Shell Scripting 2

7/31/2019 Shell Scripting 2

1/53

LINUX


2/53

Variables

process our data/information

kept in computers RAM memory

two types of variables

System variables

Created and maintained by Linux itself and is defined in

CAPITAL LETTERS.

User defined variables (UDV)

Created and maintained by user and is defined in

LOWER LETTERS


3/53

System Variables


4/53

System Variables

$ echo $USERNAME

$ echo $HOME

Caution: Do not modify System variable this can

some time create problems.


5/53

System variables

PS1 variable

Prompt string 1 variable

Contains the shell prompt, $ symbol

To change the shell prompt

$PS1=Hello>

Now your prompt becomes

Hello>


6/53

PS2 variable

Specifies the value for the secondary prompt

Secondary prompt

Displays when an incomplete command is entered

on the command line

Default value is > symbol

Example

$PS2=^


7/53

Example - PS2 variable

$ echo This is incomplete

^ close the quotes

^

$


8/53

LOGNAME variable

Users login name

echo ${LOGNAME}

Or

echo $LOGNAME


9/53

SHLVL variable Contains the shell level currently working in

$echo $SHLVL

1 this is the login shell

$sh

$echo $SHLVL

2 working in the new shell

$exit

$echo $SHLVL

1 login shell


10/53

env command

View a list of all the exported environment variables

and their respective values

$env


11/53

Rules for Naming variable name (BothUDV and System Variable)

Variable name must begin with Alphanumeric character or

underscore character (_), followed by one or more

Alphanumeric character

Don't put spaces on either side of the equal sign when

assigning value to variable.

Variables are case-sensitive


12/53

Rules for Naming variable name (BothUDV and System Variable)

You can define NULL variable as follows (NULL variable is

variable which has no value at the time of definition)

For e.g.

$ vech=

$ vech="

Do not use ?,* etc, to name your variable names.


13/53


14/53

Linux


15/53

echo

Displays text or variables value on screen.

Syntax:-

echo [options] [string, variables...]


16/53

Options of echo command

-n Do not output the trailing new line.

-e Enable interpretation of the following backslash

escaped characters in the strings:

\a alert (bell)

\b backspace

\c suppress trailing new line

\n new line

\r carriage return

\t horizontal tab

\\ backslash


17/53

Example of echo command

$ echo -e "An apple a day keeps away \a\t\tdoctor\n"


18/53

More about Quotes

There are three types of quotes

Double Quotes

' Single quotes

` Back quote


19/53

Double Quotes

Anything enclose in double quotes removed meaning of

that characters (except \ and $).

echo Hello

$ echo "Today is date"

Can't print message with today's date.


20/53

Single quotes

Enclosed in single quotes remains unchanged.


21/53

Back quote

To execute command.

Example

$ echo "Today is `date`".

it will print today's date


22/53


23/53

Examples

$ expr 2 - 1

$ expr 10 / 2

$ expr 20 % 3 # remainder read as 20 mod 3and remainder is 2)

$ expr 10 \* 3 # Multiplication use \* not *

since its wild card)

$ echo `expr 6 + 3`


24/53

Exit Status

$? variable of shell is used

$ ls

$ echo $?

It will print 0 to indicate command is successfu


25/53

Arithmetic expression

Calculate the value of an expression

Enclose the expansion in $(( )).

Syntax

$((expression))


26/53

expr command

echo `expr 10 + 2`

Var=`expr 10 + 2`


27/53

Example

$ a=25

& b=20

echo $((a+b))


28/53

Expression

combination of terms and operators

Result of an expression can be an arithmetic or a logical

result Arithmetic result is represented as a string of decimal

digits

Logical result is represented as a true condition or a

false condition


29/53

Conditional execution

Test and [ ] command

If constructs


30/53

Test command

Specify a condition that can be either true or false

test command or [ expr ] is used to see if an expression is true

Use square brackets [], instead of test command

Returns true or false

if it is true it return zero(0), otherwise returns nonzero(>0) for

false.

Syntax:

testexpression

[expression]


31/53

test command

test $user_name= rani

[ $user_name= rani ]


32/53

For Mathematics use following operator inShell Script


33/53

For string Comparisons use


34/53

Shell also test for file and directory types


35/53

Logical Operators

Logical operators are used to combine two or more condition at a time


36/53

Simple if construct

used for decision making in shell script

If given condition is true then command1 is executed.

condition is nothing but comparison between two values, for

compression we can use test or [expr ] statements or evenexist status can be also used.

An expression is nothing but combination of values,

relational operator (such as >,


37/53

Simple If

Syntax:

if condition

then

command1 if condition is true or if exit status

of condition is 0 (zero)

...

fi


38/53


39/53

Example

if test $1 -gt 0

then

echo "$1 number is positive"

else

echo "$1 number is negative"

fi

M ltil l if th l S t


40/53

Multilevel if-then-else -Syntax

if condition

then

condition is zero (true - 0)

execute all commands up to elif statement

elif condition1

condition1 is zero (true - 0) execute all commands up to elif statement

elif condition2

condition2 is zero (true - 0)

execute all commands up to elif statement


41/53

Multilevel if-then-else -Syntax

else

None of the above condtion,condtion1,condtion2

are true (i.e. all of the above nonzero or false) execute all commands up to fi

fi

L i l t l


42/53

Logical operators example-a AND-o OR

if [ $a ge 60 -a $a -lt 80 ]

then

Echo distinction

fi


43/53

exit command

Terminate the execution of a shell script and return

to the $prompt

Syntax

exit


44/53

case.....esac construct

Check multiple values of a variable


45/53

Syntax:

case $variable_name in

value1) command.

;;

value2) command.

;;

.

..*) command;;

esac


46/53

Example

case $rental in

"car") echo "For $rental Rs.20 per k/m";;

"van") echo "For $rental Rs.10 per k/m";;

"jeep") echo "For $rental Rs.5 per k/m";;

"bicycle") echo "For $rental 20 paisa per k/m";;

*) echo "Sorry, invalid data";;

esac


47/53

Linux


48/53

Loops

for loop

While loop

Until loop


49/53


50/53

Example

for i in 1 2 3 4 5

do

echo "Welcome $i times"

done


51/53

While loop

Loop is executed as long as given condition is true


52/53

Example multiplication table of a given


53/53

Example multiplication table of a givennumber

n=$1

i=1

while [ $i -le 10 ]

do

echo "$n * $i = `expr $i \* $n`"

i=`expr $i + 1`

done

Shell Scripting 2

Documents

Transcript of Shell Scripting 2