if-elif-else in Bash Scripting

Introduction

Use if-elif-else to make decisions in your script based on different conditions.

Check if a file exists before processing it.

Decide what message to show based on user input.

Run different commands depending on the time of day.

Validate if a number is positive, negative, or zero.

Syntax

Bash Scripting

if [ condition ]; then
  # commands if condition is true
elif [ another_condition ]; then
  # commands if another_condition is true
else
  # commands if none of the above conditions are true
fi

Use spaces around brackets and conditions, like [ condition ].

End the if block with fi to close it.

Examples

Checks if age is 18 or more, then prints a message.

Bash Scripting

if [ "$age" -ge 18 ]; then
  echo "You are an adult."
else
  echo "You are a minor."
fi

Prints different messages based on the color value.

Bash Scripting

if [ "$color" = "red" ]; then
  echo "Stop"
elif [ "$color" = "yellow" ]; then
  echo "Caution"
else
  echo "Go"
fi

Sample Program

This script asks the user for a number and tells if it is positive, negative, or zero.

Bash Scripting

#!/bin/bash

read -p "Enter a number: " num

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

Important Notes

Always quote variables inside [ ] to avoid errors if they are empty.

Use -eq, -ne, -gt, -lt, -ge, -le for numeric comparisons.

Use = or != for string comparisons inside [ ].

Summary

if-elif-else helps your script choose what to do based on conditions.

Remember to close the block with fi.

Use proper spacing and quoting to avoid syntax errors.