Bash - Arithmetic Expression

About

arithmetic in bash

Example

Counter

counter=0
(( counter + 1))
counter=$((counter+1))

Simple addition

echo One plus One is $((1+1))

echo One plus One is 2

Syntax

(( arithmetic expression ))
# or to capture the output
$((arithmetic_expression))
# or
let arithmetic_expression [arithmetic_expression, ...]

where let is the let command

If the value of the expression is:

non-zero, the return status is 0;
otherwise the return status is 1.

Management

Variable

If the variable has its integer attribute set (see Bash - Declare (Variable declaration and attribute)), then value is evaluated as an arithmetic expression even if the $((...)) expansion is not used.

Operator

In the context where an assignment statement is assigning a value to a shell variable, the += operator can be used to append to or add to the variable’s previous value. When += is applied to a variable for which the integer attribute has been set, value is evaluated as an arithmetic expression and added to the variable’s current value, which is also evaluated.

Comparison

As number

if (( a > b )); then
    ...
fi

As string in a conditional expression.

if [ "$a" -gt "$b" ]; then
    ...
fi