Linux Shell Scripts

8/10/2019 Linux Shell Scripts

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Linux Shell Scripts


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Road Map

To understand the types of shell.

To understand Shell variables.

To understand the programming constructs.

To understand the System Variables.

To customize the shell.


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Reference

UNIX Bourne Shell Scriptinghttp://www.esscc.uq.edu.au/~ksteube/Bshell/
http://www.esscc.uq.edu.au/~ksteube/Bshell/http://www.esscc.uq.edu.au/~ksteube/Bshell/


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What is Shell.

A program that interprets users requests to runprograms.Provides environment for user programs. A command line interpreter that

Reads user inputs.Executes commands.


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Types of ShellShell Name Developed By Remark

BASH ( Bourne- Again SHell )

Brian Fox andChet Ramey

Most common shell inLinux. It's Freeware shell.

CSH (C SHell) Bill Joy The C shell's syntax andusage are very similar tothe C programminglanguage.

KSH (KornSHell)

David Korn

TCSH See the manpage.Type $ man tcsh

TCSH is an enhanced butcompletely compatibleversion of the BerkeleyUNIX C shell (CSH).

Tip: To find all available shells in your system type following command:$ cat /etc/shells

To find your current shell type following command

$ echo $SHELL


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What Shell Does ?

Shell allows three types of commands: An internal command.

ls,cat,cd

An executable file that contains a sequence of shellcommand lines..sh file

An executable file that contains object code produced bycompilation.

.o file generated from a c file


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Why to Write Shell Script ?

Shell script can take input from user, file andoutput them on screen.

Useful to create our own commands.Save lots of time.To automate some task of day today life.

System Administration part can be alsoautomated.


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Shell Variables.

Positional Parameters Acquire values from the position of argumentsin command line

Special ParametersCreated and maintained by Linux itself.

Named variablesCreated and maintained by user.


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Positional Parameters Examples.

Acquire values from the position of argumentsin command line.

$1, $2, $3,..$9

sh file1 10 20 30

$1 $2 $3


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Special Parameters.

SpecialParameter Meaning Usage$$ PID number of the current shell echo $$ , Then type tcsh and echo $$

$#Number of Command Line

Arguments Run Special.sh in the example$* Displays all command line option Run Special.sh in the example

$? Exit statusecho $? If command is successfullyrun then it will give 0 else 1. try to copya non existant file and echo $?

$! Process number of the lastbackground command

sleep 3000 & , echo $!


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Named Variables.

User-defined variablethat can be assigneda value.Used extensively in

shell-scripts. Used for readingdata, storing anddisplaying it.

Example 1 Example 2

$> a=10 , echo $a $> name=Binaya $> echo$name


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The read/display Statement

Use to get input (data from user) from keyboard andstore (data) to variable.Syntax : read variable_name.

Example : read sname

VariableName


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Program Execution.

shCommand to execute program in Unix.Syntax : sh Example : sh file1


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Test or [expr] command.

Test command or [ expr ] is used to see if anexpression is true,If it is true it return zero(0), otherwise returns

nonzero for false.In place of writing test explicitly, the user could alsouse [ ].test or [ expr ] works with1.Integer ( Number without decimal point)2.File types3.Character strings


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Test or [expr] command for Integers

Using Test Using [ expr]

5 == 6 is equal to if test 5 -eq 6 if [ 5 -eq 6 ]5 != 6 is not equal to if test 5 -ne 6 if [ 5 -ne 6 ]5 < 6 is less than if test 5 -lt 6 if [ 5 -lt 6 ]5 6 is greater than if test 5 -gt 6 if [ 5 -gt 6 ]

5 >= 6 is greater than or equalto

if test 5 -ge 6 if [ 5 -ge 6 ]

In ShellNormal ArithmeticalStatement

Meaning

Tip: Make sure that there is a space when you use [].


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Test or [expr] command for Strings

string1 = string2 string1 is equal to string2string1 != string2 string1 is NOT equal to string2string1 string1 is NOT NULL or not defined-n string1 string1 is NOT NULL and does exist

-z string1 string1 is NULL and does exist

Operator Meaning


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Test or [expr] command for files

-s file Non empty file -f file Is File exist or normal file and not a directory-d dir Is Directory exist and not a file

-w file Is writeable file-r file Is read-only file -x file Is file is executable

Operator Meaning


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Logical Operators

! expression Logical NOTexpression1 -a expression2 Logical ANDexpression1 -o expression2 Logical OR

Operator Meaning


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expr command.

Used for evaluating shell expressions.Used for arithmetic and string operations.

Example : expr 7 + 3 would give an output 10.

When used with variables, back quotes needto be used.

Operator has to be preceded and followed by a space.


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Shell Quotes

Case Answer Single-Quote (' ') Takes Literally x=10; echo 'echo $x' echo $xSingle- Back Quote (``) Interpret the result X=èxpr $x + $y`Double Quote ("") Evaluate the variables ($) x=10; echo 'echo $x' echo 10

Quote Type Usage

Tip 1: Make sure that single command work on command line.

Tip 2 : Be careful about the space.

y =èxpr $x+$x` Not OKy =èxpr $x + $x` OK


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Program Constructs

iffor while

untilcase


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if statement.

Syntax:

i f con t ro l comm and

then

else

fi


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for statement.

Syntax.

for variable-name in value1 value2 .. .. valueN

d o

< c o m m a n d s >

done


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while statement.

Syntax.

w h i l e c o n t ro l c o m m a n d

d o

< c o m m a n d s >

d o n e


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until statement.

Syntax.

unt i l control command

d o

d o n e


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case statement.

Syntax.

case value in

cho ice1) com m ands ; ;

cho ice2) com m ands ; ;

....

....esac

The symbols ;; are used as option terminators.


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Useful Shell Scripting commands.

breakTo come out of a loop.

continue

To jump to the start of loop.exit

To prematurely terminate a program.

: or #

To interpret the rest of line as comments.


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export command.

exportTo make a variable a part of environment and also beaccessible to the child process (or shell)

Give an Example of Oracle ENV variable


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Documents

Linux Shell Scripts