kernelnewbies 4 sports 5 bofh excuses 6 magic 7 love 8 literature 9 drugs 10 education n Enter your choice or 0 for exit choice choice in 1 FORTUNE politics 2 FORTUNE startrek 3 FORTUNE kernelnewbies ji echo Go a 5 FORTUNE 1 6 FORTUNE 1 tA Is FORTUNE 1 FORTUNE 1 4A 0 9 FORTUNE 10 FORTUNE 0 echo OK break E echo Sports are a waste of time energy and money back to your keyboard echo e t t t t Unhealthy is my middle name bofh excuses magic love literature drugs education see you Soggie echo That is not a valid choice try a number from 0 to 10 r1 Mind that break exits the loop not the script This can be demonstrated by adding an echo command at the end of the script This echo will also be executed upon input that causes break to be executed when the user types 0 In nested loops break allows for specification of which loop to exit See the Bash info pages for more 9 5 2 The continue built in The continue statement resumes iteration of an enclosing for while until or select loop When used in a for loop the controlling variable takes on the value of the next element in the list When used in a while or until constru
158. ocess runs starting or stopping a daemon and so on only have to be written once in a general way If the same task is needed again the code is recycled From this functions file the checkpid function file nome edward cache fr 60kIkP bash beginners guide html 82 112 13 7 21 Bash Guide for Beginners wf CHECK LI ppU LUULU VE PLUTaL ale IUNNLNY checkpid local i for i in do d proc i amp amp return 0 done return 1 This function is reused in the same script in other functions which are reused in other scripts The daemon function for instance is used in the majority of the startup scripts for starting a server process on machines that use this system 11 2 2 Setting the path This section might be found in your etc profile file The function pathmunge is defined and then used to set the path for the root and other users pathmunge if echo PATH bin egrep q 1 then if 2 after then PATH PATH 1 else PATH 1 PATH fi fi Path manipulation if id u then pathmunge sbin pathmunge usr sbin pathmunge usr local sbin fi pathmunge usr X11R6 bin after unset pathmunge The function takes its first argument to be a path name If this path name is not yet in the current path it is added The second argument to the function defines if the path will be added in front or after the current PATH definition Normal users only get
159. of providing ample message for the users of our scripts As always when others use your scripts it is better to give too much information than not enough Here documents is a type of shell construct that allows creation of lists holding choices for the users This construct can also be used to execute otherwise interactive tasks in the background without intervention 8 4 Exercises These exercises are practical applications of the constructs discussed in this chapter When writing the scripts you may test by using a test directory that does not contain too much data Write each step then test that portion of code rather than writing everything at once 1 Write a script that asks for the user s age If it is equal to or higher than 16 print a message saying that this user is allowed to drink alcohol If the user s age is below 16 print a message telling the user how many years he or she has to wait before legally being allowed to drink As an extra calculate how much beer an 18 user has drunk statistically 100 liters year and print this information for the user 2 Write a script that takes one file as an argument Use a here document that presents the user with a couple of choices for compressing the file Possible choices could be gzip bzip2 compress and zip 3 Write a script called homebackup that automates tar so the person executing the script always uses the desired options cvp and backup destination directory var backups to
160. olds the value of the entire line This is depicted in the image below where we see six colums in the output of the df command Figure 6 1 Fields in awk Wa In the output of Is 1 there are 9 columns The print statement uses these fields as follows kelly octarine test gt ls l awk print 5 9 160o0rig 121script sed 120temp_file 126test 120twolines 441txt2html sh kelly octarine test gt This command printed the fifth column of a long file listing which contains the file size and the last column the name of the file This output is not very readable unless you use the official way of referring to columns which is to separate the ones that you want to print with a comma In that case the default output separater character usually a space will be put in between each output field 6 2 2 Formatting fields Without formatting using only the output separator the output looks rather poor Inserting a couple of tabs and a string to indicate what output this is will make it look a lot better kelly octarine test gt ls ldh grep v total awk print Size is 5 bytes for 9 Size is 160 bytes for orig Size is 121 bytes for script sed Size is 120 bytes for temp file Size is 126 bytes for test Size is 120 bytes for twolines Size is 441 bytes for txt2html sh kelly octarine test gt Note the use of the backslash which makes long input continue on the next line without the shell interpr
161. on Section 3 4 7 arithmetic operators file nhome edward cache fr 60kIkP bash beginners guide html 103 112 13 7 21 OCCUULL 4 array Section 10 2 1 awk Section 6 1 awkprogram Section 6 1 2 B bash Section 1 2 bash_login Section 3 1 2 2 bash_logout Section 3 1 2 5 bash_profile Section 3 1 2 1 bashre Section 3 1 2 4 batch editor Section 5 1 1 break Section 9 5 1 boolean operators Section 7 2 4 Bourne shell Section 1 1 2 brace expansion Section 3 4 3 built in commands Section 1 3 2 C case statements Section 7 2 5 character classes Section 4 2 2 2 Section 4 3 2 child process Section 1 3 1 combined expressions Section 7 1 1 1 command substitution Bash Guide for Beginners file nhome edward cache fr 60kIkP bash beginners guide html 104 112 13 7 21 oecuun 9 4 0 comments Section 2 2 2 conditionals Section 7 1 configuration files Section 3 1 constants Section 10 1 3 continue control signals Section 12 1 1 3 creating variables Section 3 2 2 csh The C shell Section 1 1 2 D debugging scripts Section 2 3 declare Section 10 1 2 Section 10 2 1 double quotes Section 3 3 4 E echo Section 1 5 5 Section 2 1 2 Section 2 3 2 Section 8 1 2 editors Section 2 1 1 else Section 7 2 1 emacs Section 2 1 1 env Section 3 2 1 1 esac Section 7 2 5 escape characters Section 3 3 2 escape sequences Bash Guide for Begin
162. ong as the number of arguments is greater than zero The 1 variable and the shift statement process each argument The number of arguments is reduced each time shift is executed and eventually becomes zero upon which the while loop exits The example below cleanup sh uses shift statements to process each file in the list generated by find bin bash file nhome edward cache fr 60kIkP bash beginners guide html 73 112 13 7 21 Bash Guide for Beginners wf tliLs gt tI LPL call ULetall up ILLOS LHaGAlL WCIS tadstl acllLesscu VVC JUJ uay gt ayu USAGE Usage 0 dirl dir2 dir3 dirN if 0 then echo USAGE exit 1 fi while do if 1s 1 then echo Empty directory nothing to be done else find 1 type f a atime 365 exec rm i fi shift done exec VS xargs The above find command can be replaced with the following find options xargs commands_to_execute_on_found_files The xargs command builds and executes command lines from standard input This has the advantage that the command line is filled until the system limit is reached Only then will the command to execute be called in the above example this would be rm If there are more arguments a new command line will be used until that one is full or until there are no more arguments The same thing using find exec calls on the command to execute on the found files every time a file is found Thus using
163. ormatter fortune Print a random hopefully interesting adage ftp Transfer files unsafe unless anonymous account is used services G galeon Graphical web browser gdm Gnome Display Manager min a getty Control console devices gimp Image manipulation program grep Print lines matching a pattern groff Emulate nroff command with groff grub The grub shell gv A PostScript and PDF viewer gzip Compress or expand files H halt Stop the system file nome edward cache fr 60kIkP bash beginners guide html 95 112 13 7 21 Bash Guide for Beginners ueau Output the first part of files help Display help on a shell built in command host DNS lookup utility httpd Apache hypertext transfer protocol server Print real and effective UIDs and GIDs ifconfig Configure network interface or show configuration info Read Info documents init Process control initialization iostat Display T O statistics Display change network interface status ipchains IP firewall administration iptables IP packet filter administration jar Java archive tool jobs List backgrounded tasks K kdm Desktop manager for KDE kill all Terminate process es ksh Open a Korn shell L Idapmodify file nome edward cache fr 60kIkP bash beginners guide html 96 112 13 7 21 Bash Guide for Beginners Modiry an LVAF entry Idapsearch LDAP search tool less more with features lil
164. ow the top three users of disk space in the home file system if you don t have the directory holding the homes on a separate partition make the script for the partition this is present on every UNIX system First execute the commands from the command line Then put them in a script The script should create sensible output sensible as in readable by the boss If everything proves to work have the script email its results to you use for instance mail s Disk space usage lt you your_comp gt lt result If the quota daemon is running use that information if not use find file nhome edward cache fr 60kIkP bash beginners guide html 49 112 13 7 21 Bash Guide for Beginners 3 Create XML style output from a Tab separated list in the following form Meaning very long line with a lot of description meaning another long line othermeaning more longline testmeaning Loo00000000000000000000000000000000000000000000000000000000000000000ng line but i mean really loooo000000000000 The output should read lt row gt lt entry gt Meaning lt entry gt lt entry gt very long line lt entry gt lt row gt lt row gt lt entry gt meaning lt entry gt lt entry gt long line lt entry gt lt row gt lt row gt lt entryothermeaning lt entry gt lt entry gt more longline lt entry gt lt row gt lt row gt lt entrytestmeaning lt entry gt lt entry gt 1000000000000000000000000000000000000000000
