second edition Your UNIX: The Ultimate Guide Das © 2006 The McGraw-Hill Companies, Inc. All rights reserved. UNIX – Shell Programming The activities of the shell are not restricted to command interpretation alone. shell has a whole set of internal commands that can be strung together as a language most of the constructs are borrowed from C, but there are syntactical differences shell programming is powerful because the external UNIX commands blend easily with the shell’s internal constructs in shell scripts

second edition Your UNIX: The Ultimate Guide Das © 2006 The McGraw-Hill Companies, Inc. All rights reserved. UNIX – Shell Programming We will examine the programming features of the lowest common denominator of all shells the Bourne shell (/bin/sh) and its derivatives – the Korn shell (/bin/ksh) and Bash (/bin/bash). The C shell uses totally different programming constructs

second edition Your UNIX: The Ultimate Guide Das © 2006 The McGraw-Hill Companies, Inc. All rights reserved. UNIX – Shell Programming Shell Scripts when a group of commands have to be executed regularly, they should be stored in a file, and the file executed as a shell script or a shell program. it is not mandatory using the.sh extension for shell scripts; makes them easy to match with the wild cards. shell script needs to have execute permission when invoked by its name.

second edition Your UNIX: The Ultimate Guide Das © 2006 The McGraw-Hill Companies, Inc. All rights reserved. UNIX – Shell Programming Shell Scripts it is not compiled to a separate executable file as a C program is it runs in interpretive mode and in a separate child process the calling process (often the login shell) forks a sub-shell which reads the script file and loads each statement into memory when it is to be executed shell programs are slower than compiled programs shell scripts are not recommended for number crunching, but to automate routine tasks often scheduled to run non-interactively with cron (cron daemon)

second edition Your UNIX: The Ultimate Guide Das © 2006 The McGraw-Hill Companies, Inc. All rights reserved. UNIX – Shell Programming Shell Scripts System administrative tasks are often best handled by shell scripts the reason why UNIX system administrator must be an accomplished shell programmer.

second edition Your UNIX: The Ultimate Guide Das © 2006 The McGraw-Hill Companies, Inc. All rights reserved. UNIX – Shell Programming Shell Scripts The She-Bang Line the first line of the script (#!bin/bash) contains a string beginning with #! this is not a comment line it is called the interpreter line, hash-bang, or she-bang line when the shell executes, the login shell reads this line first to determine the pathname of the program to be used for running the script

second edition Your UNIX: The Ultimate Guide Das © 2006 The McGraw-Hill Companies, Inc. All rights reserved. UNIX – Shell Programming Shell Scripts The She-Bang Line the login shell spawns a Bourne sub-shell which actually executes each statement in sequence (in interpretive mode) failure to provide the she-bang line, the login shell will spawn a child of its own type to run the script—which may not be the shell you want you can also explicitly spawn a shell of your choice by running the program representing the shell with script name as argument: sh script.sh

second edition Your UNIX: The Ultimate Guide Das © 2006 The McGraw-Hill Companies, Inc. All rights reserved. UNIX – Shell Programming Shell Scripts The She-Bang Line when using the shell with the script name as argument, the Bourne sub-shell opens the file but ignores the interpreter line

second edition Your UNIX: The Ultimate Guide Das © 2006 The McGraw-Hill Companies, Inc. All rights reserved. UNIX – Shell Programming Shell Scripts read: Making Scripts Interactive The read statement is the shell’s interactive tool for taking input from the user, making script interactive read is used with one or more variables that are assigned by keyboard input. The statement read name No $ here will pause the script to take input from the standard input a single read statement can be used with one or more variables read pname flname

second edition Your UNIX: The Ultimate Guide Das © 2006 The McGraw-Hill Companies, Inc. All rights reserved. UNIX – Shell Programming Shell Scripts #!/bin/sh echo "Enter the pattern to be searched: \c" read pname echo "Enter the file to be used: \c" read flname echo "Searching for $pname from file $flname" grep "$pname" $flname echo "Selected lines shown above"

second edition Your UNIX: The Ultimate Guide Das © 2006 The McGraw-Hill Companies, Inc. All rights reserved. UNIX – Shell Programming Shell Scripts {apache:~} emp1.sh Enter the pattern to be searched: director Enter the file to be used: shortlist Searching for director from file shortlist 9876: bill johnson :director :production:03/12/50: :john woodcock :director :personnel :05/11/47: Selected lines shown above

second edition Your UNIX: The Ultimate Guide Das © 2006 The McGraw-Hill Companies, Inc. All rights reserved. UNIX – Shell Programming Shell Scripts Using Command Line Arguments Scripts not using read can run non-interactively and be used with redirection and pipelines such scripts take their input from command line arguments assigned to certain special “variables”—positional parameters the first argument is available in $1, the second $2, and so on in addition to the positional parameters, there are a few other special parameters used by the shell

second edition Your UNIX: The Ultimate Guide Das © 2006 The McGraw-Hill Companies, Inc. All rights reserved. UNIX – Shell Programming Shell Scripts Using Command Line Arguments $* -- Stores the complete set of positional parameters as a single string $# -- It is set to the number of arguments specified; lets you design scripts that check whether the right number of arguments have been entered $0 – Holds the script filename itself.

