Writing Shell Scripts in UNIX A simple Bash-shell script
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1. The simplest type of quotes are single quotes anything between the two quote marks is treated as a simple string The shell will not attempt to execute or otherwise interpret any words within the string The script below simply prints out the message your name is fred bin bash echo Your name is fred What happens if rather than always using the name fred we want to make the name controlled by a variable We might then try writing a script like this bin bash NAME fred echo Your name is SNAME However this will not do what we want It will actually output the message Your name is name because anything between the quote marks is treated as literal text and that includes name For this reason shells also understand double quotes The text between double quotes marks is also interpreted as literal text except that any variables in it are interpreted If we change the above script to use double quotes then it will do what we want bin bash NAME fred echo Your name is NAME The above script writes out the message Your name is fred Double quotes are so useful that we normally use them rather than single quotes which are only really needed on the rare occasions when you actually want to print out a message with variable names in it The third type of quotes are called back quotes and we have already seen them in action with the expr command Back quotes cause the Shell to treat whatever is between the2. funcmul Modify the funcadd script to request from the user the number of parameters to be added receive them and then add them together Write a script which implements a single times table by requesting from the user a value for the times table an incremental value and a stop value Hint you should inplement a loop of your choice your script should then call a function from inside the loop over again sending it both the cumulative total and the value to add until the loop terminates Korn Shell Summary Variables Variables are assigned using the equals sign no spaces They are used by preceding them with a dollar character Arguments Arguments are labelled 1 2 9 indicates how many arguments there are shift moves all the arguments down so that 2 becomes 1 3 becomes 2 etc This allows more than nine arguments to be accessed if necessary Quotes Single quotes string form string literals No interpretation is performed on the string Double quotes string form string literals with limited substitution variables are replaced with their value and back quoted commands are replaced with the results of their execution A backslash V at the end of a line allows strings to be stretched over a number of lines Back quotes string execute the string in a sub shell and substitute in the results of the execution Conditionals if lt condition then elif else fi elif and else are
3. must include a detailed description of the workings of the program and justify why you have undertaken the task in this manner Write a script which will list all of the files in a users directory and add all the filesizes together it should display all of the filenames and size followed by a summary line stating Page 19 of 8 total size is Hint you will need to pipe a directory listing to a file and then cut out the relevant field and call the function with the latest value of the cumulative total You must include a detailed description of the workings of the program and justify why you have undertaken the task in this manner Page 20 of 8
4. optional case lt string in lt case1 3 lt case2 53 esac The case acts as a default for any value not matched by one of the earlier cases Looping for lt variable in lt list do done while lt condition do done Page 18 of 8 until lt condition do done Expressions The expr command will do calculations It usually needs to be enclosed in back quotes as the result of the calculation will be assigned to some variable The arguments to the expr command must be separated by spaces The value of expr 3 T is 4 whereas the value of expr 3 1 no spaces is the string 3 1 Input User input can be solicited using the read command which places what the user types into variables If users are to type several words read can be given a number of arguments Functions Can be called from within a shell script By using the return command it is possible to return a value to the main script such as a result Review Exercises Modify the DOS command interpreter above to work with the following DOS commands cd dir copy and rename Write a menu program which runs iteratively offering the user the following choices Times table menu DOS command emulator Exit to UNIX The user should be able to enter their choice and your system should call the appropriate subshell The timestable menu should be the system you created in the looping exercises You have written a number of scripts in t
5. the screen cat 1 2 gt 3 else echo Need three parameters two input and one output Sorry fi bin bash An alternative version of the join command This time we check that is exactly three is a special variable which indicates how many parameters were given to the script by the user if eq 3 then cat 1 2 gt 3 else echo Need exactly three parameters sorry fi bin bash checks whether a named file exists in a special directory stored in the dir variable If it does prints out the top of the file using the head command N B establish your own dir directory if you copy this DIR HOME safe if f DIR 1 then head DIR 1 fi Case Statements The if condition is suitable if a single possibility or at most a small number of possibilities are to be tested However it is often the case that we need to check the value of a variable against a number of possibilities The case statement is used to handle this situation The script below reacts differently depending on which name is given to it as an argument Syntax Case variablename in Option1 commands Option2 commands commands esac Page 5 of 8 Eg bin bash Script Written by Phil Irving and Andrew Hunter 18 6 98 case 1 in fred echo Hi fred Nice to see you joe echo Oh Its you is it joe harry echo Clear off echo Who are you 29 esac The case statement compares the
