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Bash Brackets ([], (), {}, $( ), $(( ))) – Types, Uses & Examples

In Bash scripting, different types of brackets are used for various operations, such as condition checking, command substitution, array handling, and arithmetic operations. Below is a detailed guide with examples.

Table of Contents

📌 Key Highlights of  Bash Brackets

Bracket Type Usage Example
[] Basic test conditions [ -f file.txt ]
[[ ]]  more advanced features compared to

single brackets, including regular expression

 [[ $str == Jo* ]]
() Subshell execution (cd /tmp && ls)
$(( )) Arithmetic operations y=$((x + 10))
{} Command grouping,Variable expansion & loops echo {1..5}
$( ) Command substitution today=$(date)
${} String manipulation ${#str} (length)

📌 1. Square Brackets [] (Test Conditions)

Used for evaluating conditions, similar to the test command.

✅ Example 1: Check if a File Exists

bash
if [ -f "file.txt" ]; then
echo "File exists"
else
echo "File does not exist"
fi

🔹 Use case: Check if file.txt exists.

✅ Example 2: String Comparison

bash
str="Hello"
if [ "$str" = "Hello" ]; then
echo "Strings match"
fi

🔹 Use case: Compare two strings.

✅ Example 3: Integer Comparisons

bash
num=10
if [ "$num" -gt 5 ]; then
echo "Number is greater than 5"
fi

🔹 Use case: Compare integers (-eq, -ne, -gt, -lt, -ge, -le).

📌 2. Double Square Brackets [[ ]] (Advanced Test Conditions)

[[ ... ]] provides enhanced string comparison and supports regex.

✅ Example 4: String Matching with Wildcards

bash
name="JohnDoe"
if [[ "$name" == Jo* ]]; then
echo "Name starts with Jo"
fi

🔹 Use case: Pattern matching (*, ?, [ ] work like globbing).

✅ Example 5: Regex Matching

bash
if [[ "hello123" =~ [0-9]+ ]]; then
echo "String contains numbers"
fi

🔹 Use case: Use regex inside conditions.

📌 3. Parentheses () (Command Grouping & Subshells)

Used for grouping commands or executing in a subshell.

✅ Example 6: Grouping Commands

bash
(cd /tmp && ls) # Runs in a subshell

🔹 Use case: Changes directory temporarily without affecting the current shell.

✅ Example 7: Background Process

bash
(sleep 5; echo "Done") &

🔹 Use case: Runs commands in the background.

📌 4. Double Parentheses $(( )) (Arithmetic Operations)

Used for integer arithmetic operations.

✅ Example 8: Basic Arithmetic

bash
x=5
y=$((x + 10))
echo "Result: $y"

🔹 Use case: Perform addition, subtraction, multiplication, division.

✅ Example 9: Increment & Decrement

bash
x=10
((x++)) # Increment
((x--)) # Decrement

🔹 Use case: Shorthand for x=x+1 or x=x-1.

📌 5. Curly Braces {} (Variable Expansion & Loops)

Used for variable expansion, command grouping, and brace expansion.

✅ Example 10: Variable Expansion

bash
name="World"
echo "Hello, ${name}!"

🔹 Use case: Avoid ambiguity in variable names.

✅ Example 11: Looping with Brace Expansion

bash
echo {1..5}

🔹 Use case: Expands to 1 2 3 4 5.

✅ Example 12: Multiple Command Execution

bash
{ echo "First"; echo "Second"; } > output.txt

🔹 Use case: Group commands without creating a subshell.

📌 6. Command Substitution $( )

Used to store the output of a command in a variable.

✅ Example 13: Store Command Output

bash
today=$(date +"%Y-%m-%d")
echo "Today is $today"

🔹 Use case: Store date command output.

✅ Example 14: Using Command Substitution in a Loop

bash
for file in $(ls *.txt); do
echo "Processing $file"
done

🔹 Use case: Loop through files.

