[[ ]] extended test in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the [[ ]] syntax represent in bash scripting?

It is an extended test command used for conditional expressions, offering more features and safer syntax than the older [ ] test.

intermediate

How does [[ ]] differ from [ ] in bash?

The [[ ]] supports pattern matching with ==, allows && and || inside without escaping, and prevents word splitting and pathname expansion.

beginner

What is the result of this command: [[ "abc" == a* ]] && echo yes || echo no

It prints 'yes' because [[ ]] supports pattern matching, and 'abc' matches the pattern 'a*'.

intermediate

Can you use regex matching inside [[ ]]? If yes, how?

Yes, using the =~ operator, e.g., [[ $var =~ ^[0-9]+$ ]] tests if $var contains only digits.

intermediate

Why is [[ ]] considered safer than [ ] when testing variables?

Because [[ ]] does not perform word splitting or pathname expansion, it avoids errors when variables contain spaces or special characters.

Which of these is a valid use of [[ ]] in bash?

A [ $name = "John" ]

B [ $name == "John" ]

C [[ $name = "John" ]]

D [[ $name == "John" ]]

[[ $name == "John" ]] is valid and supports pattern matching. Single brackets require single = for string comparison.

What does this command print? [[ 5 -gt 3 && 2 -lt 4 ]] && echo yes || echo no

A Nothing

B yes

C Syntax error

D no

Inside [[ ]], you can use && without escaping. Both conditions are true, so it prints 'yes'.

Which operator is used inside [[ ]] for regex matching?

A ==

B -eq

C =~

D !=

The =~ operator is used for regex matching inside [[ ]].

What happens if a variable with spaces is tested using [ ] without quotes?

A It causes word splitting and errors

B It automatically quotes the variable

C It works fine

D It ignores spaces

Without quotes, [ ] splits words on spaces causing errors. [[ ]] avoids this problem.

Which is true about [[ ]] in bash?

A It is a shell keyword, not a command

B It is an external program

C It behaves exactly like [ ]

D It cannot be used with variables

[[ ]] is a shell keyword providing enhanced conditional testing.

Explain the advantages of using [[ ]] over [ ] in bash scripting.

Think about safety and extra features in condition tests.

Describe how to use regex matching inside [[ ]] and give an example.

Look for the operator that tests patterns, not just equality.

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 [[ ]]