6. Writing Shell Scripts in UNIX Combining sets of commands into one file which then may be run to execute the other commands This is extremely useful for backups moving files general housekeeping A shell script is a file containing commands that could also be typed at the command line When given execute permissions and called the file is interpreted by the system and the commands are executed A simple Bash shell script To make sure that the correct shell is run the first line of the script should always indicate which one is required Below is illustrated a simple Korn shell script it just outputs the message hello world bin bash echo hello world Execute the script filename and observe the output Comments and Commands implies a comment in every line except the first line in the file Here the comment is used to indicate which shell should be used bash is the Korn Shell Good practice to use comments to indicate what the script does As a bare minimum the script must always contain the name of the author the date written a brief description of what the script does and some indication as to whther it works The script should also contain comments showing the revision history Lines not preceded by a hash character are taken to be UNIX commands and are executed Any UNIX command can be used For example the script below displays the current directory name using the pwd command and then lists the directory c
7. ditionals Review Exercises 1 Write a script called fileexists which indicates whether a named file exists or not 2 Write a modified version of the show example script the first example of the ifstatement given above which prints out the message File does not exist ifthe user gives a name which isn t a file or directory and the message You must give an argument if the user doesn t give an argument to the program 3 Write a script called save which copies a file into a special directory and another called recover which copies a file back out of the special directory The user of the script should not be aware of the location of the special directory obviously the script will be 4 Alter your scripts so that if you try to save a file which already exists in the special directory the script refuses to save the file and prints out a message to that effect Further Input the read command Further Input the read command exercises Write a shell script to input two numbers from the user using the read command and multiply them together Modify the above script to divide the two numbers Modify the above script to add the two numbers Modify the above script to subtract the two numbers Modify your menu program created earlier to accept input during execution using read rather than from the command line Further Conditional Commands the select statement The select statement may be used to quickly generate a menu it s
8. for the test command see below for example d which causes the test command to determine if the following argument is a directory Thus test d DIRNAME will check to see if a directory exists in the current directory with the name contained in the variable DIRNAME if it does then it will return a zero exit status CAUTION if you simply type the above at the command line nothing will appear to happen this is because you need to tell the Korn shell what to do if the test is TRUE or NOT TRUE This can be achieved by coupling it with the if statement Syntax if condition then commands else commands fi For example the shell below displays the contents of a file on the screen using cat but lists the contents of a directory using Is bin bash show script if test d 1 then Is 1 else Page 3 of 8 cat 1 fi As you will recall there unfortunately is an alternative way of writing a condition in the Korn shell the test command may be omitted and the round parenthesis replaced by square ones Thus the following if statement if test d 1 may be re written as if d 1 NOTE the spaces after the first and before the second There are a number of conditions supported by shell scripts for a complete list use the on line manual on the test command man test Some examples are d is a directory f is a file are two strings the same r is string set eq are two numbers eq
9. he correct type of speech marks and the arguments of the expr command 1 and 1 must be separated by spaces bin bash RESULT expr 1 1 echo Result is RESULT There is an alternative to using the expr command and that is to use the basic calculator bc This can be invoked from the command line by typing bc which places you into the interactive basic calculator don t expect too much it is not very friendly You then enter the calculation to be performed eg 2 2 and the result is displayed To exit simply press cntrl D The basic calculator may also be accessed within a shell script by piping the calculation to be performed to it bin bash N1 2 N2 5 echo N1 N2 lbc Page 2 of 8 This would cause the result 10 to be writted to standard output Similarly the result can be placed into a variable bin bash N1 2 N2 5 RESULT echo N1 N2 Ibc echo Result is RESULT The basic calculator has the following basic commands Integer remainder function eg 11 3 would give 3 raise to the power of sqrt square root function Conditional Statements in Scripts The IF Statement The most frequently used conditional operator is the if statement Before considering the if statement it is necessary to explain something of the condition arrangement The test command will return an exit status the exit status is zero if the condition is true otherwise it is non zero There are many parameters
