[[ ]] extended test in Bash Scripting - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of [[ ]] with string comparison

What is the output of the following bash script snippet?

Bash Scripting

str1="hello"
str2="hello"
if [[ $str1 == $str2 ]]; then
  echo "Match"
else
  echo "No Match"
fi

A Syntax Error

B No Match

C Match

D No output

💡 Hint

The [[ ]] test compares strings for equality.

The [[ $str1 == $str2 ]] condition checks if the two strings are equal. Since both are "hello", it prints "Match".

💻 Command Output

intermediate

Numeric comparison with [[ ]]

What will this script output?

Bash Scripting

num=5
if [[ $num -gt 3 ]]; then
  echo "Greater"
else
  echo "Smaller or equal"
fi

A Syntax Error

B Greater

C Smaller or equal

D No output

💡 Hint

Use -gt for numeric greater than comparison inside [[ ]].

The condition [[ $num -gt 3 ]] checks if num is greater than 3. Since 5 > 3, it prints "Greater".

💻 Command Output

advanced

Testing file existence with [[ ]]

What is the output of this script if the file /etc/passwd exists?

Bash Scripting

if [[ -f /etc/passwd ]]; then
  echo "File exists"
else
  echo "File missing"
fi

A File exists

B File missing

C Syntax Error

D No output

💡 Hint

The -f option tests if a file exists and is a regular file.

The [[ -f /etc/passwd ]] condition checks if the file exists and is a regular file. Usually, /etc/passwd exists, so it prints "File exists".

💻 Command Output

advanced

Logical AND with [[ ]]

What will this script output?

Bash Scripting

a=10
b=20
if [[ $a -lt 15 && $b -gt 15 ]]; then
  echo "Both true"
else
  echo "Condition failed"
fi

A Both true

B Condition failed

C Syntax Error

D No output

💡 Hint

Use && inside [[ ]] for logical AND.

The condition checks if a < 15 and b > 15. Since 10 < 15 and 20 > 15, both are true, so it prints "Both true".

💻 Command Output

expert

Complex string pattern matching with [[ ]]

What is the output of this script?

Bash Scripting

filename="report2024.txt"
if [[ $filename == report[0-9][0-9][0-9][0-9].txt ]]; then
  echo "Pattern matched"
else
  echo "No match"
fi

A Syntax Error

B No match

C No output

D Pattern matched

💡 Hint

The [[ ]] supports pattern matching with glob patterns like [0-9].

The pattern report[0-9][0-9][0-9][0-9].txt matches filenames starting with 'report' followed by exactly 4 digits and '.txt'. The filename matches, so it prints "Pattern matched".

Practice

1. What does the [[ ]] construct do in a bash script?

easy

A. It tests conditions like comparing strings or numbers.

B. It defines a function in bash.

C. It creates a new variable.

D. It runs a command in the background.

Solution

1. Step 1: Understand the purpose of [[ ]]

   The [[ ]] syntax is used in bash to test conditions such as string comparison, number comparison, and file checks.

2. Step 2: Compare with other options

   Options B, C, and D describe other bash features unrelated to [[ ]].

3. Final Answer:

   It tests conditions like comparing strings or numbers. -> Option A

4. Quick Check:

   [[ ]] = condition test [OK]

Hint: Remember: [[ ]] is for testing conditions in bash [OK]

Common Mistakes:

• Confusing [[ ]] with function definition syntax
• Thinking [[ ]] creates variables
• Assuming [[ ]] runs commands

2. Which of the following is the correct syntax to check if variable var equals "hello" using [[ ]]?

easy

A. [[ $var != "hello" ]]

B. [[ $var == "hello" ]]

C. [[ $var === "hello" ]]

D. [[ $var -eq "hello" ]]

Solution

1. Step 1: Identify correct string comparison operator in [[ ]]

   In bash, inside [[ ]], the operator for string equality is ==. -eq is for numeric equality. === is invalid.

2. Step 2: Check each option

   [[ $var != "hello" ]] checks inequality. [[ $var -eq "hello" ]] uses numeric -eq on string (error). [[ $var === "hello" ]] uses invalid ===. [[ $var == "hello" ]] uses == correctly.

3. Final Answer:

   [[ $var == "hello" ]] -> Option B

4. Quick Check:

   String equality in [[ ]] uses == [OK]

Hint: Use == for string equality inside [[ ]] [OK]

Common Mistakes:

• Using === which is invalid in bash
• Using -eq for string comparison
• Using != when checking for equality

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

var=5
if [[ $var -gt 3 && $var -lt 10 ]]; then
echo "In range"
else
echo "Out of range"
fi

medium

A. In range

B. Out of range

C. Syntax error

D. No output

Solution

1. Step 1: Evaluate numeric conditions inside [[ ]]

   The variable var is 5. The condition checks if 5 is greater than 3 and less than 10, which is true.

2. Step 2: Determine which branch runs

   Since the condition is true, the script echoes "In range".

3. Final Answer:

   In range -> Option A

4. Quick Check:

   5 > 3 and 5 < 10 = true [OK]

Hint: Check each numeric condition carefully with && [OK]

Common Mistakes:

• Confusing -gt and -lt operators
• Using single & instead of &&
• Forgetting to quote variables (not critical here)

4. Identify the error in this bash snippet:

filename="test.txt"
if [[ -f $filename && -r $filename ]]
then
echo "File exists and readable"
fi

medium

A. Variable $filename not quoted

B. Wrong operator for file check

C. Missing closing bracket "]" for [[ ]]

D. Missing then keyword

Solution

1. Step 1: Check syntax of [[ ]] condition

   The condition starts with [[ and has matching closing ]]. So brackets are correct.

2. Step 2: Verify other parts

   The then keyword is missing after the condition line, which causes a syntax error.

3. Final Answer:

   Missing then keyword -> Option D

4. Quick Check:

   Every if condition must be followed by then [OK]

Hint: Always include then after if condition [OK]

Common Mistakes:

• Forgetting the then keyword
• Using single [ with && without escaping
• Ignoring variable quoting warnings

5. You want to check if a variable input is either "yes" or "y" (case insensitive) using [[ ]]. Which snippet correctly does this?

hard

A. [[ $input =~ ^(yes|y)$ ]]

B. [[ $input == "yes" || $input == "y" ]]

C. [[ ${input,,} == "yes" || ${input,,} == "y" ]]

D. [[ $input == "yes" && $input == "y" ]]

Solution

1. Step 1: Understand case-insensitive check

   To ignore case, convert input to lowercase using ${input,,} before comparison.

2. Step 2: Check logical OR condition

   We want to accept either "yes" or "y", so use || between comparisons.

3. Step 3: Evaluate options

   [[ ${input,,} == "yes" || ${input,,} == "y" ]] correctly converts to lowercase and uses OR. [[ $input == "yes" || $input == "y" ]] lacks case conversion. [[ $input =~ ^(yes|y)$ ]] uses regex but is case-sensitive. [[ $input == "yes" && $input == "y" ]] uses AND which requires both to be true, impossible here.

4. Final Answer:

   [[ ${input,,} == "yes" || ${input,,} == "y" ]] -> Option C

5. Quick Check:

   Use ${var,,} for lowercase and || for OR [OK]

Hint: Use ${var,,} to lowercase and || for OR in [[ ]] [OK]

Common Mistakes:

• Not converting input to lowercase
• Using && instead of || for alternatives
• Incorrect regex syntax inside [[ ]]