165. p files are read in this case 1 2 2 3 Interactive shells 1 2 2 3 1 What is an interactive shell An interactive shell generally reads from and writes to a user s terminal input and output are connected to a terminal Bash interactive behavior is started when the bash command is called upon without non option arguments except when the option is a string to read from or when the shell is invoked to read from standard input which allows for positional parameters to be set see Chapter 3 1 2 2 3 2 Is this shell interactive Test by looking at the content of the special parameter it contains an i when the shell is interactive eddy gt echo himBH In non interactive shells the prompt PS1 is unset 1 2 2 3 3 Interactive shell behavior Differences in interactive mode e Bash reads startup files e Job control enabled by default e Prompts are set PS2 is enabled for multi line commands it is usually set to gt This is also the prompt you get when the shell thinks you entered an unfinished command for instance when you forget quotes command structures that cannot be left out etc e Commands are by default read from the command line using readline file nhome edward cache fr 60kIkP bash beginners guide html 8 112 13 7 21 Bash Guide for Beginners Bash interprets the shell option ignoreeof instead of exiting immediately upon receiving EOF End Of File e Command history and history expansion are en
166. p the process SIGKILL and SIGSTOP wa SIGKILL and SIGSTOP can not be caught blocked or ignored When killing a process or series of processes it is common sense to start trying with the least dangerous signal SIGTERM That way programs that care about an orderly shutdown get the chance to follow the procedures that they have been designed to execute when getting the SIGTERM signal such as cleaning up and closing open files If you send a SIGKILL to a process you remove any chance for the process to do a tidy cleanup and shutdown which might have unfortunate consequences But if a clean termination does not work the INT orKILL signals might be the only way For instance when a process does not die using Ctrl C it is best to use the kill 9 on that process ID maud gt ps ef grep stuck_process maud 5607 2214 20 05 pts 5 00 00 02 stuck process maud gt kill 9 5607 file home edward cache fr 60kIkP bash beginners guide html 85 112 13 7 21 Bash Guide for Beginners muuu po S YSP seuen_prucess maud 5614 2214 20 15 pts 5 00 00 00 grep stuck_process 1 Killed stuck_process When a process starts up several instances killall might be easier It takes the same option as the kill command but applies on all instances of a given process Test this command before using it in a production environment since it might not work as expected on some of the commercial Unices 12 2 Traps 12 2 1 General
167. pecific options or under specific conditions which are not specified in the script If you don t want to start a new shell but execute the script in the current shell you source it source script_name sh lm source The Bash source built in is a synonym for the Bourne shell dot command The script does not need execute permission in this case Commands are executed in the current shell context so any changes made to your environment will be visible when the script finishes execution willy scripts gt source scriptl sh output ommitted willy scripts gt echo VALUE 9 willy scripts gt 2 2 Script basics 2 2 1 Which shell will run the script When running a script in a subshell you should define which shell should run the script The shell type in which you wrote the script might not be the default on your system so commands you entered might result in errors when executed by the wrong shell The first line of the script determines the shell to start The first two characters of the first line should be then follows the path to the shell that should interpret the commands that follow Blank lines are also considered to be lines so don t start your script with an empty line For the purpose of this course all scripts will start with the line bin bash As noted before this implies that the Bash executable can be found in bin 2 2 2 Adding comments You should be aware of the fact that you might not
168. place actions on lines containing a pattern 5 5 Exercises These exercises are meant to further demonstrate what sed can do 1 Print a list of files in your scripts directory ending in sh Mind that you might have to unalias Is Put the result in a temporary file 2 Make a list of files in usr bin that have the letter a as the second character Put the result in a temporary file 3 Delete the first 3 lines of each temporary file 4 Print to standard output only the lines containing the pattern an 5 Create a file holding sed commands to perform the previous two tasks Add an extra command to this file that adds a string like This might have something to do with man and man pages in the line preceding every occurence of the string man Check the results 6 A long listing of the root directory is used for input Create a file holding sed commands that check for symbolic links and plain files Ifa file is a symbolic link precede it with a line like This is a symlink If the file is a plain file add a string on the same line adding a comment like lt this is a plain file 7 Create a script that shows lines containing trailing white spaces from a file This script should use a sed script and show sensible information to the user Chapter 6 The GNU awk programming language In this chapter we will discuss e What is gawk e Using gawk commands on the command line e How to format text with gawk e How gawk
169. ppearing on the same line as another command does not take effect until the next line of input is read The commands following the alias definition on that line are not affected by the new alias Aliases are expanded when a function definition is read not when the function is executed because a function definition is itself a compound command As a consequence aliases defined in a function are not available until after that function is executed We will discuss aliases in detail in Section 3 5 file home edward cache fr 60kIkP bash beginners guide html 9 112 13 7 21 Bash Guide for Beginners 1 2 2 7 Arrays Bash provides one dimensional array variables Any variable may be used as an array the declare built in will explicitly declare an array There is no maximum limit on the size of an array nor any requirement that members be indexed or assigned contiguously Arrays are zero based See Chapter 10 1 2 2 8 Directory stack The directory stack is a list of recently visited directories The pushd built in adds directories to the stack as it changes the current directory and the popd built in removes specified directories from the stack and changes the current directory to the directory removed Content can be displayed issuing the dirs command or by checking the content of the DIRSTACK variable More information about the workings of this mechanism can be found in the Bash info pages 1 2 2 9 The prompt Bash makes playing wit
170. pt the different parts of the if statement are usually well separated Below a couple of simple examples 7 1 1 3 Checking files The first example checks for the existence of a file anny gt cat msgcheck sh bin bash echo This scripts checks the existence of the messages file echo Checking if f var log messages then echo var log messages exists fi echo echo done anny gt msgcheck sh This scripts checks the existence of the messages file Checking var log messages exists done 7 1 1 4 Checking shell options To add in your Bash configuration files These lines will print a message if the noclobber option is set if o noclobber then echo Your files are protected against accidental overwriting using redirection fi The environment Ww The above example will work when entered on the command line anny gt if o noclobber then echo echo your files are protected against overwriting echo fi your files are protected against overwriting anny gt However if you use testing of conditions that depend on the environment you might get different results when you enter the same command in a script because the script will open a new shell in which expected variables and options might not be set automatically 7 1 2 Simple applications of if 7 1 2 1 Testing exit status The variable holds the exit status of the previously executed command
171. pt can be done using shell options Shell options can be used for partial debugging or for analyzing the entire script Inserting echo commands at strategic locations is also a common troubleshooting technique 2 5 Exercises This exercise will help you to create your first script 1 Write a script using your favorite editor The script should display the path to your homedirectory and the terminal type that you are using Additionally it shows all the services started up in runlevel 3 on your system hint use HOME TERM and Is etc rc3 d S 2 Add comments in your script 3 Add information for the users of your script 4 Change permissions on your script so that you can run it 5 Run the script in normal mode and in debug mode It should run without errors 6 Make errors in your script see what happens if you misspell commands if you leave out the first line or put something unintelligible there or if you misspell shell variable names or write them in lower case characters after they have been declared in capitals Check what the debug comments say about this Chapter 3 The Bash environment In this chapter we will discuss the various ways in which the Bash environment can be influenced e Editing shell initialization files e Using variables Using different quote styles e Perform arithmetic calculations e Assigning aliases e Using expansion and substitution 3 1 Shell initialization files 3 1 1 System wide configurat
172. pts script sed NAME 1 TEMPFILE var tmp sed PID tmp sed s n M 1 sed f SCRIPT sed s M n gt TEMPFILE mv TEMPFILE NAME echo done sandy gt 1 holds the first argument to a given command in this case the name of the file to convert sandy gt cat test linel line2 line3 More on positional parameters in Chapter 7 sandy gt txt2html sh test converting test done sandy gt cat test lt html gt lt head gt lt title gt sed generated html lt title gt lt head gt lt body bgcolor ffffff gt lt pre gt linel line2 line3 lt pre gt file home edward cache fr 6oklkP bash beginners guide html 43 112 13 7 21 Bash Guide for Beginners lt body gt lt html gt sandy gt This is not really how it is done this example just demonstrates sed capabilities See Section 6 3 for a more decent solution to this problem using awk BEGIN and END constructs Easy sed Advanced editors supporting syntax highlighting can recognize sed syntax This can be a great help if you tend to forget backslashes and such 5 4 Summary The sed stream editor is a powerful command line tool which can handle streams of data it can take input lines from a pipe This makes it fit for non interactive use The sed editor uses vi like commands and accepts regular expressions The sed tool can read commands from the command line or from a script It is often used to perform find and re