10. his tutorial Review each of the scripts that you have written and ensure that they contain enough comments to allow you to be able to understand them after a period of 6 months or for a novice to be able to pick up the scripts and be able to appreciate what each line does For each of the conditional commands we have discussed in this tutorial append to your user manual a description of the command together with appropriate examples You description should also justify when you would use the different commands For each of the looping commands we have discussed in this tutorial append to your user manual a description of the command together with appropriate examples You description should also justify when you would use the different commands For each of the methods of modular programming we have discussed in this tutorial append to your user manual a description of the method together with appropriate examples You description should also justify when you would use the different commands Modify the funcparams to pass the parameters one at a time by using a loop of your choice You must include a detailed description of the workings of the program and justify why you have undertaken the task in this manner Write a script containing a function to request from the user a temperature in either Fahrenheit or centigrade and then perform the calculation to the other hint you will need to make a decision based upon whether the temp is F or C You
11. impl y produces a numbered list on the standard output from which the user can select Syntax Eg select REPLY in casefriend1 casefriend2 do case REPLY in casefriend1 Page 9 of 8 casefriend1 casefriend2 casefriend 2 echo Wrong choice esac done Although this seems a quick way to generate menus it is not as elegant as an echo case solution Further Conditional Commands the select statement Exercise Implement the above example Quoting in Scripts Quoting in Scripts Exercises Write shell scripts for the above examples and try them out Write a script which checks whether a given file contains a given word If it does the script should output the message The file contains the word if not it should output the message The file doesn t contain the word Save this as wordsearch Modify the above script to determine whether a parameter was passed before commencing the grepping process Modular Programming in Shell Scripts Like any good programming language shells allow modular programming There are two basic forms of modular programming in shell scripts By calling sub shells Functions Calling Sub shells Possibly unbeknown to you you actually undertook this in the variable tutorial to experiment with variable inheritence To call a sub shell you simply enter its name on a line of the shell script as you would any other UNIX command For example let us suppose we wish to ca
12. ing the exit value addnumfunc FUNCRESULT expr 1 2 return FUNCRESULT Main part of the script addnumfunc 3 7 SCRIPTRES echo The result is SCRIPTRES It is perfectly possible to write recursive functions it just needs some careful thought Below is an example to implement a recursive factorial script bin bash Script Written by Phil Irving and Andrew Hunter 18 6 98 This script calls a function recursively to solve factorial for a number input facfunc factorial function if 1 le 1 if a value of 1 or less is entered we dont need to solve then return 1 return a value of 1 else FUNCTEMPVAR 0 create two temporary variables FUNCRESULT 0 FUNCTEMPVAR expr 1 1 subtract one from the number sent facfunc SFUNCTEMPVAR call the function recursively FUNCRESULT expr 1 multiply the result returned from the fuction by the value sent return SFUNCRESULT return the result fi Main script echo Please enter a positive value for factorial n read INPUTFAC funcfac SINPUTFAC echo The factorial of SINPUTFAC is Page 17 of 8 Functions Exercises Try out the examples given above save first as funcfour the second as funcparams the third as funcadd and the final one as recfunc Modify the funcfour script to run 10 times Modify the funcadd script to multiply the numbers and save as
13. ll the today script created in the above examples after we have given the present working directory we would use pwd today Should parameters require to be passed eg in wordsearch then these can be passed by placing them on the command line pwd today wordsearch and andfile The above script will call the wordsearch script and pass to it the values and as the search word and andfile as the file to be searched Alternatively these parameters could themselves be replaced by variables eg pwd today wordsearch SEARCHWORD SEARCHFILE Functions Will be dealt with later in the tutorial Modular Programming in Shell Scripts Exercises Write a shell script which calls the today shell script Write a scripts which calls the name script Page 10 of 8 Looping Commands while Whereas conditional statements allow programs to make choices about what to do looping commands support repetition Many scripts are written precisely because some repetitious processing of many files is required so looping commands are extremely important The simplest looping command is the while command The syntax of which lis given below while condition do commands done Eg bin bash Script Written by Phil Irving and Andrew Hunter 18 6 98 Output my intention followed by a new line echo I am going to count up to 10 n start a 1 NUMBER 1 while number is less than 12 while test NUMBER le 10 do print out the numbe
