Bash Brackets Explained in Simple Words (With 8 Examples)

When learning Bash scripting, one thing that often confuses beginners is the use of different brackets and symbols. You may see (), {}, [], [[]], $(), ${}, or (()) and wonder why Bash needs so many of them.

We’ll break down these symbols with clear explanations and simple examples so you can confidently read and write Bash scripts.

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$() – Run a Command and Use Its Output

The $() syntax runs a command and stores its output.

today=$(date)
echo "Today is $today"

today=$(date)
echo "Today is $today"

Here, the date command runs first, and its output is saved in the variable today.

Use $() when you need the result of a command inside your script.

() – Subshell and Arrays

Subshell

Commands inside () run in a separate shell.

(cd /var/log && ls)

(cd /var/log && ls)

Any directory change inside the parentheses does not affect your current shell.

Useful when you want to isolate commands.

Arrays

Parentheses are also used to create arrays.

files=(app.log error.log access.log)

for file in "${files[@]}"; do
  echo "Processing $file"
done

files=(app.log error.log access.log)

for file in "${files[@]}"; do
  echo "Processing $file"
done

Arrays let you store multiple values in one variable.

{} – Group Commands and Create Sequences

Grouping Commands

Curly braces group commands that run in the same shell.

{
  echo "Starting task"
  echo "Task running"
  echo "Task completed"
}

{
  echo "Starting task"
  echo "Task running"
  echo "Task completed"
}

Variables created inside remain available afterward.

Brace Expansion

Brace expansion quickly creates multiple files or strings.

touch file{1..3}.txt

touch file{1..3}.txt

This creates:

file1.txt file2.txt file3.txt

file1.txt file2.txt file3.txt

Very useful for bulk operations.

$var and ${var}—Using Variables

name="John"
echo "$name"
echo "${name}_admin"

name="John"
echo "$name"
echo "${name}_admin"

${} is safer when you attach extra text to a variable.

${} – Modify Variable Values

Bash allows you to modify variables without using external commands.

file="report.txt"
backup="${file%.txt}.bak"
echo "$backup"

file="report.txt"
backup="${file%.txt}.bak"
echo "$backup"

Output:

report.bak

report.bak

Commonly used for renaming files and string manipulation.

[] – Basic Condition Check

Single brackets are used to test conditions.

file="/etc/passwd"

if [ -f "$file" ]; then
  echo "File exists"
fi

file="/etc/passwd"

if [ -f "$file" ]; then
  echo "File exists"
fi

This is the traditional way of writing conditions in shell scripts.

[[]] – Advanced Condition Check (Recommended)

Double brackets are more powerful and safer in Bash.

if [[ $USER == "root" ]]; then
  echo "You are root"
fi

if [[ $USER == "root" ]]; then
  echo "You are root"
fi

Benefits:

• Supports pattern matching
• Handles logical operators better
• Fewer quoting issues

(()) – Math Operations

Double parentheses are used for arithmetic.

a=5
b=3
result=$((a * b + 1))
echo "$result"

a=5
b=3
result=$((a * b + 1))
echo "$result"

Only works with numbers, not strings.

Author:
Krishna T
Linux | DevOps | Cloud Engineer