Control program flow with if statement in shell scripting

In the last post, we learned about tests in shell scripting.

Variables and Tests in shell scripting

Twinkle lahariya ・ Apr 6 '22

#shellscripting #bash #beginners

In this post, let's see how we can use them with if statement to control the flow of the script.

What is if statement?

If statement allows us to make decisions in our script. It starts with the keyword if followed by a test expression, then list of commands followed by the closing fi keyword.

A basic if statement says that if the test expression is true, then perform the following commands.

Syntax

if []
then
    command 1
    command 2
    .
    .
    command n
fi

Let's understand if with an example where we check if a given number is greater than 10.

#!/bin/bash

VAR=20
if [ $VAR -gt 10 ]
then
    echo "Variable is greater than 10"
fi

Output:

> ./IfDemo.sh
Variable is greater than 10

Let's see what we have done here:

if [ $VAR -gt 10 ] : if followed by a test condition $VAR -gt 10
$VAR -gt 10 : returns true if the var is greater (-gt) than 10
then : if the condition is true, then execute the following commands
fi : end of if statement

What if you want to log both the scenarios, i.e. if the number is greater than 10 and then the number is less than 10.

If..else statement

If-else statement follows the following form:

Syntax

if []
then
    command 1
else 
    command 2
fi

If-else tells that if the test is true, then run command 1, else run command 2.

Let's extend the same example of a number greater than 10 and print a message if the number is not.

#!/bin/bash

VAR=5
if [ $VAR -gt 10 ]
then
    echo "Variable is greater than 10"
else 
    echo "Variable is less than 10"
fi

Output:

> ./IfDemo.sh

Variable is less than 10

Now, if we want to check the second test condition if the first one fails and handle it differently, then we use if-elif-else ladder.

If..elif..else statement

Let's check if the number is greater than 10, then print 'greater than'. Else, if the number is equals 10, print 'equals to', else print 'less than'.

#!/bin/bash

VAR=5
if [ $VAR -gt 10 ]
then
    echo "Variable is greater than 10"
elif [ $VAR -eq 10]
    echo "Variable is equals to 10"
else 
    echo "Variable is less than 10"
fi

Output:

> ./IfDemo.sh
Variable is equals to 10

There is more to if statements. To list some of the scenarios:

1: Nested if statements:

if [[ Test condition ]]
then
  if [[ Test condition ]]
  then
    command 1
  else
    command 2
  fi
fi

2: Multiple test conditions

if [[ Test condition ]] && [[ Test condition ]]
then
  command 1
elif [[ Test condition ]] || [[ Test condition ]]
then
  command 2
else
  command 3
fi

Cool, I'll go now and try these scenarios out, do let me know if you try them and have any questions about it.

Up next is loops in shell scripting. Stay tuned.

Until next time. Love ❤️