If takes the form:

if CONDITION 
then
STATEMENTS
fi

The statements are only executed given the CONDITION is true.

The fi keyword is used for marking the end of the if statement.

#!/bin/bash
 
echo -n "Enter a number: "
read num
 
if [[ $num -gt 10 ]]
then
echo "Number is greater than 10."
fi

The above program will only show the output if the number provided via input is greater than ten.

The -gt stands for greater than; similarly -lt for less than; -le for less than equal; and -ge for greater than equal. The [[ ]] are required.

Combining the else construct with if allows much better control over the script’s logic.

#!/bin/bash
 
read n
if [ $n -lt 10 ];
then
echo "It is a one digit number"
else
echo "It is a two digit number"
fi

The else part needs to be placed after the action part of if and before fi.

The elif statement stands for 'else if' and offers a convenient means for implementing chain logic.

#!/bin/bash
 
echo -n "Enter a number: "
read num
 
if [[ $num -gt 10 ]]
then
echo "Number is greater than 10."
elif [[ $num -eq 10 ]]
then
echo "Number is equal to 10."
else
echo "Number is less than 10."
fi

See Switch