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Shell Programming, or Scripting

Shirley MooreCPS 5401 Fall 2013

www.cs.utep.edu/svmooresvmoore@utep.edu

August 29, 2013

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What is a Shell Script?• Normally shells are interactive – a shell accepts a

command from you and executes it.• You can store a sequence of commands in a text file,

called a shell script, and tell the shell to execute the file.• In addition to commands, you can use – functions– control flow statements

• if..then..else• loops

• Shell scripts are useful for automating repetitive workflows.

• Shell scripting is fun!

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Disadvantages of Shell Scripting

• Incompatibilities between different platforms• Slow execution speed• A new process is launched for almost every

shell command executed.

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Learning Objectives

• After completing this lesson, you should be able to – Explain the basics of Linux shell scripting– Write shell scripts and use them to save time– Customize your shell startup files

• We will use the Bash shell

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Bash initialization and startup files1. /etc/profile – the systemwide initialization file, executed for

login shells2. /etc/bash.bashrc – the systemwide per-interactive-shell

startup file– may not exist or may not get sourced– might be named /etc/bashrc

3. /etc/bash.logout – systemwide login shell cleanup file, executed when a login shell exits

4. $HOME/.bash_profile – your personal initialization file5. $HOME/.bashrc – your individual per-interactive-shell startup

file6. $HOME/.bash_logout – your login shell cleanup file7. $HOME/.inputrc – individual readline initialization file

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Bash init and startup files (2)

• A login shell calls the following when a user logs in– /etc/profile runs first when a user logs in– $HOME/.bash_profile runs second– $HOME/.bash_profile calls $HOME/.bashrc, which

calls /etc/bash.bashrc

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To Create a Shell Script

• Use a text editor such as vi to create the file.• Save and close the file.• Make the file executable.• Test the script.

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Try this Example

• Save the following into a file named hello.sh and close the file:#!/bin/bashecho “Hello, World!”echo “Knowledge is power.”

• Make the file executable$ chmod +x hello.sh

• Execute the file$ ./hello.sh

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Shell Comments

• Example:#!/bin/bash# A Simple Shell Script To Get Linux Network Information# Vivek Gite - 30/Aug/2009echo "Current date : $(date) @ $(hostname)"echo "Network configuration"/sbin/ifconfig

• Lines beginning with # are ignored by Bash• Explanatory text about the script• Make the script easier to understand and maintain

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Formatted Output with printf

• Use the printf command to format output to appear on the screen (similar the C printf() )

• Example:printf “%s\n” $PATH

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Quoting

• Try these$ echo $PATH$ echo “$PATH”$ echo ‘$PATH’$ echo \$PATH$ echo /etc/*.conf$ echo “/etc/*.conf”$ echo ‘/etc/*.conf’$ echo “PATH is $PATH”$ echo “PATH is \$PATH”

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Export and Unset• The export builtin exports environment variables to child processes.• Try the following:

$ myvar=“Hello, world”$ echo $myvar$ export myvar2=“Hello, world2”$ echo $myvar2$ bash$ echo $myvar$ echo $myvar2$ exit$ export$ echo $myvar$ unset myvar$ echo $myvar

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Getting User Input

• Create a script called greet.sh as follows:#!/bin/bashread -p "Enter your name : " nameecho "Hi, $name. Let us be friends!"

• Save and close the file. Run it as follows:chmod +x greet.sh./greet.sh

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Arithmetic Operations

• Tryecho $((10 + 5))

• Create and run a shell script called add.sh:#!/bin/bashread -p “Enter first number : “ xread -p “Enter second number : “ yans=$(( x + y ))echo "$x + $y = $ans"

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Variable Existence Check

• Create a shell script called varcheck.sh:#!/bin/bash# varcheck.sh: Variable sanity check with :?path=${1:?Error command line argument not passed}echo "Backup path is $path."echo "I'm done if \$path is set.”