14. o execute the Is 1 command We do this by creating an alias for dir alias dir ls Il From now on whenever we type dir the Is 1 command will run until we log out Commands themselves can also be aliased For instance if we don t want anyone to have access to the talk command we can simply alias it to null alias talk null If this is included in a startup script then it simply stops the user being able to access the talk command by effectively hiding it Of course the systems administrator could also remove user privileges to that command To remove an alias we use the unalias command thus to remove the talk alias we would use unalias talk Aliasing Exercises Alais the dir command Unalias the dir command Alias the talk command Unalias the talk command Try implementing the DOS interpreter as alias commands Unalias these commands Functions Page 15 of 8 The Korn shell supports functions in a similar way to C Within a script a function may be declared and sunsequently used It may also have parameters passed to it in a similar fashion to C Syntax function name list of commands Eg bin bash Script Written by Phil Irving and Andrew Hunter 18 6 98 Function Declaration without parameters greeting echo hello echo world Set count variable ITCOUNT 1 Iteration to run funciton 4 times while ITCOUNT le 4 do Call greeting function g
15. ommand is considered by the case statement which executes an appropriate UNIX command depending on which DOS command has been given bin bash Script Written by Phil Irving and Andrew Hunter 18 6 98 DOS interpreter Impersonates DOS as follows DOS command UNIX equivalent Action dir Is List directory contents type cat List file contents echo Welcome to the DOS interpreter echo Type exit to leave Setting the variable command to ensure the loop operates COMMANDE Notexit loop while test S COMMAND exit do Show DOS prompt c stops a new line from being issued echo DOS c Read in user s command read COMMAND ARG1 ARG2 Do a UNIX command corresponding to the DOS command case COMMAND in type cat ARG1 Page 14 of 8 del rm ARGI1 exit The leave option therefore do nothing echo DOS does not recognise the command COMMAND esac done More Sample Scripts Exercises Implement the DOS command interpreter given above Once you have it operational write a user guide to explain to a novice user how the system works Aliasing In the above exercise we created a command interpreter for DOS which made UNIX emulate DOS This works fine whilst running the script but doesn t allow users to use any UNIX commands or call shell scripts There is an alternative however we can alias commands to be something else For instance we can tell the Korn shell that whenever someone types dir t
16. ontents using the ls command bin bash Script Written by Phil Irving and Andrew Hunter 18 6 98 purpose print out current directory name and contents pwd Is Page of 8 Shell Variables Shells support variables They may be assigned values manipulated and used Convention dictates that the variables used in scripts are in UPPERCASE The script below shows how to assign and use a variable In general shell variables are all treat as strings i e bits of text Shells are extremely fussy about getting the syntax exactly right in the assignment there must be no space between the variable name and the equals sign or the equals sign and the value To use the variable it is prefixed by a dollar character bin bash NAME is a variable NAME fred echo The name is NAME The special variables 1 9 correspond to the arguments passed to the script when it is invoked For example if we rewrite the script above as shown below calling the script name and then invoke the command name Dave Smith the message Your name is Dave Smith will be printed out bin bash echo Your name is 1 2 Arithmetic in Shell Scripts Shell scripts can also do arithmetic although this does not come particularly naturally to them The script below adds one to the number passed to it as an argument To do this it must use the expr command enclosed in back quote characters Once again precise syntax is critical You must use t
17. own location and therefore paths inside the script will need to be spcified using environmental variables Looping Commands until The third looping command available to us is until loops These are very similar to the while loop except that it executes until the condition is true al bin bash Script Written by Phil Irving and Andrew Hunter 18 6 98 Output my intention followed by a new line echo I am going to count up to 10 n start a 1 NUMBER 1 until number is greater than 12 until test SNUMBER gt 12 do print out the number echo NUMBER add 1 to the number NUMBER expr SNUMBER 1 done Looping Commands until Exercises Write a script to count backwards from 100 to 1 Modify the above to count forwards Rewrite the times tables exercises from the while command to work with the until command Modify the above to produce the times table in reverse 60 to 1 Modify your menu program previously modified in the while exercises Looping Commands Review Exercises Write a menu program which allows the user to select form the following Times table from 1 to 12 Times table from to 60 Times table form 12 to 1 Times table from 60 to 1 Add two numbers together you specify Add together X numbers you specify you also tell me X Ascending times table from X to Y you specify X and Y Descending times table from X to Y you specify X and Y Ascending times table from X to Y you specify X Y and I inc
18. p must be set to one each time we pass through the outer loop INNER 1 echo echo OUTER times table The INNER loop while test SINNER le 12 do Working out the value of inner times outer The is the syntax for multiplication with the expr command RESULT expr INNER SOUTER echo INNER x OUTER RESULT Page 11 of 8 Incrementing the inner count by 1 INNER expr INNER 1 done Incrementing the outer loop by 1 OUTER expr SOUTER 1 done Looping Commands while exercises Write a shell script which will count from 1 to 100 Modify the above script to count backwards from 100 to 1 Write a shell script to produce times tables up to 60 Modify the above to produce the times table in reverse 60 to 1 Modify your menu program to repeat until the user enters 9 for exit Hint you may need to include an option 9 in your case statement Looping Commands for Another form of looping command which is useful in other circumstances is the for command The for command sets a variable to each of the values in a list and executes the body of the command once for each value The syntax is given below for var in list do commands done Eg simple bin bash Script Written by Phil Irving and Andrew Hunter 18 6 98 for NAME in fred joe harry do echo Hello NAME done The script above prints out the messages Hello fred Hello joe and Hello harry The command consists of the keywo
