String comparisons (=, !=, -z, -n) in Bash Scripting

Introduction

String comparisons help you check if words or texts are equal, different, empty, or not empty in scripts.

Check if a user entered the correct password.

See if a file name matches a specific pattern.

Verify if a variable has any value or is empty.

Decide what to do based on user input text.

Compare two strings to control script flow.

Syntax

Bash Scripting

if [ "$string1" = "$string2" ]; then
  # commands
fi

if [ "$string1" != "$string2" ]; then
  # commands
fi

if [ -z "$string" ]; then
  # commands for empty string
fi

if [ -n "$string" ]; then
  # commands for non-empty string
fi

Always put strings in double quotes to avoid errors with spaces or empty values.

Use single brackets [ ] with spaces around operators for string tests in bash.

Examples

This checks if the variable name is exactly "Alice".

Bash Scripting

if [ "$name" = "Alice" ]; then
  echo "Hello, Alice!"
fi

This checks if input is not empty.

Bash Scripting

if [ "$input" != "" ]; then
  echo "You typed something."
fi

This runs if password has no characters.

Bash Scripting

if [ -z "$password" ]; then
  echo "Password is empty!"
fi

This runs if filename has one or more characters.

Bash Scripting

if [ -n "$filename" ]; then
  echo "Filename is set."
fi

Sample Program

This script asks for your name. It checks if you typed nothing, if you typed "Alice", or something else, and prints a message.

Bash Scripting

#!/bin/bash

read -p "Enter your name: " name

if [ "$name" = "" ]; then
  echo "You did not enter a name."
elif [ "$name" = "Alice" ]; then
  echo "Welcome back, Alice!"
else
  echo "Hello, $name!"
fi

Important Notes

Use = for equality and != for inequality in string tests.

-z means the string is empty; -n means it is not empty.

Always quote variables to avoid errors when they are empty or contain spaces.

Summary

Use = and != to compare if strings are equal or not.

Use -z to check if a string is empty, and -n to check if it is not empty.

Always put quotes around variables in comparisons to keep your script safe.