Ch10 Linux Shell Bash Script

8/9/2019 Ch10 Linux Shell Bash Script

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SAIGONLAB 69-3 Nguyen Thi Nho, P9, Q.TBinh, Tp. HCM LPI 102

SAIGONLAB 83 Nguyn Th Nh, P9, Q.Tn Bnh, Tp. HCM LPI 102

Chapter 10Chapter 10

THE LINUX SHELLTHE LINUX SHELL

BASH SCRIPTBASH SCRIPT


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Objectives

Identify different Linux shell environments

Understand the redirection of input and outputUnderstand and utilize command substitution

Write and configure BASH script using variables,

flow controls interactive input, functions, arithmeticand arrays


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The Linux Shell

Shellis a interface between OS and user. It provides :

A facility for launchingand managingcommands andprograms

An operating environment

Aprogramming language


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The Linux Shell

Type of shell : Bourne shell (sh), Bourne Again shell(bash), Korn shell (ksh), C shell (csh, tcsh),

Programs start from command line have separateenvironments : parameters, variables, functions,

aliases


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Shell Environment Customize

Type of variables: local(shell variable),global(environment variable)

List all local variables : use set commandList all global variables : use env command

Default environment variables : PS1, PS2,

HOME, LOGNAME, SHELL, PATH, PAGER,

LPDEST, PWD, DISPLAY, MAIL,...


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Shell Environment Customize

bash configuration files :

/etc/profile

~/.bash_profile, ~/.bash_login, ~/.profile

set : define a new variable

unset : undefine a variable

export : make a localvariable becomes aglobalvariable


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SAIGONLAB 69-3 Nguyen Thi Nho, P9, Q.TBinh, Tp. HCM LPI 102SAIGONLAB 83 Nguyn Th Nh, P9, Q.Tn Bnh, Tp. HCM LPI 102

Using the bash shell

Alt+Fn : switch between virtual consoles

gpm : mouse server daemon, use it to copy and pasteeven between different consoles

Auto complete : use TAB key

Up and down arrow keys : get history commands

(store in ~/.bash_history)Ctrl+Z, Ctrl+C, Ctrl+D, *, ?


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Redirecting Input and Output


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Redirecting Input and Output

Redirect input : use ()

# ls l > list_file

( Use set o noclobber : prevent file overwriting )

Append : use (>>)

Redirect error : use (2>)


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Pipe and Back ticks (` ,`)

Pipe (|): command1 | command2

Output of command1 becomes input of command2

# ls l |grep sambaBack ticks (``) or $()

1. # which passwd

/usr/bin/passwd2. # ls l /usr/bin/passwd

3. # ls l `which passwd`


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Background jobs

Job ( process) :

# ls l | grep *.jpg

Run job in the background :

Append with & :

# sleep 1000 &

[1] 1234

If a job is running in foreground, suspend it by Ctrl+Z

then run the following command to switch to

background: # bg %job_id


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Shell Script

Shell script : is a textfile contains shell commands,functions, aliases, flow control structures, loops,comments

All comments begin with # but #! is used toidentify a commands interpreter

$ more example_script

#!/bin/bash/usr/bin/smbd -D

/usr/bin/nmbd -D


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Variables

Naming : not begin with a digit, usually in upper caseletters

Assigning : not SPACES around =

VAR=value : assign value string to VAR

VAR=`cmd` : the same VAR=$(cmd) ,assign outputofcmd to VAR

# VAR1=`ls /var/log | wc l`

# echo $VAR1

65


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Variables

Quotes (single, double) :

# VAR=Hello World# echo $VAR

Hello World

# echo $VAR

$VAR


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Variable Notation

Expand variables : use ${VAR}

# VAR1=This is a String ; echo $VAR1

This is a String

# VAR2=$VAR1xyz ; echo $VAR2

Nothing #default

# VAR3=${VAR1}xyz ; echo $VAR3

This is a Stringxyz

# VAR4=${VAR1}xyz ; echo $VAR4

${VAR1}xyz


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Passing Information to Sript

On the command line, information can be passed toscript through pre-set positional parameters

$0 The name of the script

$1-$9 Parameters are being passed to script$* A string contains ALL parameters passed to script,

separated by the first character defined in IFS (InternalFile Separator) variable

$@ A list of ALL parameters as separate string$# Number of parameters included on the command line

The shift command will shiftthe positional parametersone or more position from leftto right


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Flow control

Loop : do something more than one time

Loop commands : for ,while ,until


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The for Loop

Syntax :

for in do

# list of commands to do

done


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The for Loop Example

This script will rename all file .txt in current directoryto .html

#!/bin/bashforfile in $(ls *.txt)

do

newname=$(basename $file .txt).html;

mv $file $newname;

done


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The while and until Loop

Syntax :while

do

# list of commands to do

done

--------------------------------------------------------

until

do# list of commands to do

done


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The while Loop Example

count=0

while [ $count lt 4 ]

do

echo $countcount=$(($count +1))

done

Output :

0

1

2

3


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The until Loop Example

count=0

until [ $count ge 4 ]

do

echo $countcount=$(($count +1))

done

Output :

01

2

3


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Return codes/Exit status

The variable $? contains the return code of the

previous executed command or application.0 Success

0 Failure

The exit n command will cause the script to quit andassign the value of n to $? variable


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Tests and Conditions

Test : use [ ] around expression

If-then-else structure:

if [ ] # include SPACEsthen

#commands to do ifthe exp1 is true

else #commands to do ifthe exp1 is NOT true

fi


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case Structure

case expression in

pattern1 )

action

;;

pattern2)

action

;;

esac


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Input Interactive

We can input information into script when executingthe script

Commands :

read

select


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read Command

Allow to read values into variables

Syntax :read VAR1 VAR2

If there is more input than you are looking

for, all the extras are put in the lastvariable.


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read Command Example

#!/bin/bashecho Enter 2 number, I will add them

read VAR1 VAR2

echo $VAR1 + $VAR2 = $(($VAR1+$VAR2))


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select Command

It is great for creating menu

Syntax :

select in

do

# commands

done


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Case Command example

#!/bin/bash

VAR=$(ls)

select i in $VAR

do

echo $i;

exit;done


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Functions

Syntax :

functionfunction_name ()

{

#commands}

Or

function_name ()

{

#commands

}


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Functions

Functions can be called in main script by functionsname.

It inherits ALL parameters in the main script

We can change the return code of the function by using

return n command


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Summary

Step 1 : create script file (cat, vi, mc, ),enter script

codes.Step 2 : add execute permission mode to file ( chmod u+xfile )

Step 3 : run it (add script directory to PATHenvironment or use absolute path)

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Ch10 Linux Shell Bash Script