173. r and continue the rest onto adjacent pages If you publish or distribute Opaque copies of the Document numbering more than 100 you must either include a machine readable Transparent copy along with each Opaque copy or state in or with each Opaque copy a publicly accessible computer network location containing a complete Transparent copy of the Document free of added material which the general network using public has access to download anonymously at no charge using public standard network protocols If you use the latter option you must take reasonably prudent steps when you begin distribution of Opaque copies in quantity to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy directly or through your agents or retailers of that edition to the public It is requested but not required that you contact the authors of the Document well before redistributing any large number of copies to give them a chance to provide you with an updated version of the Document B 5 Modifications You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above provided that you release the Modified Version under precisely this License with the Modified Version filling the role of the Document thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it In addition yo
174. r grouping A couple of examples Is more gunzip file tar gz tar xvf 1 4 1 3 Shell functions file nhome edward cache fr 60kIkP bash beginners guide html 11 112 13 7 21 Bash Guide for Beginners Shell functions are a way to group commands for later execution using a single name for the group They are executed just like a regular command When the name of a shell function is used as a simple command name the list of commands associated with that function name is executed Shell functions are executed in the current shell context no new process is created to interpret them Functions are explained in Chapter 11 1 4 1 4 Shell parameters A parameter is an entity that stores values It can be a name a number or a special value For the shell s purpose a variable is a parameter that stores a name A variable has a value and zero or more attributes Variables are created with the declare shell built in command If no value is given a variable is assigned the null string Variables can only be removed with the unset built in Assigning variables is discussed in Section 3 2 advanced use of variables in Chapter 10 1 4 1 5 Shell expansions Shell expansion is performed after each command line has been split into tokens These are the expansions performed e Brace expansion e Tilde expansion e Parameter and variable expansion e Command substitution e Arithmetic expansion e Word splitting e Filename expansion We
175. r sh ls LIST test Test TEST test upper continue Test upper ORIG Test echo Test tr A Z a z NEW test mv b Test test echo new name for Test is test new name for Test is test TEST upper ORIG TEST echo TEST tr A Z a z NEW test mv b TEST test echo new name for TEST is test new name for TEST is test carol octarine test ls a test test The tr is part of the textutils package it can perform all kinds of character transformations 9 6 Making menus with the select built in 9 6 1 General 9 6 1 1 Use of select The select construct allows easy menu generation The syntax is quite similar to that of the for loop select WORD in LIST do RESPECTIVE COMMANDS done LIST is expanded generating a list of items The expansion is printed to standard error each item is preceded by a number If in LIST is not present the positional parameters are printed as if in would have been specified LIST is only printed once Upon printing all the items the PS3 prompt is printed and one line from standard input is read If this line consists of a number corresponding to one of the items the value of WORD is set to the name of that item If the line is empty the items and the PS3 prompt are displayed again If an EOF End Of File character is read the loop exits Since most users don t have a clue which key combinatio
176. ram Functions are like mini scripts and just like a script they generate exit or return codes While this was a short chapter it contains important knowledge needed for achieving the ultimate state of laziness that is the typical goal of any system administrator 11 4 Exercises Here are some useful things you can do using functions 1 Add a function to your bashrc config file that automates the printing of man pages The result should be that you type something like printman lt command gt upon which the first appropriate man page rolls out of your printer Check using a pseudo printer device for testing purposes As an extra build in a possibility for the user to supply the section number of the man page he or she wants to print 2 Create a subdirectory in your home directory in which you can store function definitions Put a couple of functions in that directory Useful functions might be amongs others that you have the same commands as on DOS or a commercial UNIX when working with Linux or vice versa These functions should then be imported in your shell environment when bashrc is read Chapter 12 Catching signals In this chapter we will discuss the following subjects e Available signals e Use of the signals e Use of the trap statement e How to prevent users from interrupting your programs 12 1 Signals 12 1 1 Introduction 12 1 1 1 Finding the signal man page file nome edward cache fr 60kIkP bash b
177. rator user a comment describing this user choice from a list of shells test for acceptability else exit printing a message expiration date for this account extra groups of which the new user should be a member o With the obtained information add a line to etc passwd etc group and etc shadow create the user s home directory with correct permissions add the user to the desired secondary groups o Set the password for this user to a default known string 5 Rewrite the script from Section 7 2 1 4 so that it reads input from the user instead of taking it from the first argument Chapter 9 Repetitive tasks Upon completion of this chapter you will be able to e Use for while and until loops and decide which loop fits which occasion e Use the break and continue Bash built ins e Write scripts using the select statement e Write scripts that take a variable number of arguments 9 1 The for loop 9 1 1 How does it work The for loop is the first of the three shell looping constructs This loop allows for specification of a list of values A list of commands is executed for each value in the list The syntax for this loop is for NAME in LIST do COMMANDS done If in LIST is not present it is replaced with in and for executes the COMMANDS once for each positional parameter that is set see Section 3 2 5 and Section 2 2 1 2 The return status is the exit status of the last command that executes If no comman
178. read upon login as well which means you don t have to enter the same settings in multiple files In this user s bashrc a couple of aliases are defined and variables for specific programs are set after the system wide etc bashrc is read franky gt cat bashrc home franky bashrc Source global definitions if f etc bashrc then etc bashrec fi shell options set o noclobber my shell variables export PS1 033 1 44m u w 033 Om export PATH PATH bin scripts my aliases alias cdrecord cdrecord dev 0 0 0 speed 8 alias ss ssh octarine alias ll ls la mozilla fix MOZILLA _FIVE_HOME usr lib mozilla LD_LIBRARY_PATH usr lib mozilla usr lib mozilla plugins MOZ DIST BIN usr lib mozilla MOZ_PROGRAM usr lib mozilla mozilla bin export MOZILLA FIVE HOME LD_LIBRARY_PATH MOZ DIST BIN MOZ PROGRAM font fix alias xt xterm bg black fg white amp BitchX settings file nhome edward cache fr 60kIkP bash beginners guide html 22 112 13 7 21 Bash Guide for Beginners export IRCNAME frnk THE END franky gt More examples can be found in the Bash package Remember that sample files might need changes in order to work in your environment Aliases are discussed in Section 3 5 3 1 2 5 bash_logout This file contains specific instructions for the logout procedure In the example the terminal window is cleared upon logout This is useful for remote connectio
179. replace text w Write to a file Apart from editing commands you can give options to sed An overview is in the table below Table 5 2 Sed options Option Effect e SCRIPT Add the commands in SCRIPT to the set of commands to be run while processing the input f Add the commands contained in the file SCRIPT FILE to the set of commands to be run while processing the input n Silent mode V Print version information and exit The sed info pages contain more information we only list the most frequently used commands and options here 5 2 Interactive editing 5 2 1 Printing lines containing a pattern This is something you can do with grep of course but you can t do a find and replace using that command This is just to get you started This is our example text file sandy gt cat n example 1 This is the first line of an example text 2 It is a text with erors 3 Lots of erors 4 So much erors all these erors are making me sick 5 This is a line not containing any errors 6 This is the last line sandy gt We want sed to find all the lines containing our search pattern in this case erors We use the p to obtain the result sandy gt sed erors p example This is the first line of an example text It is a text with erors It is a text with erors Lots of erors Lots of erors So much erors all these erors are making me sick
180. results of the other expansions into words on these characters If IFS is unset or its value is exactly lt space gt lt tab gt lt newline gt the default then any sequence of IFS characters serves to delimit words If IFS has a value other than the default then sequences of the whitespace characters space and Tab are ignored at the beginning and end of the word as long as the whitespace character is in the value of IFS an IFS whitespace character Any character in IFS that is not IFS whitespace along with any adjacent IF whitespace characters delimits a field A sequence of IFS whitespace characters is also treated as a delimiter If the value of IFS is null no word splitting occurs Explicit null arguments or are retained Unquoted implicit null arguments resulting from the expansion of parameters that have no values are removed If a parameter with no value is expanded within double quotes a null argument results and is retained Expansion and word splitting If no expansion occurs no splitting is performed 3 4 9 File name expansion After word splitting unless the f option has been set see Section 2 3 2 Bash scans each word for the characters and If one of these characters appears then the word is regarded as a PATTERN and replaced with an alphabetically sorted list of file names matching the pattern If no matching file names are found and the shell option nullglob is disabled the word is