📌 7. Dollar Sign & Curly Brackets ${} (Variable Manipulation)

Used for string modifications.

✅ Example 15: Get String Length

bash
str="Hello"
echo "Length: ${#str}"

🔹 Use case: Get the length of a string.

✅ Example 16: Remove Part of a String

bash
filename="document.txt"
echo "${filename%.txt}" # Removes .txt

🔹 Use case: Extract filename without extension.

🔥 Bash Brackets Interview Questions & Answers

Brackets ([], [[ ]], {}, (), $(( )), $( ), ${}) are commonly used in Bash scripting for different operations. Here are top interview questions on Bash brackets with detailed answers.

📌 Basic Questions

1️⃣ What is the difference between [ ] and [[ ]] in Bash?

✅ Answer:

  • [ ] is a POSIX-compliant test command (same as test).

  • [[ ]] is a Bash built-in, supporting regex, pattern matching, and logical operators.

📌 Example:

bash
var="hello"
[ "$var" = "hello" ] && echo "POSIX test"
[[ "$var" == hello ]] && echo "Bash test"

📌 Key Differences:

Feature [ ] [[ ]]
String Comparison = ==
Logical Operators -a, -o &&, `
Pattern Matching ❌ ✅ (== pattern*)
Regex Support ❌ ✅ (=~ regex)

2️⃣ What is the use of {} in Bash?

✅ Answer:

  • Brace expansion for sequences {1..5}.

  • Variable expansion ${var}.

  • Grouping commands { cmd1; cmd2; }.

📌 Examples:

bash
echo {1..5} # Output: 1 2 3 4 5
echo "Hello ${USER}!" # Expands variable
{ echo "First"; echo "Second"; } > output.txt # Group commands

3️⃣ What is the difference between ( ) and { } in Bash?

✅ Answer:

  • ( ) creates a subshell (isolated execution).

  • { } groups commands without a subshell.

📌 Example:

bash
(cd /tmp && ls) # Runs in subshell (directory change is temporary)
{ cd /tmp && ls; } # Runs in the same shell (directory change persists)

📌 Intermediate Questions

4️⃣ How do $(( )) and $( ) differ in Bash?

✅ Answer:

  • $(( )) is used for arithmetic operations.

  • $( ) is used for command substitution.

📌 Examples:

bash
echo $((5 + 10)) # Output: 15 (Arithmetic)
echo "Today is $(date +%Y-%m-%d)" # Command substitution

5️⃣ What happens if you don’t use curly braces ${} for variables in Bash?

✅ Answer:
Without curly braces, ambiguity can occur.

📌 Example (Incorrect Use):

bash
file="data"
echo "$file123" # Will not work (Bash tries to expand `$file123`)

📌 Correct Use:

bash
echo "${file}123" # Expands correctly to "data123"

6️⃣ How do you use [[ ]] for pattern matching?

✅ Answer:
You can use == with wildcards inside [[ ]].

📌 Example:

bash
var="hello123"
if [[ $var == hello* ]]; then
echo "Starts with hello"
fi

📌 Key Features:

  • == allows wildcards (*, ?).

  • =~ allows regular expressions.

📌 Advanced Questions

7️⃣ How do you use [[ ]] with regex?

✅ Answer:
Use =~ inside [[ ]] for regex matching.

📌 Example:

bash
var="abc123"
if [[ $var =~ [0-9]+ ]]; then
echo "Contains numbers"
fi

🚀 Tip: No need to quote the regex!

8️⃣ How do you compare numbers correctly in Bash?

✅ Answer:
Use arithmetic comparison operators (-eq, -ne, -gt, -lt).

📌 Incorrect:

bash
if [[ $num > 5 ]]; then # WRONG: This is a string comparison!

📌 Correct:

bash
if [[ $num -gt 5 ]]; then # Correct: Numeric comparison

9️⃣ How do you use brace expansion {} in Bash loops?

