Variables in Bash or Shell script  

variable is used to temporary store data.In shell script variable has no data types. A variable in shell script can contain a number( 0 to 9) , a character (a to z or A to Z) , a string of characters or the underscore character ( _).

Defining Variables

VarName=Value

e.g

VarName=”Hi all”

TotalNumber=234

Example of Variable in Shell Script

#!/bin/sh

NameDisplay="Mohamadsiraj Raza"

echo $NameDisplay

OUTPUT :

Mohamadsiraj Raza

Reading user input with read
Some time we need input from user so we use read command in bash

#!/bin/bash
echo "Where do you live:"
read Place
echo "Your Home town is  $Place!"

From the above example we can get user input and store in variable ( Place) and display in the screen.

It may happen that we want to take more input from user.

#!/bin/bash
echo "Eneter your firstname and lastname:"
read firstname lastname
echo "Hi! $firstname, $lastname !"

Variable Types in bash script

1) Local Variables

We use keyword local for denoting local variable its present in current instance

Example:

#!/bin/bash
Value=outside_function
function test {
local Value=local_inside_fnction
echo $Value
}
echo $Value
test
echo $Value

Output

outside_function
local_inside_fnction
outside_function

2) Environment Variables
This variable is variable to any child process of the shell.

Special variables in bash script or shell script

$0 – Output will be filename of the script.

Example: 
#!/bin/sh
echo "Output: $0"

Then
./File.sh
Output

./File.sh

$n – display the argument where n is number ($1 first argument and $2 second argument)
$# – number of argument
$? – stats of last command run. 0 if command successful, and 1 if command unsuccessful.
$$ – It gives process id of the current working  shell
$! – It will give Process id of last background command which you have run.

Example

#!/bin/sh

echo "Name of File: $0"
echo "First Parameter : $1"
echo "Second Parameter : $2"
echo "Total Number of Parameters : $#"

Run the script : ./File.sh ken corner

Output

Name of File: ./File.sh
First Parameter : ken
Second Parameter : corner
Total Number of Parameters : 2

Special Parameters $* and $@

Both will take and display unknown number of argument.

Difference between $@ and $*
$* – All the arguments are double quoted
$@ – All the arguments are individually double quoted

For example

#!/bin/bash
echo "Output for *:"
for argument in "$*"; do echo "$argument"; done
echo "-----\n"
echo "Output for @:"
for argument in "$@"; do echo "$argument"; done

$ ./test.sh 1 2 "3 4"
Output for *:
1 2 3 4

Output for @:
1
2
3 4

Script For Addition of two number

#!/bin/bash
echo "Enter the First Number: "
read a
echo "Enter the Second Number: "
read b
x=$(expr "$a" + "$b")
echo $a + $b = $x

Bash script for $? which give status of last run command.

#!/bin/bash

cp -R fileName /opt/
RESULT=$?
if [ $RESULT -eq 0 ];
then
echo "File copied"

else
echo "Unable to copy the file"
fi

In this above example we used $? to check that command cp -R worked or not ,it command is successful then it will return 0 or if it fail then it will return 1