181. rgument to one of the primaries is of the form dev fd N then file descriptor N is checked stdin stdout and stderr and their respective file descriptors may also be used for tests 7 1 1 1 Expressions used with if file nome edward cache fr 60kIkP bash beginners guide html 50 112 13 7 21 Bash Guide for Beginners The table below contains an overview of the so called primaries that make up the TEST COMMAND command or list of commands These primaries are put between square brackets to indicate the test of a conditional expression Table 7 1 Primary expressions Primary Meaning a FILE True if FILE exists b FILE True if FILE exists and is a block special file cFILE True if FILE exists and is a character special file d FILE True if FILE exists and is a directory e FILE True if FILE exists f FILE True if FILE exists and is a regular file g FILE True if FILE exists and its SGID bit is set h FILE True if FILE exists and is a symbolic link k FILE True if FILE exists and its sticky bit is set p FILE True if FILE exists and is a named pipe FIFO r FILE True if FILE exists and is readable s FILE True if FILE exists and has a size greater than zero tFD True if
182. rompting for user input The following example shows how you can use prompts to explain what the user should enter michel test gt cat friends sh bin bash This is a program that keeps your address book up to date friends var tmp michel friends echo Hello USER This script will register you in Michel s friends database echo n Enter your name and press ENTER read name echo n Enter your gender and press ENTER read n 1 gender file home edward cache fr 60kIkP bash beginners guide html 61 112 13 7 21 Bash Guide for Beginners ecno grep i name friends if 0 then echo You are already registered quitting exit 1 elif gender m then echo You are added to Michel s friends list exit 1 else echo n How old are you read age if age lt 25 then echo n Which colour of hair do you have read colour echo name age colour gt gt friends echo You are added to Michel s friends list Thank you so much else echo You are added to Michel s friends list exit 1 fi fi michel test gt cp friends sh var tmp cd var tmp michel test gt touch friends chmod a w friends michel test gt friends sh Hello michel This script will register you in Michel s friends database Enter your name and press ENTER michel Enter your gender and press ENTER m You are added to Michel s friends list michel test gt cat friends Note t
183. ry name If the number of arguments is more or less than one print a usage message If the argument is not a directory print another message For the given directory print the five biggest files and the five files that were most recently modified 3 Can you explain why it is so important to put the variables in between double quotes in the example from Section 9 4 2 4 Write a script similar to the one in Section 9 5 1 but think of a way of quitting after the user has executed 3 loops 5 Think of a better solution than move b for the script from Section 9 5 3 to prevent overwriting of existing files For instance test whether or not a file exists Don t do unnecessary work 6 Rewrite the whichdaemon sh script from Section 7 2 4 so that it o Prints a list of servers to check such as Apache the SSH server the NTP daemon a name daemon a power management daemon and so on o For each choice the user can make print some sensible information like the name of the web server NTP trace information and so on o Optionally build in a possibility for users to check other servers than the ones listed For such cases check that at least the given process is running o Review the script from Section 9 2 2 4 Note how character input other than q is processed Rebuild this script so that it prints a message if characters are file nhome edward cache fr 60kIkP bash beginners guide html 74 112 13 7 21 Bash Guide for Beginners given as mpu
184. s load average 0 12 0 22 0 15 TY USER T FROM LOGIN IDLE JCPU PCPU WHAT root tty2 Sat 2pm 4 25m 0 24s 0 05s bash willy 0 Sat 2pm 0 00s willy pts 3 Sat 2pm 3 33m 36 395 36 39s BitchX willy ir willy pts 2 Sat 2pm 3 33m 0 13s 0 06s usr bin screen I m setting two variables now This is a string black And this is a number 9 I m giving you back your prompt now willy scripts gt echo COLOUR willy scripts gt echo VALUE willy scripts gt This is the most common way to execute a script It is preferred to execute the script like this in a subshell The variables functions and aliases created in this subshell are only known to the particular bash session of that subshell When that shell exits and the parent regains control everything is cleaned up and all changes to the state of the shell made by the script are forgotten If you did not put the scripts directory in your PATH and the current directory is not in the PATH either you can activate the script like this script_name sh A script can also explicitly be executed by a given shell but generally we only do this if we want to obtain special behavior such as checking if the script works with another shell or printing traces for debugging rbash script_name sh sh script_name sh bash x script_name sh The specified shell will start as a subshell of your current shell and execute the script This is done when you want the script to start up with s
185. s in which the shell looks for commands PS1 The primary prompt string The default value is s v PS2 The secondary prompt string The default value is gt 3 2 4 2 Bash reserved variables These variables are set or used by Bash but other shells do not normally treat them specially Table 3 2 Reserved Bash variables Variable name Definition auto_resume This variable controls how the shell interacts with the user and job control BASH The full pathname used to execute the current instance of Bash BASH_ENV before executing the script If this variable is set when Bash is invoked to execute a shell script its value is expanded and used as the name of a startup file to read The version number of the current instance of Bash BASH_VERSION BASH_VERSINFO A read only array variable whose members hold version information for this instance of Bash COLUMNS Used by the select built in to determine the terminal width when printing selection lists Automatically set upon receipt of a SIGWINCH signal COMP CWORD An index into COMP_WORDS of the word containing the current cursor position COMP_LINE The current command line COMP_POINT The index of the current cursor position relative to the beginning of the current command COMP WORDS An array variable consisting of the individual words in the current command
186. s may only be referenced assignment to them is not allowed Table 3 3 Special bash variables Character Definition Expands to the positional parameters starting from one When the expansion occurs within double quotes it expands to a single word with the value of each parameter separated by the first character of the IFS special variable Expands to the positional parameters starting from one When the expansion occurs within double quotes each parameter expands to a separate word Expands to the number of positional parameters in decimal Expands to the exit status of the most recently executed foreground pipeline A hyphen expands to the current option flags as specified upon invocation by the set built in command or those set by the shell itself such as the i Expands to the process ID of the shell Expands to the process ID of the most recently executed background asynchronous command 0 Expands to the name of the shell or shell script The underscore variable is set at shell startup and contains the absolute file name of the shell or script being executed as passed in the argument list _ Subsequently it expands to the last argument to the previous command after expansion It is also set to the full pathname of each command executed and placed in the environment exported to that command When checking mail this parameter holds the
187. s to it you have the option of following the terms and conditions either of that specified version or of any later version that has been published not as a draft by the Free Software Foundation If the Document does not specify a version number of this License you may choose any version ever published not as a draft by the Free Software Foundation B 12 How to use this License for your documents To use this License in a document you have written include a copy of the License in the document and put the following copyright and license notices just after the title page Copyright c YEAR YOUR NAME Permission is granted to copy distribute and or modify this document under the terms of the GNU Free Documentation License Version 1 1 or any later version published by the Free Software Foundation with the Invariant Sections being LIST THEIR TITLES with the Front Cover Texts being LIST and with the Back Cover Texts being LIST A copy of the license is included in the section entitled GNU Free Documentation License If you have no Invariant Sections write with no Invariant Sections instead of saying which ones are invariant If you have no Front Cover Texts write no Front Cover Texts instead of Front Cover Texts being LIST likewise for Back Cover Texts If your document contains nontrivial examples of program code we recommend releasing these examples in parallel under your choice of free software license such as the GNU General Publ
188. sitional parameter 0 is unchanged The Bash variable FUNCNAME is set to the name of the function while it is executing If the return built in is executed in a function the function completes and execution resumes with the next command after the function call When a function completes the values of the positional parameters and the special parameter are restored to the values they had prior to the function s execution If a numeric argument is given to return that status is returned A simple example lydia cointreau test cat showparams sh bin bash echo This script demonstrates function arguments echo echo Positional parameter 1 for the script is 1 echo test echo Positional parameter 1 in the function is 1 RETURN VALUE echo The exit code of this function is RETURN VALUE test other param lydia cointreau test showparams sh parameterl1 This script demonstrates function arguments Positional parameter 1 for the script is parameterl Positional parameter 1 in the function is other param The exit code of this function is 0 lydia cointreau test Note that the return value or exit code of the function is often storen in a variable so that it can be probed at a later point The init scripts on your system often use the technique of probing the RETVAL variable in a conditional test like this one if RETVAL eq 0 J then lt start the daemon gt Or like this example from the
