Integer Arithmetic

Arithmetic Expansion: $(( ))

$(( expression )) evaluates a C-like arithmetic expression and expands to the resulting integer, which can be stored in a variable or used directly in text. Inside $(( )) you can reference variables without a leading $, though using it is also allowed.

Warning: Integer division truncates toward zero: $((7 / 2)) is 3, not 3.5.

Example: Arithmetic Expansion: $(( ))

#!/bin/bash
a=7
b=3
sum=$((a + b))
product=$((a * b))
quotient=$((a / b))
remainder=$((a % b))
echo "sum=$sum product=$product quotient=$quotient remainder=$remainder"

((...)) for Side Effects

The (( expression )) command (no leading $) evaluates the expression purely for its exit status and side effects, such as incrementing a variable, rather than producing output. Its exit status is 0 (true) if the expression evaluates to a non-zero number, and 1 (false) if it evaluates to zero.

Example: ((...)) for Side Effects

#!/bin/bash
count=0
(( count = count + 1 ))
echo "count is now $count"
if (( count > 0 )); then
    echo "count is positive"
fi

The let Builtin

let performs arithmetic assignment similarly to $(( )), letting you write let x=x+1 instead of x=$((x + 1)). It is older style and less commonly used in modern scripts, but you will still see it in existing code.

Example: The let Builtin

#!/bin/bash
x=10
let x=x+5
let "x -= 2"
echo "x is now $x"

Increment/Decrement Shortcuts

Inside (( )) or $(( )), Bash supports C-style shortcuts like +=, -=, ++, and -- for updating a variable in place. These are purely arithmetic-context conveniences and do not work the same way outside double parentheses.

Example: Increment/Decrement Shortcuts

#!/bin/bash
score=0
(( score += 10 ))
(( score++ ))
echo "Final score: $score"