String comparisons (=, !=, -z, -n) in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the '=' operator do in bash string comparisons?

The '=' operator checks if two strings are exactly equal. It returns true if they match.

beginner

How do you check if two strings are NOT equal in bash?

Use the '!=' operator inside a test condition. It returns true if the strings differ.

beginner

What does '-z' check in bash string tests?

The '-z' operator checks if a string is empty (zero length). It returns true if the string has no characters.

beginner

What does '-n' check in bash string tests?

The '-n' operator checks if a string is not empty. It returns true if the string has one or more characters.

beginner

Write a simple bash if statement to check if variable 'name' is empty.

if [ -z "$name" ]; then echo "Name is empty" fi This uses '-z' to test if 'name' has zero length.

Which operator checks if two strings are equal in bash?

A -z

B !=

C =

D -n

The '=' operator tests if two strings are exactly equal.

What does the '-z' operator test for in bash?

A String is not empty

B String is empty

C Strings are equal

D Strings are not equal

The '-z' operator returns true if the string length is zero (empty).

How do you test if two strings are NOT equal in bash?

A !=

B -n

C -z

D =

The '!=' operator returns true if the strings differ.

Which operator returns true if a string is NOT empty?

A -n

B -z

C =

D !=

The '-n' operator checks if a string has one or more characters.

What will this bash test return? [ -z "hello" ]

A True

B Error

C Depends on shell

D False

Since 'hello' is not empty, '-z' returns false.

Explain how to check if a string variable is empty or not in bash using string comparison operators.

Think about testing zero length strings.

Describe the difference between '=' and '!=' operators in bash string comparisons.

One checks if strings match, the other if they differ.

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