189. specifying the KILL signal nothing will happen bin bash traptest sh trap echo Booh SIGINT SIGTERM echo pid is while This is the same as while true do sleep 60 This script is not really doing anything done 12 2 2 How Bash interprets traps When Bash receives a signal for which a trap has been set while waiting for a command to complete the trap will not be executed until the command completes When Bash is waiting for an asynchronous command via the wait built in the reception of a signal for which a trap has been set will cause the wait built in to return immediately with an exit status greater than 128 immediately after which the trap is executed 12 2 3 More examples 12 2 3 1 Detecting when a variable is used When debugging longer scripts you might want to give a variable the trace attribute and trap DEBUG messages for that variable Normally you would just declare a variable using an assignment like VARIABLE value Replacing the declaration of the variable with the following lines might provide valuable information about what your script is doing declare t VARIABLE value trap echo VARIABLE is being used here DEBUG rest of the script 12 2 3 2 Removing rubbish upon exit The whatis command relies on a database which is regularly built using the makewhatis cron script with cron bin bash LOCKFILE var lock makewhatis lock Previous makewhatis should execute successfully f
190. ss has the same environment as its parent only the process ID number is different This procedure is called forking After the forking process the address space of the child process is overwritten with the new process data This is done through an exec call to the system The fork and exec mechanism thus switches an old command with a new while the environment in which the new program is executed remains the same including configuration of input and output devices environment variables and priority This mechanism is used to create all UNIX processes so it also applies to the Linux operating system Even the first process init with process ID 1 is forked during the boot procedure in the so called bootstrapping procedure 1 3 2 Shell built in commands file nhome edward cache fr 60kIkP bash beginners guide html 10 112 13 7 21 Bash Guide for Beginners Built in commands are contained within the shell itself When the name of a built in command is used as the first word of a simple command the shell executes the command directly without creating a new process Built in commands are necessary to implement functionality impossible or inconvenient to obtain with separate utilities Bash supports 3 types of built in commands e Bourne Shell built ins break cd continue eval exec exit export getopts hash pwd readonly return set shift test times trap umask and unset Bash built in commands alias bind builtin
191. stance u which will treat unset variables as an error when set and exits a non interactive shell upon encountering such errors willy gt echo VAR willy gt set u willy gt echo VAR bash VAR unbound variable This option is also useful for detecting incorrect content assignment to variables the same error will also occur for instance when assigning a character string to a variable that was declared explicitly as one holding only integer values One last example follows demonstrating the noglob option which prevents special characters from being expanded willy testdir gt set o noglob willy testdir gt touch willy testdir gt ls l rw rw r 1 willy willy Feb 27 13 37 3 7 Summary The Bash environment can be configured globally and on a per user basis Various configuration files are used to fine tune the behavior of the shell These files contain shell options settings for variables function definitions and various other building blocks for creating ourselves a cosy environment Except for the reserved Bourne shell Bash and special parameters variable names can be chosen more or less freely Because a lot of characters have double or even triple meanings depending on the environment Bash uses a system of quoting to take away special meaning from one or multiple characters when special treatment is not wanted Bash uses various methods of expanding command line entries in order to determine which
192. sts for instance in this one z COLUMNS amp amp COLUMNS 80 It is a shorter notation for if z COLUMNS then COLUMNS 80 fi See Section 7 1 2 3 for more information about this type of condition testing If the hyphen is replaced with the equal sign the value is assigned to the parameter if it does not exist bob in echo TEST2 bob in echo TEST2 TEST a_string bob in echo TEST2 a_string The following syntax tests the existence of a variable If it is not set the expansion of WORD is printed to standard out and non interactive shells quit A demonstration bob in cat vartest sh bin bash This script tests whether a variable is set If not it exits printing a message echo TESTVAR There s so much I still wanted to do echo TESTVAR is set we can proceed bob in testdir vartest sh vartest sh line 6 TESTVAR There s so much I still wanted to do bob in testdir export TESTVAR present bob in testdir vartest sh present TESTVAR is set we can proceed Using instead of the exclamation mark sets the variable to the expansion of WORD if it does not exist nothing happens 10 3 3 2 Removing substrings To strip a number of characters equal to OFFSET from a variable use this syntax S VAR OFFSET LENGTH file nhome edward cache fr 60kIkP bash beginners guide html 79 112 13 7 21 Bash Guide for Beginners lne LENG H parameter
193. stty sane esac This user configures the backspace character for login on different operating systems Apart from that the user s bashrc and bash_login are read 3 1 2 2 bash_login This file contains specific settings that are normally only executed when you log in to the system In the example we use it to configure the umask value and to show a list of connected users upon login This user also gets the calendar for the current month FBT AHH HHH HHRHH AHH HHHHHRHHHHHHHHHHHHE cal date m date Y In the absence of bash_profile this file will be read 3 1 2 3 profile Bash_login file commands to perform from the bash shell at login time sourced from bash profile HAE AE AEA EAE AE AEE AEE AE EEE AEA AEE AE EE AE EE EE UHAR RH HARU HRRHHH file protection umask 002 all to me read to group and others miscellaneous w Se t t H In the absence of bash_profile and bash_login profile is read It can hold the same configurations which are then also accessible by other shells Mind that other shells might not understand the Bash syntax 3 1 2 4 bashre Today it is more common to use a non login shell for instance when logged in graphically using X terminal windows Upon opening such a window the user does not have to provide a user name or password no authentication is done Bash searches for bashrc when this happens so it is referred to in the files
194. sysadmin for setting us up such a great printing infrastructure Text to be filtered This text is printed at the end of each print job File descriptor 5 wa Using this file descriptor might cause problems see the Advanced Bash Scripting Guide chapter 16 You are strongly advised not to use it 8 2 4 2 2 Read in scripts The following is an example that shows how you can alternate between file input and command line input michel testdir gt cat sysnotes sh bin bash This script makes an index of important config files puts them together in a backup file and allows for adding comment for each file CONFIG var tmp sysconfig out rm CONFIG 2 gt dev null echo Output will be saved in CONFIG exec 7 lt amp 0 exec lt etc passwd Read the first line of etc passwd read rootpasswd echo Saving root account info echo Your root account info gt gt CONFIG echo rootpasswd gt gt CONFIG exec 0 lt amp 7 7 lt amp echo n Enter comment or ENTER for no comment read comment echo comment gt gt CONFIG echo Saving hosts information first prepare a hosts file not containing any comments TEMP var tmp hosts tmp cat etc hosts grep v gt TEMP exec 7 lt amp 0 exec lt TEMP read ipl namel alias1l read ip2 name2 alias2 echo Your local host configuration gt gt CONFIG echo ipl namel alias1 gt gt CONFIG echo ip2 name
195. t Chapter 10 More on variables In this chapter we will discuss the advanced use of variables and arguments Upon completion you will be able to e Declare and use an array of variables Specify the sort of variable you want to use e Make variables read only e Use set to assign a value to a variable 10 1 Types of variables 10 1 1 General assignment of values As we already saw Bash understands many different kinds of variables or parameters Thus far we haven t bothered much with what kind of variables we assigned so our variables could hold any value that we assigned to them A simple command line example demonstrates this bob in VARIABLE 12 bob in echo VARIABLE 12 bob in VARIABLE string bob in echo VARIABLE string There are cases when you want to avoid this kind of behavior for instance when handling telephone and other numbers Apart from integers and variables you may also want to specify a variable that is a constant This is often done at the beginning of a script when the value of the constant is declared After that there are only references to the constant variable name so that when the constant needs to be changed it only has to be done once A variable may also be a series of variables of any type a so called array of variables VAROVAR1 VAR2 VARN 10 1 2 Using the declare built in Using a declare statement we can limit the value assignment to variables The syntax for
196. t file This kind of redirection also works with other kinds of loops 9 4 2 Output redirection In the example below output of the find command is used as input for the read command controlling a while loop carol octarine testdir cat archiveoldstuff sh bin bash This script creates a subdirectory in the current directory to which old files are moved Might be something for cron if slightly adapted to execute weekly or monthly ARCHIVENR date Y m d DESTDIR PWD archive ARCHIVENR mkdir DESTDIR find PWD type f a mtime 5 while read file do gzip file mv file gz DESTDIR echo file archived done file nhome edward cache fr 60kIkP bash beginners guide html 70 112 13 7 21 Fues are compressed betore they are moved into the archive directory 9 5 Break and continue 9 5 1 The break built in Bash Guide for Beginners The break statement is used to exit the current loop before its normal ending This is done when you don t know in advance how many times the loop will have to execute for instance because it is dependent on user input The example below demonstrates a while loop that can be interrupted This is a slightly improved version of the wisdom sh script from Section 9 2 2 3 bin bash This script provides wisdom You can now exit in a decent way FORTUNE usr games fortune while true do echo echo echo echo echo echo echo echo echo ech
