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Shell Programming

Shell Programming. Any command or a sequence of UNIX commands stored in a text file is called a shell program. It is common to call this file a command file or a script file . Usually the terms command or script is used when the file contains a sequence of commands.

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Shell Programming

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  1. Shell Programming • Any command or a sequence of UNIX commands stored in a text file is called a shell program. • It is common to call this file a command file or a script file. Usually the terms command or script is used when the file contains a sequence of commands. • The term shell program usually identifies a file containing a more complicated arrangements of commands, often using the shell's conditional commands and other advanced features. • Three ways to execute a command file: 1. Redirection 2. Supply the script name as an argument to the sh command 3.

  2. Executing a command file • 1.        Since the shell reads commands interactively, use redirection to get the shell to read the commands from a file: ie $sh < lsdir • This is a bit clumsy and also does not allow you to supply command line arguments to the script which may be required in some cases. • 2.        The second method is to supply the script name as an argument to the sh command. This reflects the special capablity that is built into the shell. Ie • $ sh lsdir • One advantage is that you can supply arguments to the script ie • $ sh lsdir /home/gb/exams • The drawback of the two methods shown is that the script has to be explicitly run in order to execute the commands. • You cannot avoid using the shell to execute a script but you can have the script executed automatically if you set the execute privileges to the script. • When you create a script in text editor, the execute permission is turned off. This can be turned on to make the script text executable.

  3. Executing scripts • 3.        Thus method 3 makes the script executable by changing the permission to the script file • $ ls -l lsdir • -rw-r--r-- etc • $ chmod a+x lsdir • -rwxr-xr-x - etc • When you make the script executable, it must be located somewhere in the search path. • Most search paths are constructed to look for programs in the current directory. However, if this is not the case the script will not execute. • You can also pass arguments to the executable script.

  4. Shells can store variables • Shell can also use variables to store values: ie • $ kc=goran • $echo $kc • goran • (but no spaces are allowed) • If spaces are needed use quotes • $ kc="goran is here" • $echo $kc • goran is here • Curly brackets are needed when to surround the variable when it is immediately followed by characters that are not part of the name. • $ kc=goran • $echo ${kc}Bez • goranBez

  5. Given an example Bourne Shell script – explain what is going on • TMP=/tmp • PIDLOG=${TMP}/pidlog$$ • DATAFILE=${TMP}/date$$ • DATEFILE=${TMP}/date$$ • ERRLOG=${TMP}/ERRLOG • #shell function to clean up and exit after an error • errexit() • { echo $1 • date>> $ERRLOG • rm –f $PIDLOG $DATAFILE • exit • } • ok() • { while true • do • read ans • case $ans in • [yY]*) return 0 ;; • [nN]*) return 1;; • *) echo please answer y or n ;; • esac • done • } • echo $$ > $PIDLOG • date > $DATAFILE • echo –n “Testing the errexit function” • echo normal termination • echo Please remove $PIDLOG and $DATAFILE

  6. Example Shell Script • This program calls two functions errexit() and ok(). Comments: • ------------------------------------------------------- • #files used in the program are given here • TMP=/tmp • PIDLOG=${TMP}/pidlog$$ • DATAFILE=${TMP}/data$$ • DATEFILE=${TMP}/date$$ • ERRLOG=${TMP}/ERRLOG • ------------------------------------------------------- • #shell function to clean up and exit after an error, • #this is the first function call. • errexit() • { • echo $1 • date>> $ERRLOG • rm –f $PIDLOG $DATAFILE • exit • } • Within the shell function the arguments are accessed using the notation $1, $2 etc.. Errexit is used to exit from the program when an error is encountered. • In this event, errexit deletes temporary files, logs the error in a log file, and prints a message.

  7. The other function call • # read a y/n response, this is the second function call • ok() • { • while true • do • read ans • case $ans in • [yY]*) return 0 ;; • [nN]*) return 1;; • *) echo please answer y or n ;; • esac • done • } • Function ok() reads a line from the standard input, and returns TRUE or FALSE, depending on user input y/n.

  8. Continued • echo $$ > $PIDLOG • date > $DATAFILE • echo –n “Testing the errexit function” • echo normal termination • echo Please remove $PIDLOG and $DATAFILE • The body of the shell program asks if you want to test the errexit function, in which case user responds with y, conversely user responds with a n. • Function run: • The following are the possible results when the program runs, Two options user Y or n • $ function_ssi • Test errexit function [y/n] y • Testing the errexit function • $ function_ssi • Test errexit function [y/n] n • Normal Termination • Please remove $PIDLOG and $DATAFILE

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