19. quotes as a command which is executed then to substitute the output of the command in its place This is the main way to get the results of commands into your script for further manipulation Use of back quotes is best described by an example bin bash TODA Y date echo Today is TODAY The date command prints out today s date The above script attempts to use it to print out today s date However it does not work The message printed out is Today is date The reason for this is that the assignment today date simply puts the string date into the Page 7 of 8 variable today What we actually want to do is to execute the date command and place the output of that command into the today variable We do this using back quotes bin bash TODA Y date echo Today is STODAY Back quotes have innumerable uses Here is another example This uses the grep command to check whether a file includes the word and bin bash Check for the word and in a file RESULT grep and 1 if RESULT then echo The file 1 includes the word and fi The grep command will output any lines in the file which do include the word and We assign the results of the grep command to the variable result by using the back quotes so if the file does include any lines with the word and in them resu t will end up with some text in it but if the file doesn t include any lines with the word and in them result will end up empty The if s
20. r echo NUMBER add 1 to the number NUMBER expr NUMBER 1 done The above script repeats the while loop 10 times with the number stepping through from 1 to 10 The body of the loop is enclosed between the do and done commands Every time the while command is executed it checks whether the condition in the square brackets is true If it is then the body of the while loop is executed and the computer loops back to the while statement again If it isn t then the body of the loop is skipped If a while loop is ever to end something must occur to make the condition become untrue The above example is a typical example of how a loop can end Here the number variable is initially set to one Each time through the loop it is incremented i e has one added to it once it reaches 10 the condition fails and the loop ends This is the standard technique for repeating something a set number of times If you get inadvertently stuck in such a loop you can always press Ctrl C to break out Just as if statements can be nested so can while statements In the example below the script causes all the times tables to be printed up to 12 bin bash Script Written by Phil Irving and Andrew Hunter 18 6 98 Output my intention followed by a new line echo I am going to print out the times tables from 1 to 12 n set the outer loop at 1 OUTER 1 The OUTER loop while test SOUTER le 12 do Set the inner loop at 1 dont forget the inner loo
21. rd for followed by the name of a variable in this case name but you don t use the dollar in the for statement itself followed by the keyword in followed by a list of values The variable is set to each value in turn and the code between the do and done keywords is executed once for each value The for loop is most successful when combined with the ability to use wildcards to match file names in the current directory The for loop below uses the wildcard to match all files and sub directories in the current directory Thus the loop below is executed once for each file or directory with file set to each one s name This script checks whether each one is a directory using the d option and only writes out the name if it is The effect is to list all the sub directories but not the files in the current directory bin bash Script Written by Phil Irving and Andrew Hunter 18 6 98 for FILE in do if d FILE then echo FILE Page 12 of 8 fi done Looping Commands for Exercises Write a script to output to standard output 4 files named in the script ensure the four files all begin with the prefix bul Modify the above script to automatically output any files in the current directory which begin with the prefix bul Create a directory called bupdir and another called copydir Write a script which will copy files from the copydir into the bupdir and save it as backup Hint your script must work irrespective of its
22. reeting ITCOUNT expr SITCOUNT 1 done Function may also use parameters in a similar fashion to calling a script ie by the positional method Eg bin bash Script Written by Phil Irving and Andrew Hunter 18 6 98 Script to demonstrate use of functions in shell scripts mynamefuncl echo My name is SNAME display the contents of variable NAME echo All parameters are display all parameters in the list Main part of the script Calling script with one parameter mynamefuncl Phil with many mynamefuncl Phil Bill Geoff Returning from a Function To return from a function you may either leave as above or you can use the return command By using the return command it is possible to control what is returned from the function If for instance we want the function to add two numbers together and return the Page 16 of 8 value back to the main part of the script we can do so by setting the exit code of the function to a value be careful if you are testing the variable for true you could get some interesting results The exit code is accessible in the main part of the script by using the Variable Things to note here if no argument is used the function exits with the exit code set to the exit code of the last command executed Eg bin bash Script Written by Phil Irving and Andrew Hunter 18 6 98 This script adds two values in a function and returns the result us