197. t Shell Expansions gt Filename Expansion gt Pattern Matching 4 4 Summary Regular expressions are powerful tools for selecting particular lines from files or output A lot of UNIX commands use regular expressions vim perl the PostgreSQL database and so on They can be made available in any language or application using external libraries and they even found their way to non UNIX systems For instance regular expressions are used in the Excell spreadsheet that comes with the MicroSoft Windows Office suite In this chapter we got the feel of the grep command which is indispensable in any UNIX environment The grep command can do much more than the few tasks we discussed here we only used it as an example for regular expressions The GNU grep version comes with plenty of documentation which you are strongly advised to read Bash has built in features for matching patterns and can recognize character classes and ranges file nhome edward cache fr 60kIkP bash beginners guide html 39 112 13 7 21 Bash Guide for Beginners 4 5 Exercises These exercises will help you master regular expressions 1 Display a list of all the users on your system who log in with the Bash shell as a default 2 From the etc group directory display all lines starting with the string daemon 3 Print all the lines from the same file that don t contain the string 4 Display localhost information from the etc hosts file display the line number s matchin
198. t containing any errors This is the last line sandy gt To print the file starting from a certain line until the end of the file use a command similar to this sandy gt sed 3 d example This is the first line of an example text It is a text with erors sandy gt This only prints the first two lines of the example file The following command prints the first line containing the pattern a text up to and including the next line containing the pattern a line sandy gt sed n a text This p example It is a text with erors Lots of erors So much erors all these erors are making me sick This is a line not containing any errors sandy gt 5 2 4 Find and replace with sed In the example file we will now search and replace the errors instead of only de selecting the lines containing the search string sandy gt sed s erors errors example This is the first line of an example text It is a text with errors Lots of errors So much errors all these erors are making me sick This is a line not containing any errors This is the last line sandy gt As you can see this is not exactly the desired effect in line 4 only the first occurrence of the search string has been replaced and there is still an eror left Use the g command to indicate to sed that it should examine the entire line instead of stopping at the first occurrence of your string sandy gt sed s erors errors g example Th
199. t does it really mean Back in the 1970s three programmers got together to create this language Their names were Aho Kernighan and Weinberger They took the first character of each of their names and put them together So the name of the language might just as well have been wak 6 1 2 Gawk commands When you run awk you specify an awk program that tells awk what to do The program consists of a series of rules It may also contain function definitions loops conditions and other programming constructs advanced features that we will ignore for now Each rule specifies one pattern to search for and one action to perform upon finding the pattern There are several ways to run awk If the program is short it is easiest to run it on the command line awk PROGRAM inputfile s If multiple changes have to be made possibly regularly and on multiple files it is easier to put the awk commands in a script This is read like this awk f PROGRAM FILE inputfile s 6 2 The print program 6 2 1 Printing selected fields The print command in awk outputs selected data from the input file When awk reads a line of a file it divides the line in fields based on the specified input field separator FS which is an awk variable see Section 6 3 2 This variable is predefined to be one or more spaces or tabs The variables 1 2 3 N hold the values of the first second third until the last field of an input line The variable 0 zero h
200. t notices and the license notice saying this License applies to the Document are reproduced in all copies and that you add no other conditions whatsoever to those of this License You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute However you may accept compensation in exchange for copies If you distribute a large enough number of copies you must also follow the conditions in section 3 You may also lend copies under the same conditions stated above and you may publicly display copies B 4 Copying in quantity If you publish printed copies of the Document numbering more than 100 and the Document s license notice requires Cover Texts you must enclose the copies in covers that carry clearly and legibly all these Cover Texts Front Cover Texts on the front cover and Back Cover Texts on the back cover Both covers must also clearly and legibly identify you as the publisher of these copies The front cover must present the full title with all words of the title equally prominent and visible You may add other material on the covers in addition Copying with changes limited to the covers as long as they preserve the title of the Document and satisfy these conditions can be treated as verbatim copying in other respects If the required texts for either cover are too voluminous to fit legibly you should put the first ones listed as many as fit reasonably on the actual cove
201. t separator OFS Any character string may be used as the output field separator by setting this built in variable 6 3 2 2 The output record separator The output from an entire print statement is called an output record Each print command results in one output record and then outputs a string called the output record separator ORS The default value for this variable is n a newline character Thus each print statement generates a separate line To change the way output fields and records are separated assign new values to OFS and ORS kelly octarine test gt awk BEGIN OFS ORS n gt n print 1 2 test record1 datal gt record2 data2 gt kelly octarine test gt If the value of ORS does not contain a newline the program s output is run together on a single line 6 3 3 The number of records The built in NR holds the number of records that are processed It is incremented after reading a new input line You can use it at the end to count the total number of records or in each output record kelly octarine test gt cat processed awk BEGIN OFS ORS n gt done n print Record number NR t 1 2 END print Number of records processed NR kelly octarine test gt awk f processed awk test Record number 1 recordl datal gt done Record number 2 record2 data2 gt done Number of records processed 2 gt done kelly octarine test gt
202. td conf kelly is in etc gt The END statement can be added for inserting text after the entire input is processed kelly is in etc gt ls l awk lt a x conf print 9 END print Can I do anything else for you mistress amd conf antivir conf xcdroast conf xinetd conf Can I do anything else for you mistress kelly is in etc gt file nhome edward cache fr 60kIkP bash beginners guide html 46 112 13 7 21 Bash Guide for Beginners 6 2 5 Gawk scripts As commands tend to get a little longer you might want to put them in a script so they are reusable An awk script contains awk statements defining patterns and actions As an illustration we will build a report that displays our most loaded partitions See Section 6 2 2 kelly is in gt cat diskrep awk BEGIN print WARNING WARNING WARNING lt 8 9 0 9 print Partition 6 t 5 full END print Give money for new disks URGENTLY kelly is in gt df h awk f diskrep awk WARNING WARNING WARNING Partition usr 97 full Give money for new disks URGENTLY kelly is in gt awk first prints a begin message then formats all the lines that contain an eight or a nine at the beginning of a word followed by one other number and a percentage sign An end message is added B Syntax highlighting Awk is a programming language Its syntax is recognized by most editors that
203. te line of input before executing any of the commands on that line Aliases are expanded when a command is read not when it is executed Therefore an alias definition appearing on the same line as another command does not take effect until the next line of input is read The commands following the alias definition on that line are not affected by the new alias This behavior is also an issue when functions are executed Aliases are expanded when a function definition is read not when the function is executed because a function definition is itself a compound command As a consequence aliases defined in a function are not available until after that function is executed To be safe always put alias definitions on a separate line and do not use alias in compound commands Aliases are not inherited by child processes Bourne shell sh does not recognize aliases More about functions is in Chapter 11 Functions are faster wa Aliases are looked up after functions and thus resolving is slower While aliases are easier to understand shell functions are preferred over aliases for almost every purpose 3 6 More Bash options 3 6 1 Displaying options We already discussed a couple of Bash options that are useful for debugging your scripts In this section we will take a more in depth view of the Bash options Use the o option to set to display all shell options file nhome edward cache fr 60kIkP bash beginners guide html 34 112 13 7 2
204. terminal window If you want to save the output to a file redirect it sed option some expression file_to_process gt sed_output_in_a_file More examples wa Plenty of sed examples can be found in the startup scripts for your machine which are usually in etc init d or etc rc d init d Change into the directory containing the initscripts on your system and issue the following command grep sed 5 3 Non interactive editing 5 3 1 Reading sed commands from a file Multiple sed commands can be put in a file and executed using the f option When creating such a file make sure that No trailing white spaces exist at the end of lines No quotes are used e When entering text to add or replace all except the last line end in a backslash 5 3 2 Writing output files Writing output is done using the output redirection operator gt This is an example script used to create very simple HTML files from plain text files sandy gt cat script sed 1i lt html gt lt head gt lt title gt sed generated html lt title gt lt head gt lt body bgcolor ffffff gt lt pre gt a lt pre gt lt body gt lt html gt sandy gt cat txt2html sh bin bash This is a simple script that you can use for converting text into HTML First we take out all newline characters so that the appending only happens once then we replace the newlines echo converting 1 SCRIPT home sandy scri
