if statements

If statements are conditional statements used to test whether the condition is true or false and then continue executing the script.

if statements in Linux Shell Scripting

if condition

Syntax:

 
if condition
	then
		execute some commands
		...
		...
fi
	      

Example: if.sh

 
# SOME VARIABLES
x=7

# TESTING...
if [ $x -eq 7 ]
then
{
        echo "$x is equal to 7!"
}
fi
          

In the above example if condition was used to check if the value of x is equal to 7.
If the condition is true it will print the result.

If else condition

Syntax:

 
if condition
           then
                       condition is true
                       execute some commands

           else
                       if condition is not true
                       execute some commands up
           fi
		   

Example: if-else.sh

 
# Get the value from the user
echo " Enter a value for the variable x:"
read x

# TESTING the value of x is 7 or not!
if [ $x -eq 7 ]
then
{
        echo "$x is equal to 7!"
}
else
{
echo "$x is not equal to 7!"
}
fi
		   

running the script: if-else.sh
sh if-else.sh
Enter a value for the variable x:
3
3 is not equal to 7!

In the above example, based on the value of x given by you, the script will determine if the value of x is 7 or not.

Nested If else condition

Syntax:

 
if condition
	then
		if condition
		then
			.....
			..
			do this
		else
			....
			..
			do this
		fi
	else
		...
		.....
		do this
	fi
		   

Example: nested-if-else.sh

 
# Get the value from the user
echo " Enter a value for the variable x:"
read x

# TESTING the value of x is 0 or not!
if [ $x -eq 0 ]
then
{
echo "$x is equal to 0!"
}
else
{
if [ $x -gt 0 ]
then
{
echo "$x is greater than 0!"
}
else
{
echo "$x is less than 0!"
}
fi
}
fi
		   

running the script: nested-if-else.sh
./nested-if-else.sh
Enter a value for the variable x:
-10
-10 is less than 0!

Here the script can determine the value entered by you is less than 0 , greater than 0 or equal to 0. Try running this script with different values.