How to Use let in Bash: Syntax and Examples

In bash, let is used to perform arithmetic operations on variables. You write expressions inside let without spaces around operators, and it updates the variable values directly.

Syntax

The basic syntax of let is: let expression. The expression can include variables and arithmetic operators like +, -, *, /, and %. No spaces should be around the operators inside the expression.

Example parts:

• let: the command to evaluate arithmetic
• expression: the math operation, e.g., count=5+3

bash

let count=5+3

Example

This example shows how to use let to add, subtract, multiply, and divide variables in bash.

bash

count=10
let count=count+5
let count=count-3
let count=count*2
let count=count/4
echo $count

Output

6

Common Pitfalls

Common mistakes include putting spaces around operators, which causes errors, or forgetting to use variables properly.

Wrong: let count = count + 1 (spaces cause error)

Right: let count=count+1

bash

count=1
# Wrong usage (will cause error)
# let count = count + 1

# Correct usage
let count=count+1
echo $count

Output

2

Quick Reference

Usage	Description
let var=expression	Evaluate arithmetic expression and assign to var
let var+=value	Add value to var
let var-=value	Subtract value from var
let var*=value	Multiply var by value
let var/=value	Divide var by value

Key Takeaways

Use let to perform arithmetic operations on bash variables without spaces around operators.

Always write expressions like let count=count+1, not with spaces.

let updates variables directly and returns success or failure based on the result.

You can use let for addition, subtraction, multiplication, division, and modulo operations.

Remember to echo variables after let to see the updated values.