✅ Answer:
You can expand ranges using {}.

📌 Example:

bash
for i in {1..5}; do
echo "Number: $i"
done

🔹 Output:

javascript
Number: 1
Number: 2
...

🔟 How do you use ${} for string operations?

✅ Answer:
Bash provides built-in string manipulation.

📌 Examples:

bash
str="hello world"
echo ${#str} # Get length (Output: 11)
echo ${str:0:5} # Substring (Output: hello)
echo ${str/world/universe} # Replace (Output: hello universe)

🔥 Bonus: Quick Comparison Table

Bracket Type Use Case Example
[ ] Basic test [ -f file.txt ]
[[ ]] Advanced test [[ $str == hello* ]]
{} Expansion & grouping echo {1..5}
() Subshell execution (cd /tmp && ls)
$(( )) Arithmetic operations y=$((x + 10))
$( ) Command substitution today=$(date)
${} Variable manipulation ${#str} (length)

Hands-On Bash Brackets Exercises

These exercises will help you practice different types of brackets ([], [[ ]], {}, (), $(( )), $( ), ${}) in Bash scripting.

📌 1. Using [ ] and [[ ]] for Condition Checking

🔹 Task:

  • Write a script to check if a file exists and is not empty using [ ].

  • Modify it to use [[ ]] instead.

📌 Example Output:

arduino
File exists and is not empty!

📌 2. Using {} for Brace Expansion

🔹 Task:

  • Create a script that generates file names:

    • file1.txt, file2.txt, file3.txt, ... file5.txt

  • Use {} for sequence expansion.

📌 Example Output:

file1.txt file2.txt file3.txt file4.txt file5.txt

📌 3. Using () for Subshells

🔹 Task:

  • Run pwd before and after changing to /tmp inside a subshell.

  • Print both results.

📌 Expected Output:

pgsql
Current Directory: /home/user
Inside Subshell: /tmp
Back to Original: /home/user

📌 4. Using $(( )) for Arithmetic Operations

🔹 Task:

  • Write a script that takes two numbers from the user and performs:

    • Addition, Subtraction, Multiplication, and Division.

  • Use $(( )) for calculations.

📌 Example Output:

yaml
Enter first number: 10
Enter second number: 5
Sum: 15
Difference: 5
Product: 50
Quotient: 2

📌 5. Using $( ) for Command Substitution

🔹 Task:

  • Print today’s date and time inside a sentence.

  • Use $(date) for command substitution.

📌 Example Output:

mathematica
Today is Sunday, March 24, 2025, at 12:30 PM

📌 6. Using ${} for String Manipulation

🔹 Task:

  • Store a full name in a variable and perform:

    • Extract first name.

    • Convert to uppercase.

    • Get length of the name.

📌 Example Output:

yaml
Full Name: John Doe
First Name: John
Uppercase: JOHN DOE
Name Length: 8

📌 7. Using [[ ]] for Regex Matching

🔹 Task:

  • Write a script that checks if a given string contains numbers using regex (=~).

📌 Example Output:

csharp
Enter a string: hello123
Contains numbers!

📌 8. Using {} for Grouping Commands

🔹 Task:

  • Write a script that:

    • Runs echo "Hello" and ls inside {}.

    • Redirects both outputs to a file.

📌 Expected Behavior:

  • Both Hello and the ls output should be saved in output.txt.

📌 9. Using $( ) and $(( )) Together

🔹 Task:

  • Get current year using $(date +%Y).

  • Calculate the year when the user will turn 100 years old.

📌 Example Output:

yaml
Enter your birth year: 1990
You will turn 100 in the year 2090.

📌 10. Using Brackets for File Processing

🔹 Task:

  • Read a file line-by-line using a while loop and:

    • Print only lines longer than 5 words.

  • Use awk inside $( ) to count words.

📌 Example Output:

vbnet
Skipping: Hello
Processing: This is a long sentence.
Processing: Another example line with words.