if-then-else in Bash Scripting

Introduction

Use if-then-else to make decisions in your script. It helps your script choose what to do based on conditions.

Check if a file exists before using it.

Decide what message to show based on user input.

Run different commands depending on system status.

Verify if a number is positive or negative.

Choose actions based on the time of day.

Syntax

Bash Scripting

if [ condition ]
then
  commands
else
  other_commands
fi

Use spaces around brackets: [ condition ] is required.

End the if block with fi (which is if spelled backward).

Examples

Checks if the variable name is Alice, then prints a greeting or a question.

Bash Scripting

if [ "$name" = "Alice" ]
then
  echo "Hello, Alice!"
else
  echo "Who are you?"
fi

Checks if file.txt exists and prints a message accordingly.

Bash Scripting

if [ -f "file.txt" ]
then
  echo "File exists."
else
  echo "File not found."
fi

Sample Program

This script asks the user for a number and tells if it is positive or not.

Bash Scripting

#!/bin/bash

read -p "Enter a number: " num

if [ "$num" -gt 0 ]
then
  echo "The number is positive."
else
  echo "The number is zero or negative."
fi

Important Notes

Always put spaces after [ and before ] in conditions.

Use -gt for 'greater than' when comparing numbers.

Quotes around variables prevent errors if they are empty.

Summary

if-then-else lets your script choose between two paths.

Conditions go inside [ ] with spaces around them.

End the block with fi to close the if statement.