Shell Programming, or Scripting - PowerPoint Presentation

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Shell Programming, or Scripting

Shirley MooreCPS 5401 Fall 2013www.cs.utep.edu/svmooresvmoore@utep.eduAugust 29, 2013

1Slide2

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

loopsShell scripts are useful for automating repetitive workflows.Shell scripting is fun!

2Slide3

Disadvantages of Shell Scripting

Incompatibilities between different platformsSlow execution speedA new process is launched for almost every shell command executed.3Slide4

Learning Objectives

After completing this lesson, you should be able to Explain the basics of Linux shell scriptingWrite shell scripts and use them to save timeCustomize your shell startup filesWe will use the Bash shell

4Slide5

Bash initialization and startup files

/etc/profile – the systemwide initialization file, executed for login shells/etc/bash.bashrc – the

systemwide

per-interactive-shell startup file

may not exist or may not get sourcedmight be named /etc/bashrc

/etc/

bash.logout

–

systemwide

login shell cleanup file, executed when a login shell exits

$HOME/.

bash_profile

– your personal initialization file

$HOME/.bashrc – your individual per-interactive-shell startup file$HOME/.bash_logout – your login shell cleanup file$HOME/.inputrc – individual readline initialization file

5Slide6

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.bashrc6Slide7

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.7Slide8

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.shExecute the file$ ./

hello.sh

8Slide9

Shell Comments

Example:#!/bin/bash# A Simple Shell Script To Get Linux Network Information# Vivek Gite - 30/Aug/2009

echo "Current date : $(date) @ $(hostname)"

echo "Network configuration"

/sbin/ifconfigLines beginning with # are ignored by Bash

Explanatory text about the script

Make the script easier to understand and maintain

9Slide10

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

10Slide11

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”

11Slide12

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 $myvar12Slide13

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

13Slide14

Arithmetic Operations

Tryecho $((10 + 5))Create and run a shell script called add.sh:#!/bin/bashread -p “Enter first number : “

x

read -

p “Enter second number : “ yans=

$((

x

+

y

))

echo "$

x

+ $

y = $ans"14Slide15

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

15Slide16

Conditional Execution

Test commandtest –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

16Slide17

If statement

Create and execute a file named number.sh:#!/bin/bashread -p "Enter # 5 : " numberif test $number == 5then

echo "Thanks for entering # 5"

fi

if test $number != 5thenecho "I told you to enter # 5. Please try again."

fi

17Slide18

If statement (2)

Create and execute a file named number.sh:#!/bin/bashread -p "Enter # 5 : " numberif test $number == 5then

echo "Thanks for entering #

5”

elseecho "I told you to enter # 5. Please try again."fi

18Slide19

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 ]

then

echo "$

n

is negative."elif [ $n -eq 0 ]thenecho "$n is zero.”fi19Slide20

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.

20Slide21

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.

21Slide22

Exit Command

exit NThe 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 022Slide23

Exit Status of a Command

Create and run the following script called finduser.sh#!/bin/bash# set varPASSWD_FILE=/etc/

passwd

# get user name

read -p "Enter a user name : " username# try to locate username in in /etc/

passwd

grep

"^$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.sh

OPT=$1

# option

FILE=$2 # filename# test command line args matching

case $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

24Slide25

For loop

#!/bin/bash# testforloop.shfor i in 1 2 3 4 5do echo

"Welcome $

i

times."done25Slide26

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

26Slide27

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 $

ndone27Slide28

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.

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