205. th other documents released under this License under the terms defined in section 4 above for modified versions provided that you include in the combination all of the Invariant Sections of all of the original documents unmodified and list them all as Invariant Sections of your combined work in its license notice The combined work need only contain one copy of this License and multiple identical Invariant Sections may be replaced with a single copy If there are multiple Invariant Sections with the same name but different contents make the title of each such section unique by adding at the end of it in parentheses the name of the original author or publisher of that section if known or else a unique number Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work In the combination you must combine any sections entitled History in the various original documents forming one section entitled History likewise combine any sections entitled Acknowledgements and any sections entitled Dedications You must delete all sections entitled Endorsements B 7 Collections of documents You may make a collection consisting of the Document and other documents released under this License and replace the individual copies of this License in the various documents with a single copy that is included in the collection provided that you follow the rules of this License for verbatim copyi
206. than less than IARG2 or equal to greater than or greater than or equal to ARG2 respectively ARG1 and ARG2 are integers Expressions may be combined using the following operators listed in decreasing order of precedence Table 7 2 Combining expressions Operation Effect EXPR True if EXPR is false EXPR Returns the value of EXPR This may be used to override the normal precedence of operators EXPR a EXPR2 True if both EXPR1 and EXPR2 are true EXPR o EXPR2 True if either EXPR1 or EXPR2 is true The or test built in evaluates conditional expressions using a set of rules based on the number of arguments More information about this subject can be found in the Bash documentation Just like the if is closed with fi the opening angular bracket should be closed after the conditions have been listed file home edward cache fr 6oklkP bash beginners guide html 51 112 13 7 21 Bash Guide for Beginners 7 1 1 2 Commands following the then statement The CONSEQUENT COMMANDS list that follows the then statement can be any valid UNIX command any executable program any executable shell script or any shell statement with the exception of the closing fi It is important to remember that the then and fi are considered to be separated statements in the shell Therefore when issued on the command line they are separated by a semi colon In a scri
207. the if statement if TEST COMMANDS then CONSEQUENT COMMANDS elif MORE TEST COMMANDS then MORE CONSEQUENT COMMANDS else ALTERNA TE CONSEQUENT COMMANDS fi The TEST COMMANDS list is executed and if its return status is zero the CONSEQUENT COMMANDS list is executed If TEST COMMANDS returns a non zero status each elif list is executed in turn and if its exit status is zero the corresponding MORE CONSEQUENT COMMANDS is executed and the command completes If else is followed by an ALTERNATE CONSEQUENT COMMANDS list and the final command in the final if or elif clause has a non zero exit status then ALTERNA TE CONSEQUENT COMMANDS is executed The return status is the exit status of the last command executed or zero if no condition tested true 7 2 2 2 Example This is an example that you can put in your crontab for daily execution anny etc cron daily gt cat disktest sh bin bash This script does a very simple test for checking disk space space df h awk print 5 grep grep v Use sort n tail 1 cut d f1 alertvalue 80 if space ge alertvalue then echo At least one of my disks is nearly full mail s daily diskcheck root else echo Disk space normal mail s daily diskcheck root fi 7 2 3 Nested if statements Inside the if statement you can use another if statement You may use as many levels of nested ifs as you can logically manage This is an example testing le
208. the parent a variable can be exported after it has been assigned a value Then a new subshell is started in which the variable exported from the parent is visible The variable is changed to hold another name but the value for this variable in the parent stays the same 3 2 4 Reserved variables 3 2 4 1 Bourne shell reserved variables Bash uses certain shell variables in the same way as the Bourne shell In some cases Bash assigns a default value to the variable The table below gives an overview of these plain shell variables Table 3 1 Reserved Bourne shell variables Variable name Definition CDPATH A colon separated list of directories used as a search path for the ed built in command HOME The current user s home directory the default for the ed built in The value of this variable is also used by tilde expansion IFS A list of characters that separate fields used when the shell splits words as part of expansion MAIL If this parameter is set to a file name and the MAILPATH variable is not set Bash informs the user of the arrival of mail in the specified file MAILPATH A colon separated list of file names which the shell periodically checks for new mail OPTARG The value of the last option argument processed by the getopts built in OPTIND The index of the last option argument processed by the getopts built in PATH A colon separated list of directorie
209. this one which uses a here document email get tester address sh Find who to e mail hostname hostname This machine s name Send e mail to the right person mail email s Demo Testing lt lt EOF The lucky tester this week is email Reminder the list of demos is here http web example com 8080 DemoSites This e mail was generated by 0 on hostname EOF 10 3 Operations on variables 10 3 1 Arithmetic on variables We discussed this already in Section 3 4 6 10 3 2 Length of a variable file nhome edward cache fr 60kIkP bash beginners guide html 78 112 13 7 21 Bash Guide for Beginners Using the SLAVARS syntax will calculate the number of characters 1n a variable It VAR 1s 7 or this value 1s substituted with the number ot positional parameters or number of elements in an array in general This is demonstrated in the example below bob in echo SHELL bin bash bob in echo SHELL 9 bob in ARRAY one two three bob in echo ARRAY 3 10 3 3 Transformations of variables 10 3 3 1 Substitution VAR WORD If VAR is not defined or null the expansion of WORD is substituted otherwise the value of VAR is substituted bob in echo TEST test test bob in echo TEST bob in export TEST a_string bob in echo TEST test a_string bob in echo TEST2 TEST a_string This form is often used in conditional te
210. thy 4096 Mar 22 2002 xml ngn n tom This lists all files in cathy s home directory starting with a b c x y or z myn If the first character within the braces is or any character not enclosed will be matched To match the dash include it as the first or last character in the set The sorting depends on the current locale and of the value of the LC_COLLATE variable if it is set Mind that other locales might interpret a cx z as aBbCcXxYyZz if sorting is done in dictionary order If you want to be sure to have the traditional interpretation of ranges force this behavior by setting LC_COLLATE or LC_ALL to C 4 3 2 Character classes Character classes can be specified within the square braces using the syntax CLASS where CLASS is defined in the POSIX standard and has one of the values alnum alpha ascii blank cntrl digit graph lower print punct space upper word or xdigit Some examples cathy gt ls ld digit drwxrwxr x 2 cathy cathy 4096 Apr 20 13 45 2 cathy gt ls ld upper drwxrwxr 3 cathy cathy 4096 Sep 30 2001 Nautilus drwxrwxr x 4 cathy cathy 4096 Jul 11 2002 OpenOffice orgl 0 rw rw r 1 cathy cathy 997376 Apr 18 15 39 Schedule sdc When the extglob shell option is enabled using the shopt built in several extended pattern matching operators are recognized Read more in the Bash info pages section Basic shell features g
211. tter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document you may at your option designate some or all of these sections as invariant To do this add their titles to the list of Invariant Sections in the Modified Version s license notice These titles must be distinct from any other section titles You may add a section entitled Endorsements provided it contains nothing but endorsements of your Modified Version by various parties for example statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard You may add a passage of up to five words as a Front Cover Text and a passage of up to 25 words as a Back Cover Text to the end of the list of Cover Texts in the Modified Version Only one passage of Front Cover Text and one of Back Cover Text may be added by or through arrangements made by any one entity If the Document already includes a cover text for the same cover previously added by you or by arrangement made by the same entity you are acting on behalf of you may not add another but you may replace the old one on explicit permission from the previous publisher that added the old one The author s and publisher s of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version B 6 Combining documents You may combine the Document wi
212. u must do these things in the Modified Version A Use in the Title Page and on the covers if any a title distinct from that of the Document and from those of previous versions which should if there were any be listed in the History section of the Document You may use the same title as a previous version if the original publisher of that version gives permission B List on the Title Page as authors one or more persons or entities responsible for authorship of the modifications in the Modified Version together with at least five of the principal authors of the Document all of its principal authors if it has less than five C State on the Title page the name of the publisher of the Modified Version as the publisher D Preserve all the copyright notices of the Document E Add an appropriate copyright notice for your modifications adjacent to the other copyright notices F Include immediately after the copyright notices a license notice giving the public permission to use the Modified Version under the terms of this License in the form shown in the Addendum below G Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document s license notice H Include an unaltered copy of this License I Preserve the section entitled History and its title and add to it an item stating at least the title year new authors and publisher of the Modified Version as given on the Title Pag