• Run it as follows:chmod +x varcheck.sh./varcheck.sh /home./varcheck.sh

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Conditional Execution• Test command

test –f /etc/autofs.conf && echo “File autofs.conf found” || echo “File autofs.conf not found”

• Can also use [ ][ -f /etc/autofs.conf ] && echo “file autofs.conf found” || echo “file

autofs.conf not found”• For more information:

$ man test• command1 && command2 – execute command2 if command1

is successful• command1 || command2 – execute command2 if command1 is

not successful

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If statement

• Create and execute a file named number.sh:#!/bin/bashread -p "Enter # 5 : " numberif test $number == 5thenecho "Thanks for entering # 5"fiif test $number != 5thenecho "I told you to enter # 5. Please try again."fi

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If statement (2)

• Create and execute a file named number.sh:#!/bin/bashread -p "Enter # 5 : " numberif test $number == 5thenecho "Thanks for entering # 5”elseecho "I told you to enter # 5. Please try again."fi

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Nested If• Create and execute numest.sh:

#!/bin/bashread -p "Enter a number : " nif [ $n -gt 0 ]; thenecho "$n is positive."elif [ $n -lt 0 ]thenecho "$n is negative."elif [ $n -eq 0 ]thenecho "$n is zero.”fi

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Command-line Arguments

• Create and run the following script called cmdargs.sh giving it some arguments:

#!/bin/bashecho "The script name : $0"echo "The value of the first argument to the script : $1"echo "The value of the second argument to the script : $2"echo "The value of the third argument to the script : $3"echo "The number of arguments passed to the script : $#"echo "The value of all command-line arguments (\$* version) : $*"echo "The value of all command-line arguments (\$@ version) : $@”

• Try adding IFS=“,” as the second line of the script.

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Shell Parameters• All command line parameters or arguments can be accessed

via $1, $2, $3,..., $9.• $* holds all command line parameters or arguments.• $# holds the number of positional parameters.• $- holds flags supplied to the shell.• $? holds the return value set by the previously executed

command.• $$ holds the process number of the shell (current shell).• $! hold the process number of the last background command.• $@ holds all command line parameters or arguments.

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Exit Command• exit N

– The exit statement is used to exit from a shell script with a status of N.– Use the exit statement to indicate successful or unsuccessful shell script

termination.– The value of N can be used by other commands or shell scripts to take their

own action.– If N is omitted, the exit status is that of the last command executed.– Use the exit statement to terminate a shell script upon an error.– N set to 0 means normal shell exit.

• Create a shell script called exitcmd.sh:#!/bin/bashecho "This is a test."# Terminate our shell script with success messageexit 0

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Exit Status of a Command

• Create and run the following script called finduser.sh#!/bin/bash# set varPASSWD_FILE=/etc/passwd# get user nameread -p "Enter a user name : " username# try to locate username in in /etc/passwdgrep "^$username" $PASSWD_FILE > /dev/null# store exit status of grep# if found grep will return 0 exit stauts# if not found, grep will return a nonzero exit statusstatus=$?if test $status -eq 0thenecho "User '$username' found in $PASSWD_FILE file."elseecho "User '$username' not found in $PASSWD_FILE file."fi

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Command arg processing using case#!/bin/bash# casecmdargs.shOPT=$1 # optionFILE=$2 # filename# test command line args matchingcase $OPT in -e|-E) echo "Editing file $2..." # make sure filename is passed else an error displayed [ -z $FILE ] && { echo "File name missing"; exit 1; } || vi $FILE ;; -c|-C) echo "Displaying file $2..." [ -z $FILE ] && { echo "File name missing"; exit 1; } || cat $FILE ;; -d|-D) echo "Today is $(date)" ;; *) echo "Bad argument!" echo "Usage: $0 -ecd filename" echo " -e file : Edit file." echo " -c file : Display file." echo " -d : Display current date and time." ;;esac

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For loop

#!/bin/bash# testforloop.shfor i in 1 2 3 4 5do echo "Welcome $i times."done

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Nested for loop#!/bin/bash# chessboard.sh – script to display a chessboard on the screenfor (( i = 1; i <= 8; i++ )) ### Outer for loop ###do for (( j = 1 ; j <= 8; j++ )) ### Inner for loop ### do total=$(( $i + $j)) # total tmp=$(( $total % 2)) # modulus # Find out odd and even number and change the color # alternating colors using odd and even number logic if [ $tmp -eq 0 ]; then echo -e -n "\033[47m " else echo -e -n "\033[40m " fi done echo "" #### print the new line ###done

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While loop

#!/bin/bash# while.sh# set n to 1n=1# continue until $n equals 5while [ $n -le 5 ]do echo "Welcome $n times." n=$(( n+1 )) # increments $ndone

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Until loop

#!/bin/bashuntil.shi=1until [ $i -gt 6 ]do echo "Welcome $i times." i=$(( i+1 ))done

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Exercises

• Write a shell script that counts the number of files in each of the sub-directories of your home directory.

• Write a shell script that accepts two directory names as arguments and deletes those files in the first directory that have the same names in the second directory.