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.

Practice

1. Which operator in bash scripting checks if two strings are equal?

easy

A. Using = inside double brackets [[ ]]

B. Using -z to check string length

C. Using != inside double brackets [[ ]]

D. Using -n to check if string is not empty

Solution

1. Step 1: Understand string equality operator

   In bash, = inside [[ ]] compares if two strings are equal.

2. Step 2: Differentiate from other operators

   -z checks if string is empty, != checks inequality, -n checks non-empty string.

3. Final Answer:

   Using = inside double brackets [[ ]] -> Option A

4. Quick Check:

   String equality = = [OK]

Hint: Use = inside [[ ]] to check if strings are equal [OK]

Common Mistakes:

• Confusing = with -z or -n
• Using = outside [[ ]] without quotes
• Mixing != with = for equality

2. Which of the following is the correct syntax to check if a variable var is not empty in bash?

easy

A. [ -z "$var" ]

B. [ "$var" = "" ]

C. [ -n "$var" ]

D. [ $var != "" ]

Solution

1. Step 1: Understand -n operator

   -n returns true if the string length is greater than zero (not empty).

2. Step 2: Check syntax correctness

   Using quotes around $var prevents errors if variable is empty or contains spaces.

3. Final Answer:

   [ -n "$var" ] -> Option C

4. Quick Check:

   Not empty check = -n [OK]

Hint: Use [ -n "$var" ] to test if var is not empty [OK]

Common Mistakes:

• Using -z instead of -n for not empty
• Forgetting quotes around variable
• Using = "" to check emptiness incorrectly

3. What will be the output of this bash script snippet?

var="hello"
if [[ "$var" != "world" ]]; then
  echo "Not world"
else
  echo "World"
fi

medium

A. Not world

B. World

C. Syntax error

D. No output

Solution

1. Step 1: Evaluate string comparison

   Variable var is "hello", which is not equal to "world".

2. Step 2: Determine which branch runs

   The condition [[ "$var" != "world" ]] is true, so it echoes "Not world".

3. Final Answer:

   Not world -> Option A

4. Quick Check:

   hello != world -> Not world [OK]

Hint: If strings differ, != condition is true [OK]

Common Mistakes:

• Confusing = and != operators
• Missing quotes causing syntax errors
• Assuming else branch runs when condition is true

4. Identify the error in this bash snippet that checks if input is empty:

if [ -z $input ]; then
  echo "Empty"
else
  echo "Not empty"
fi

medium

A. Using -z instead of -n

B. Missing quotes around $input in test

C. Using single brackets instead of double brackets

D. No error, script is correct

Solution

1. Step 1: Check variable quoting in test

   Variable $input is not quoted, which can cause errors if empty or contains spaces.

2. Step 2: Understand correct syntax

   Quotes around $input like "$input" are needed to safely test emptiness.

3. Final Answer:

   Missing quotes around $input in test -> Option B

4. Quick Check:

   Always quote variables in tests [OK]

Hint: Always quote variables in [ ] tests to avoid errors [OK]

Common Mistakes:

• Forgetting quotes around variables
• Thinking -z and -n are interchangeable
• Believing single brackets can't test strings

5. You want to write a bash script that prints "Empty string" if variable text is empty, and "Has content" otherwise. Which snippet correctly does this?

hard

A. if [ "$text" != "" ]; then echo "Empty string"; else echo "Has content"; fi

B. if [[ "$text" = "" ]]; then echo "Has content"; else echo "Empty string"; fi

C. if [ -n "$text" ]; then echo "Empty string"; else echo "Has content"; fi

D. if [[ -z "$text" ]]; then echo "Empty string"; else echo "Has content"; fi

Solution

1. Step 1: Understand -z usage

   -z "$text" returns true if text is empty.

2. Step 2: Check echo outputs match condition

   If empty, print "Empty string"; else print "Has content". if [[ -z "$text" ]]; then echo "Empty string"; else echo "Has content"; fi matches this logic correctly.

3. Final Answer:

   if [[ -z "$text" ]]; then echo "Empty string"; else echo "Has content"; fi -> Option D

4. Quick Check:

   -z empty -> print "Empty string" [OK]

Hint: Use if [[ -z "$var" ]] for empty string check [OK]

Common Mistakes:

• Swapping outputs for empty and non-empty cases
• Using = "" without quotes or wrong logic
• Confusing -n and -z meanings