[[ ]] extended test in Bash Scripting

Introduction

The [[ ]] extended test lets you check conditions in bash scripts easily. It helps decide what to do next based on facts.

Check if a file exists before reading it.

Compare two numbers to choose a path.

Test if a string is empty or not.

Verify if a directory is writable.

Decide actions based on user input.

Syntax

Bash Scripting

[[ condition ]]

# Example:
if [[ $var -eq 5 ]]; then
  echo "Equal to 5"
fi

Use [[ ]] inside if statements or loops to test conditions.

Supports many operators like -eq, -ne, <, >, string tests, and file tests.

Examples

Tests if variable num is greater than 10.

Bash Scripting

[[ $num -gt 10 ]]

Checks if the file /etc/passwd exists and is a regular file.

Bash Scripting

[[ -f /etc/passwd ]]

Tests if the string variable str equals "hello".

Bash Scripting

[[ $str == "hello" ]]

Checks if /tmp is a directory.

Bash Scripting

[[ -d /tmp ]]

Sample Program

This script asks for a number and tells if it is positive, zero, or negative using [[ ]] tests.

Bash Scripting

#!/bin/bash

read -p "Enter a number: " num

if [[ $num -gt 0 ]]; then
  echo "Positive number"
elif [[ $num -eq 0 ]]; then
  echo "Zero"
else
  echo "Negative number"
fi

Important Notes

[[ ]] is safer and more powerful than [ ] in bash.

Inside [[ ]], you don't need to quote variables to prevent word splitting.

Use && and || inside [[ ]] for combining conditions.

Summary

[[ ]] tests conditions in bash scripts simply and clearly.

It supports numbers, strings, and file checks.

Use it to control script flow based on facts.

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 [[ ]]