Arithmetic expansion $(( )) in Bash Scripting

Introduction

Arithmetic expansion lets you do math calculations inside your script easily.

When you want to add, subtract, multiply, or divide numbers in a script.

When you need to update a counter or index in a loop.

When you want to calculate values based on user input or variables.

When you want to perform simple math without calling external programs.

Syntax

Bash Scripting

result=$(( expression ))

The expression inside $(( )) can use +, -, *, /, %, and more.

No spaces are needed around the arithmetic operators, but they are allowed.

Examples

Adds 3 and 5, then prints 8.

Bash Scripting

sum=$((3 + 5))
echo $sum

Subtracts 1 from variable count and prints 9.

Bash Scripting

count=10
count=$((count - 1))
echo $count

Multiplies 4 by 7 and prints 28.

Bash Scripting

product=$((4 * 7))
echo $product

Finds remainder of 10 divided by 3 and prints 1.

Bash Scripting

remainder=$((10 % 3))
echo $remainder

Sample Program

This script calculates the area by multiplying length and width using arithmetic expansion.

Bash Scripting

#!/bin/bash

# Calculate the area of a rectangle
length=8
width=5
area=$((length * width))
echo "Area of rectangle: $area"

Important Notes

You can use variables inside $(( )) without the $ sign, like $((length * width)).

Arithmetic expansion only works with integers, not decimals.

You can use parentheses inside for grouping, like $(((2 + 3) * 4)).

Summary

Arithmetic expansion $(( )) lets you do math inside bash scripts easily.

Use it to calculate values, update counters, or perform math with variables.

It works only with whole numbers and supports common math operators.