When coding, you might need to make decisions based on certain conditions. Conditions are expressions that evaluate to a boolean expression (true or false).

Statements that help to execute different code branches based on certain conditions are known as conditional statements.

if...else is one of the most commonly used conditional statements. Like other programming languages, Bash scripting also supports if...else statements. And we will study that in detail in this blog post.

Syntax of if Statements

You can use if statements in a variety of ways. The generic structure of if statements is as follows:

  • Using an if statement only: if...then...fi
  • Using and if with an else statement: if...then...else...fi statements
  • Using multiple else statements with if: if..elif..else..fi

How to Use the if Statement

When you are using a single if statement, the syntax is as follows:

if [ condition ]
then
	statement
fi
Note that the spaces are part of the syntax and should not be removed.

Let's go through an example where we are comparing two numbers to find if the first number is the smaller one.

#! /bin/sh

a=5
b=30

if [ $a -lt $b ]
then
        echo "a is less than b"
fi

If you run the above snippet, the condition if [ $a -lt $b ] evaluates to True , and the statement inside the if statement executes

Output:

a is less than b

How to Use the if .. else Statement

When you are using an if statement and you want to add another condition, the syntax is as follows:

if [ condition ]
then
	statement
else
	do this by default
fi

Let's see an example where we want to find if the first number is greater or smaller than the second one. Here, if [ $a -lt $b ] evaluates to false, which causes the else part of the code to run. 

#! /bin/sh

a=99
b=45

if [ $a -lt $b ]
then
        echo "a is less than b"
else
        echo "a is greater than b"
fi

Output:

a is greater than b

How to Use if..elif..else Statements

Let's say you want to add further conditions and comparisons to make the code dynamic. In this case, the syntax would look like this:

if [ condition ]
then
	statement
elif [ condition ] 
then
	statement 
else
	do this by default
fi

To create meaningful comparisons, we can use AND -a and OR -o as well.

In this example, we will determine the type of triangle using these conditions:

  • Scalene:  A triangle where every side is different in length.
  • Isosceles: A triangle where 2 sides are equal.
  • Equilateral: A triangle where all sides are equal.
read a
read b
read c

if [ $a == $b -a $b == $c -a $a == $c ]
then
echo EQUILATERAL

elif [ $a == $b -o $b == $c -o $a == $c ]
then
echo ISOSCELES
else
echo SCALENE

fi

In the example above, the script would ask the user to enter the three sides of the triangle. Next, it would compare the sides and decide the triangle type.

3
4
5
SCALENE

Conclusion

You can easily branch your code based on conditions like if..else and make the code more dynamic. In this tutorial, you learned the syntax of if...else along with some examples.  

I hope you found this tutorial helpful.

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