How to Use Arithmetic in Bash: Simple Guide with Examples

In bash, you can perform arithmetic using the $(( expression )) syntax or the let command. These allow you to add, subtract, multiply, divide, and use other math operations directly in your scripts.

Syntax

Bash supports arithmetic using the $(( expression )) syntax, where expression is the math operation you want to perform. You can also use the let command for arithmetic evaluation.

• $(( expression )): Evaluates the expression and returns the result.
• let expression: Evaluates the expression and assigns the result to variables if used.

bash

result=$(( 5 + 3 ))
let result=5+3

Example

This example shows how to add, subtract, multiply, and divide two numbers using $(( )) syntax and print the results.

bash

#!/bin/bash

num1=10
num2=4

sum=$(( num1 + num2 ))
diff=$(( num1 - num2 ))
prod=$(( num1 * num2 ))
div=$(( num1 / num2 ))

echo "Sum: $sum"
echo "Difference: $diff"
echo "Product: $prod"
echo "Division: $div"

Output

Sum: 14
Difference: 6
Product: 40
Division: 2

Common Pitfalls

Common mistakes include:

• Using single brackets [ ] instead of $(( )) for arithmetic, which treats values as strings.
• Forgetting spaces around operators inside $(( )) is allowed but can cause confusion.
• Using floating-point numbers, which bash arithmetic does not support natively.

Here is a wrong and right way example:

bash

# Wrong way (string concatenation):
echo $[ 5 + 3 ]  # Deprecated but works

# Right way:
echo $(( 5 + 3 ))

Output

8

Quick Reference

Operation	Syntax Example	Description
Addition	$(( a + b ))	Adds a and b
Subtraction	$(( a - b ))	Subtracts b from a
Multiplication	$(( a * b ))	Multiplies a by b
Division	$(( a / b ))	Divides a by b (integer division)
Modulus	$(( a % b ))	Remainder of a divided by b
Increment	let a++	Increases a by 1
Decrement	let a--	Decreases a by 1

Key Takeaways

Use $(( expression )) for clean and easy arithmetic in bash.

Bash arithmetic only supports integers, no floating-point math.

Avoid using single brackets [ ] for math; they are for tests.

The let command is an alternative but less common than $(( )).

Remember division in bash truncates decimals (integer division).