second edition Your UNIX: The Ultimate Guide Das © 2006 The McGraw-Hill Companies, Inc. All rights reserved. UNIX – Shell Programming Shell Scripts Using Command Line Arguments #!/bin/sh echo "Program: $0" echo "The number of arguments specified is $#" echo "The argumetns are $*" grep $1 $2 echo "\nJob Over"

second edition Your UNIX: The Ultimate Guide Das © 2006 The McGraw-Hill Companies, Inc. All rights reserved. UNIX – Shell Programming Shell Scripts Using Command Line Arguments {apache:~} emp2.sh director shortlist Program:./emp2.sh The number of arguments specified is 2 The argumetns are director shortlist 9876: bill johnson :director :production:03/12/50: :john woodcock :director :personnel :05/11/47:120000

second edition Your UNIX: The Ultimate Guide Das © 2006 The McGraw-Hill Companies, Inc. All rights reserved. UNIX – Shell Programming Shell Scripts Using Command Line Arguments $1, $2…Positional parameters representing arguments $#Number of arguments $0Name of executed command $*Complete set of positional parameters in a string quoted string is treated as a separate arg $?Exit status of the last command $$PID of current shell $!PID of last background job

second edition Your UNIX: The Ultimate Guide Das © 2006 The McGraw-Hill Companies, Inc. All rights reserved. UNIX – Shell Programming Shell Scripts exit and $? : Exit status of a Command All programs and shell scripts return a value called the exit status to the caller, often the shell The shell waits for a command to complete execution and then picks up this value from the process table exit 0 – True, everything went fine exit 1 – False; something went wrong

second edition Your UNIX: The Ultimate Guide Das © 2006 The McGraw-Hill Companies, Inc. All rights reserved. UNIX – Shell Programming Shell Scripts exit and $? : Exit status of a Command A program is designed to return a true exit status when it runs successfully and false otherwise what constitutes success or failure is determined by the designer of the porgram for example, when grep could not locate a pattern; we said that the command failed; meaning the designer of grep made the program returns false exit status on failing to locate a pattern

second edition Your UNIX: The Ultimate Guide Das © 2006 The McGraw-Hill Companies, Inc. All rights reserved. UNIX – Shell Programming Shell Scripts exit and $? : Exit status of a Command A program is designed to return a true exit status when it runs successfully and false otherwise what constitutes success or failure is determined by the designer of the porgram for example, when grep could not locate a pattern; we said that the command failed; meaning the designer of grep made the program returns false exit status on failing to locate a pattern

second edition Your UNIX: The Ultimate Guide Das © 2006 The McGraw-Hill Companies, Inc. All rights reserved. UNIX – Shell Programming Shell Scripts exit and $? : Exit status of a Command A program is designed to return a true exit status when it runs successfully and false otherwise the pattern $? stores the exit status of the last command; the value of zero (0) indicates success and non-zero when the command fails if no exit is specified, then $? is set to to zero (true)

second edition Your UNIX: The Ultimate Guide Das © 2006 The McGraw-Hill Companies, Inc. All rights reserved. UNIX – Shell Programming Shell Scripts exit and $? : Exit status of a Command /dev/null  the null device that discards all data written to it {apache:~} grep director emp.lst > /dev/null; echo $? grep: can't open emp.lst 2 {apache:~} grep director shortlist > /dev/null; echo $? 0 {apache:~} grep manager shortlist > /dev/null; echo $? 1

second edition Your UNIX: The Ultimate Guide Das © 2006 The McGraw-Hill Companies, Inc. All rights reserved. UNIX – Shell Programming Shell Scripts exit and $? : Exit status of a Command The if and while constructs implicitly check $? to control the flow of execution. it is good programming practice to place exit statements with meaningful exit values at appropriate points in script for example, if an important file doesn’t exist or can’t be read, there is not point in continuing with script execution use exit 1 at that point so the next program knows that the previous program failed

second edition Your UNIX: The Ultimate Guide Das © 2006 The McGraw-Hill Companies, Inc. All rights reserved. UNIX – Shell Programming Shell Scripts The Logical Operators && and || -- Conditional Execution cmd1 && cmd2cmd2 executed if cmd1 succeeds cmd1 || cmd2cmd2 executed if cmd1 fails When && is used to delimit two commands, cmd2 is executed only when cmd1 succeeds. {apache:~} grep director shortlist > /dev/null && echo "pattern found in file" pattern found in file

second edition Your UNIX: The Ultimate Guide Das © 2006 The McGraw-Hill Companies, Inc. All rights reserved. UNIX – Shell Programming Shell Scripts The Logical Operators && and || -- Conditional Execution cmd1 && cmd2cmd2 executed if cmd1 succeeds cmd1 || cmd2cmd2 executed if cmd1 fails The || operators does the opposite; the second command is executed only when the first fails: {apache:~} grep manager shortlist > /dev/null || echo "pattern found in file" pattern found in file

second edition Your UNIX: The Ultimate Guide Das © 2006 The McGraw-Hill Companies, Inc. All rights reserved. UNIX – Shell Programming Shell Scripts The Logical Operators && and || -- Conditional Execution To display a message before invoking exit, you need to group commands, but remember to use only curly braces this will execute the enclosed commands in the current shell: grep joker /etc/passwd || { echo "Pattern not found" ; exit 0; } Use of paranthesis would not terminte the script. If the {} is executed at the shell prompt, you would be logged out. The logical operators are recommended for making simple decisions; otherwise use the if statement.