Shell Scripting – Checking Conditions with if

http://www.linuxtechi.com/shell-scripting-checking-conditions-with-if

In Bourne Shell if statement checks whether a condition is true or not. If so , the shell executes the block of code associated with the if statement. If the statement is not true , the shell jumps beyond the end of the if statement block & Continues on.
Syntax of if Statement :
if [ condition_command ]
then
        command1
        command2
        ……..
        last_command
fi
Example:

#!/bin/bash
number=150
if [ $number -eq 150 ]
then
    echo "Number is 150"
fi

if-else Statement :

In addition to the normal if statement , we can extend the if statement with an else block. The basic idea is that if the statement is true , then execute the if block. If the statement is false , then execute the else block.
Syntax :
if [ condition_command ]
then
       command1
       command2
       ……..
       last_command
else
       command1
       command2
       ……..
       last_command
fi
Example:

#!/bin/bash
number=150
if [ $number -gt 250 ]
then
    echo "Number is greater"
else
    echo "Number is smaller"
fi

If..elif..else..fi Statement (Short for else if)

The Bourne shell syntax for the if statement allows an else block that gets executed if the test is not true. We can nest if statement , allowing for multiple conditions. As an alternative, we can use the elif construct , shot for else if.
Syntax :
if [ condition_command ]
then
       command1
       command2
       ……..
       last_command
elif [ condition_command2 ]
then
        command1
        command2
        ……..
        last_command
else
command1
command2
……..
last_command
fi
Example :

#!/bin/bash
number=150
if [ $number -gt 300 ]
then
    echo "Number is greater"
elif [ $number -lt 300 ]
then
    echo "Number is Smaller"
else
    echo "Number is equal to actual value"
fi

Nested if statements :

If statement and else statement can be nested in a bash script. The keyword ‘fi’ shows the end of the inner if statement and all if statement should end with the keyword ‘fi’.
Basic syntax of nested if is shown below :
if [ condition_command ]
then
        command1
        command2
        ……..
        last_command
else
if [ condition_command2 ]
then
        command1
        command2
        ……..
        last_command
else
        command1
        command2
         ……..
         last_command
      fi
fi
Example:

#!/bin/bash
number=150
if [ $number -eq 150 ]
then
   echo "Number is 150"
else
if [ $number -gt 150 ]
then
    echo "Number is greater"
else
    echo "'Number is smaller"
   fi
fi