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

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the variable name is equal to "Alice".

Bash Scripting

if [ "$name" [1] "Alice" ]; then
  echo "Hello, Alice!"
fi

Drag options to blanks, or click blank then click option'

A=

B-n

C-z

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' instead of '=' when checking for equality.

Using '-z' or '-n' which check string length, not equality.

The operator '=' checks if two strings are equal in bash test expressions.

2fill in blank

medium

Complete the code to check if the variable input is not empty.

Bash Scripting

if [ [1] "$input" ]; then
  echo "Input received."
fi

Drag options to blanks, or click blank then click option'

A!=

B-z

C-n

D=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-z' which checks if the string is empty.

Using '=' or '!=' which compare strings but don't check emptiness.

The '-n' operator returns true if the string length is not zero (not empty).

3fill in blank

hard

Fix the error in the code to check if var is empty.

Bash Scripting

if [ [1] "$var" ]; then
  echo "Variable is empty."
fi

Drag options to blanks, or click blank then click option'

A!=

B=

C-n

D-z

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' or '!=' without a second string to compare.

Using '-n' which checks for non-empty strings.

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

4fill in blank

hard

Fill both blanks to check if user is not "admin" and is not empty.

Bash Scripting

if [ "$user" [1] "admin" ] && [ [2] "$user" ]; then
  echo "Access denied."
fi

Drag options to blanks, or click blank then click option'

A!=

B-z

C-n

D=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' instead of '!=' for inequality.

Using '-z' instead of '-n' to check non-empty string.

Use '!=' to check user is not 'admin' and '-n' to check user is not empty.

5fill in blank

hard

Fill all three blanks to create a condition that checks if file is empty or equals "default.txt".

Bash Scripting

if [ [1] "$file" ] || [ "$file" [2] [3] ]; then
  echo "Using default file."
fi

Drag options to blanks, or click blank then click option'

A-z

B=

C"default.txt"

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' instead of '=' for equality check.

Not quoting the string "default.txt".

Use '-z' to check if file is empty, '=' to check equality, and '"default.txt"' as the string to compare.

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