IF, ELSE or ELIF (known as else if in other programming) are conditional statements which are used for execution of different-2 programmes depends on output true or false. All the if statements are started with then keyword and ends with fi keyword. To define conditions there are two ways, one is using test keyword (Eg: if test < condition > ) and second is using brackets (Eg: if [ condition ] ).

1. Bash if Statement

Single if statements are useful where we have a single program for execution.

if [ condition ]
then
    < code block >
fi

For example – If we need to check if input value is equal to 10 or not. If value is equal to 10, then it will print “Value of i is 10″, but if not nothing will be printed.

#!/bin/bash
read -p "Enter value of i :" i

if [ $i -eq 10 ]
then
    echo "Value of i is 10"
fi

2. Bash if else Statement

if else statements are useful where we have a two programs for execution, and need to execute only one based on results of if condition.

if [ condition ]
then
    < code block >
else
    < code block >
fi

For example – If input taken value is 10 then it will print “Value of i is 10″, if not program will execute else block statement and print “Value of i is not equal to 10″.

#!/bin/bash
read -p "Enter value of i :" i

if [ $i -eq 10 ]
then
    echo "Value of i is 10"
else
    echo "Value of i is not equal to 10"
fi

3. Bash if elif Statement

if elif and else statements are useful where we have more than two programs for execution, and need to execute only one based on results of if and elif condition.

if [ condition ]
then
    < code block >
elif [ condition ]
then
    < code block >
else
    < code block >
fi

For example – Below example will check input value if equal to 5, if its true then program will print “Value of i is 5″ otherwise program will go to elif statement where one more conditional will be checked and based of these results elif or elase block code will be executed.

#!/bin/bash
read -p "Enter value of i :" i

if [ $i -eq 5 ]
then
    echo "Value of i is 5"
elif [ $i -eq 10 ]
then
    echo "Value of i is 10"
else
    echo "Value of i is not equal to 5 or 10"
fi

4. Bash elif Ladder Statements

This is something similar to above one where we are adding multiple elif statements together. elif (else if) ladder are useful where we have multiple programs for execution and need to execute only one based on results of if and elif condition.

if [ condition ]
then
    < code block >

elif [ condition ]
then
    < code block >

elif [ condition ]
then
    < code block >

elif [ condition ]
then
    < code block >

else
    < code block >
fi

For example –

#!/bin/bash
read -p "Enter value of i :" i

if [ $i -eq 5 ]
then
    echo "Value of i is 5"
elif [ $i -eq 10 ]
then
    echo "Value of i is 10"
elif [ $i -eq 20 ]
then
    echo "Value of i is 20"
elif [ $i -eq 30 ]
then
    echo "Value of i is 30"
else
    echo "Value of i is not equal to 5,10,20 or 30"
fi

5. Bash nested if Statements

Nested if are useful in the situation where one condition will be checked based on results of outer condition.

if [ condition ]
then
    if [ condition ]
    then
        < code block >
    else
        < code block >
    fi
else
    if [ condition ]
    then
        < code block >
    fi
fi

For example below small shell program is for finding the greatest value between 3 values taken input by user. This program will work with numeric values only. If two values are similar it will print only one value.

#!/bin/bash

read -p "Enter value of i :" i
read -p "Enter value of j :" j
read -p "Enter value of k :" k

if [ $i -gt $j ]
then
    if [ $i -gt $k ]
    then
        echo "i is greatest"
    else 
        echo "k is greatest"
    fi
else
    if [ $j -gt $k ]
    then
        echo "j is greatest"
    else
	    echo "k is greatest"
    fi
fi