Variable Assignment and Substitution in Linux Shell Scripting

Variable

A variable is a value that can change, depending on conditions or on information passed to the program.

Assigning a Variable 

A variable can be assigned by variable name after that = and then variable value.

$ variable=Hello

Note that there should not be any space on either side of =

If there is any space then see how it work

Variable  =52

The shell runs variable as a command and =52 as option

Variable=  52

Here shell treats variable as environment variable and runs value as a command.

Variable can be assigned by different ways

1. Normal Assignment

a=5

2. Using let

let a=10+5

3. using for loop

for a in 1 2 3 4

do

echo -n "$a "

done

4. using read

echo -n "Enter \"a\" "

read a

5. Using back tilt (`)

a=`echo hi`

echo $a

output: hi

a=`pwd`

echo $a

it prints present working dorectory

a=`echo pwd`

echo $a

output: pwd

6. Null value assignment.

Setting a null value.

hello= 

echo "\$hello = $hello"

$hello =

Retrieving a variable

A variable can be retrieved by $variablename.

variable=hello

$ echo variable                  #variable cannot be retrieved here

variable

$ echo $variable

Hello

Variable substitution

Assign variable

x=52

Substitute variable

foo=$x

shell scripting