23. rement Page 13 of 8 Descending times table from X to Y you specify X Y and I increment You must write all the associated shell scripts Modify the above script so that the increment is specified in the calling shell and inherited by the sub shell Consolidation Exercises Alter your save script so that if a file has previously been saved the user is asked whether it should be overwritten Hint use the read command to get the user s decision Write a script which lists all files in the current directory which have also been stored in your special directory by the save script Adapt your answer to exercise five so that you list all files in your current directory which include a given word Write an interactive script which allows the user to repeatedly apply the save and restore scripts It should continuously prompt the user for commands which can be either of the form save lt file restore lt file or quit Hint use while read and case Right a script which searches all the sub directories of your current directory for files with a given name Hint use for if cd More Sample Scripts Below is a more complex script which acts as a DOS command interpreter DOS uses the commands cd dir type del renand copy to do the same functions as the UNIX commands cd ls cat rm mv and cp This script loops continuously allowing the user to type in DOS commands which are stored in the variables command arg1 and arg2 The c
24. string given to it in this case 1 the first argument passed to the script with the various strings each of which is followed by a closing bracket Once a match is found the statements up to the double semi colon are executed and the case statement ends The asterix character matches anything so having this as the last case provides a default case handler that is what to do if none of the other cases are matched The keywords are case in and esac end of case Further Input the Read Command So far we have only passed parameters to the shell script from the command line however it is possible for the script to interact with the user This is achieved using the read command which places what the user enters into variables If users are to type several words or values read can be given a number of arguments Syntax read varl var2 var3 Egs bin bash echo Please enter your name n read NAME echo Hello NAME bin bash echo Please enter your firstname followed by a space and your lastname n read FIRSTNAME LASTNAME echo Hello FIRSTNAME SLASTNAME Page 6 of 8 Quoting in Scripts You will recall that shells and also scripts no less than three different types of quotes are used all of which have special meanings We have already met two of these and will now consider all three in detail Two types of quotes are basically designed to allow you to construct messages and strings
25. tatement then checks whether result has actually got any text in it Page 8 of 8 Conditional Statements Exercise 1 Write a shell script to accept a name passed to it as a parameter If the name exists then output hello followed by the name Modify the above script to give a suitable error message if the name is not supplied Modify the above script to determine that if the name exists that it is John Write a shell script to accept two numbers from the command line compare them and determine which is the larger It should output a suitable message e g The first number X is greater than the 2nd number Y X and Y represent the parameters passed in Modify the above script to check whether the numbers exist before commencing the test Try entering the following shell scripts and note their output Write a short user guide for these utitlities and provide sample input and expected output Conditional Statements Continued Case Statement Exercises Write a shell script to accept the name of your friend passed at the command line and issue an appropriate greeting to that friend Save this as casefriend Modify the above script to determine whether the parameter exists before determining your friends name and giving the greeting Save this as casefriend2 Write a menu shell script which will take a parameter from the command line and tell the user the option they have chosen We will modify this later to become more interactive Con
26. ual gt is first number greater than second You can also test whether a variable is set to anything simply by enclosing it in quotes in the condition part of the if statement The script below gives an example More Samples bin bash Script to check that the user enters one argument fred if test 1 then echo Found an argument to this script if 1 fred then echo The argument was fred else echo The argument was not fred fi else echo This script needs one argument fi It is possible to nest constructs which means to put them inside one another Here there is an outer if statement and an inner one The inner one checks whether 7 is fred and says whether it is or not The outer one checks whether has been given at all and only goes on to check whether it is fred if it does exist Note that each if statement has its own corresponding condition then else and fi part The inner if statement is wholly contained between the then and else parts of the outer one which means that it happens only when the first condition is passed Page 4 of 8 bin bash join command joins two files together to create a third Three parameters must be passed two to join the third to create If 3 doesn t exist then the user can t have given all three if test 3 then this cat command will write out 1 and 2 the operator redirects the output into the file 3 otherwise it would appear on
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