213. uct amp gt FILE This is the equivalent of gt FILE 2 gt amp 1 the construct used in the previous set of examples It is also often combined with redirection to dev nul1 for instance when you just want a command to execute no matter what output or errors it gives 8 2 4 File input and output 8 2 4 1 Using dev fd The dev fd directory contains entries named 0 1 2 and so on Opening the file dev fd N is equivalent to duplicating file descriptor N If your system provides dev stdin dev stdout and dev stderr you will see that these are equivalent to dev fd 0 dev fd 1 and dev fd 2 respectively The main use of the dev fd files is from the shell This mechanism allows for programs that use pathname arguments to handle standard input and standard output in the same way as other pathnames If dev fd is not available on a system you ll have to find a way to bypass the problem This can be done for instance using a hyphen to indicate that a program should read from a pipe An example michel gt filter body txt gz cat header txt footer txt This text is printed at the beginning of each print job and thanks the sysadmin for setting us up such a great printing infrastructure Text to be filtered This text is printed at the end of each print job file nhome edward cache fr 60kIkP bash beginners guide html 63 112 13 7 21 Bash Guide for Beginners Lhe cat command tirst reads the tile header txt next its stan
214. ur documents Glossary Index List of Tables 1 Typographic and usage conventions 1 1 Overview of programming terms 2 1 Overview of set debugging options Bash Guide for Beginners file nhome edward cache fr 60kIkP bash beginners guide html 2 112 13 7 21 Bash Guide for Beginners 3 1 Reserved Bourne shell variables 3 2 Reserved Bash variables 3 3 Special bash variables 3 4 Arithmetic operators 4 1 Regular expression operators 5 1 Sed editing commands 5 2 Sed options 6 1 Formatting characters for gawk 7 1 Primary expressions 7 2 Combining expressions 8 1 Escape sequences used by the echo command 8 2 Options to the read built in 10 1 Options to the declare built in 12 1 Control signals in Bash 12 2 Common kill signals A 1 Common Shell Features A 2 Differing Shell Features List of Figures 1 Bash Guide for Beginners front cover 2 1 scriptl sh 3 1 Different prompts for different users 6 1 Fields in awk 7 1 Testing of a command line argument with if 7 2 Example using Boolean operators Introduction 1 Why this guide The primary reason for writing this document is that a lot of readers feel the existing HOWTO to be too short and incomplete while the Bash Scripting guide is too much of a reference work There is nothing in between these two extremes I also wrote this guide on the general principal that not enough free b
215. ur scripts Chapter 4 Regular expressions In this chapter we discuss e Using regular expressions Regular expression metacharacters e Finding patterns in files or output e Character ranges and classes in Bash 4 1 Regular expressions 4 1 1 What are regular expressions A regular expression is a pattern that describes a set of strings Regular expressions are constructed analogously to arithmetic expressions by using various operators to combine smaller expressions The fundamental building blocks are the regular expressions that match a single character Most characters including all letters and digits are regular expressions that match themselves Any metacharacter with special meaning may be quoted by preceding it with a backslash 4 1 2 Regular expression metacharacters A regular expression may be followed by one of several repetition operators metacharacters Table 4 1 Regular expression operators Operator Effect Matches any single character He The preceding item is optional and will be matched at most once x The preceding item will be matched zero or more times The preceding item will be matched one or more times N The preceding item is matched exactly N times N The preceding item is matched N or more times N M The preceding item is matched at least N times but not more than M times represents the range if it s not first or last i
216. users Ww 5 00pm up 18 days 7 00 4 users load average 0 79 0 39 0 33 USER TTY FROM LOGIN IDLE JCPU PCPU WHAT root tty2 Sat 2pm 5 47m 0 24s 0 05s bash willy 10 Sat 2pm 0 00s willy pts 3 Sat 2pm 54 02 36 885 36 885 BitchX willyke willy pts 2 Sat 2pm 54 02 0 13s 0 06s usr bin screen set x I m setting two variables now This is a string And this is a number I m giving you back your prompt now willy scripts gt You can switch debugging mode on and off as many times as you want within the same script The table below gives an overview of other useful Bash options Table 2 1 Overview of set debugging options Short notation Long notation Result set f set o noglob Disable file name generation using metacharacters globbing set v set o verbose Prints shell input lines as they are read set x set o xtrace Print command traces before executing command The dash is used to activate a shell option and a plus to deactivate it Don t let this confuse you In the example below we demonstrate these options on the command line willy scripts gt set v willy scripts gt ls ls commented scripts sh scriptl sh willy scripts gt set v set v willy scripts gt ls commented scripts sh scriptl sh willy scripts gt set f willy scripts gt ls ls No such file or directory willy scripts gt touch willy scripts
217. vision 1 1 1 1 2004 02 11 16 59 50 tille initiele bash import Revision 1 3 2003 02 05 09 52 53 mbounine httpd restarting added Revision 1 2 2003 02 05 08 11 32 mbounine Bug fixes Revision 1 1 2003 02 04 15 41 35 mbounine Script for syncing httpd config between web farm hosts Initial release H k HEH HR HE HH HH HHH HHH HHH RHR HR HHH HR HHH HHH HH f whoami root then echo Must be root to run 0 exit 1 fi if z 1 then echo Usage 0 lt path to httpd conf gt exit 1 fi httpd _conf_new 1 httpd_conf_path usr local apache conf Login htuser farm_hosts web03 web04 web05 web06 web07 file nhome edward cache fr 60kIkP bash beginners guide html 77 112 13 7 21 Bash Guide for Beginners for i in farm_hosts do su login c scp httpd_conf_new i httpd_conf_path su login c ssh i sudo usr local apache bin apachectl graceful done exit 0 First two tests are performed to check whether the correct user is running the script with the correct arguments The names of the hosts that need to be configured are listed in the array farm_hosts Then all these hosts are provided with the Apache configuration file after which the daemon is restarted Note the use of commands from the Secure Shell suite encrypting the connections to remote hosts Thanks Eugene and colleague for this contribution Dan Richter contributed the following example This is the problem he was confronted wit
218. working directory as set by the ed built in command RANDOM Each time this parameter is referenced a random integer between 0 and 32767 is generated Assigning a value to this variable seeds the random number generator REPLY The default variable for the read built in SECONDS This variable expands to the number of seconds since the shell was started SHELLOPTS A colon separated list of enabled shell options SHLVL Incremented by one each time a new instance of Bash is started Ruaa The value of this parameter is used as a format string specifying how the timing information for pipelines prefixed with the time reserved word should be displayed If set to a value greater than zero TMOUT is treated as the default timeout for the read built in In an interative shell the value is interpreted as TMOUT the number of seconds to wait for input after issuing the primary prompt when the shell is interactive Bash terminates after that number of seconds if input does not arrive UID The numeric real user ID of the current user Check the Bash man info or doc pages for extended information Some variables are read only some are set automatically and some lose their meaning when set to a different value than the default 3 2 5 Special parameters file home edward cache fr 6oklkP bash beginners guide html 27 112 13 7 21 Bash Guide for Beginners The shell treats several parameters specially These parameter
219. xargs greatly speeds up your scripts and the performance of your machine In the next example we modified the script from Section 8 2 4 4 so that it accepts multiple packages to install at once bin bash if lt 1 then echo Usage 0 package s exit 1 Ti while do yum install 1 lt lt CONFIRM y CONFIRM shift done 9 8 Summary In this chapter we discussed how repetitive commands can be incorporated in loop constructs Most common loops are built using the for while or until statements or a combination of these commands The for loop executes a task a defined number of times If you don t know how many times a command should execute use either until or while to specify when the loop should end Loops can be interrupted or reiterated using the break and continue statements A file can be used as input for a loop using the input redirection operator loops can also read output from commands that is fed into the loop using a pipe The select construct is used for printing menus in interactive scripts Looping through the command line arguments to a script can be done using the shift statement 9 9 Exercises Remember when building scripts work in steps and test each step before incorporating it in your script 1 Create a script that will take a recursive copy of files in etc so that a beginning system administrator can edit files without fear 2 Write a script that takes exactly one argument a directo
220. yntax of the read command is as follows read options NAME1 NAME2 NAMEN One line is read from the standard input or from the file descriptor supplied as an argument to the u option The first word of the line is assigned to the first name NAME1 the second word to the second name and so on with leftover words and their intervening separators assigned to the last name NAMEN If there are fewer words read from the input stream than there are names the remaining names are assigned empty values The characters in the value of the IFS variable are used to split the input line into words or tokens see Section 3 4 8 The backslash character may be used to remove any special meaning for the next character read and for line continuation If no names are supplied the line read is assigned to the variable REPLY The return code of the read command is zero unless an end of file character is encountered if read times out or if an invalid file descriptor is supplied as the argument to the u option The following options are supported by the Bash read built in Table 8 2 Options to the read built in Option Meaning SAN AME The words are assigned to sequential indexes of the array variable ANAME starting at 0 All elements are removed from ANAME before the assignment Other NAME arguments are ignored ae The first character of DELIM is used to terminate the input li

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