Arithmetic expansion

$(( <EXPRESSION> ))

$[ <EXPRESSION> ]

The arithmetic expression <EXPRESSION> is evaluated and expands to the result. The output of the arithmetic expansion is guaranteed to be one word and a digit in Bash.

Please do not use the second form $[ … ]! It's deprecated. The preferred and standardized form is $(( ... ))!

Example

function printSum {
    typeset -A args
    typeset name
    for name in first second; do
        [[ -t 0 ]] && printf 'Enter %s positive integer: ' "$name" >&2
        read -r ${BASH_VERSION+-e} "args[$name]"
        [[ ${args[$name]} == +([[:digit:]]) ]] || return 1 # Validation is extremely important whenever user input is used in arithmetic.
    done
    printf 'The sum is %d.' $((${args[first]} + ${args[second]}))
}

Note that in Bash you don't need the arithmetic expansion to check for the boolean value of an arithmetic expression. This can be done using the arithmetic evaluation compound command:

printf %s 'Enter a number: ' >&2
read -r number
if ((number == 1234)); then
    echo 'Good guess'
else
    echo 'Haha... :-P'
fi

Variables used inside the arithmetic expansion, as in all arithmetic contexts, can be used with or without variable expansion:

x=1
 
echo $((x))       # Good.
echo $(($x))      # Ok. Avoid expansions within arithmetic. Use variables directly.
echo $(("$x"))    # Error. There is no quote-removal in arithmetic contexts. It expands to $(("1")), which is an invalid arithmetic expression.
echo $((x[0]))    # Good.
echo $((${x[0]})) # Ok. Nested expansion again.
echo $((${x[$((${x[!$x]}-$x))]})) # Same as above but more ridiculous.
echo $(($x[0]))   # Error. This expands to $((1[0])), an invalid expression.

• The original Bourne shell doesn't have arithmetic expansions. You have to use something like expr(1) within backticks instead. Since expr is horrible (as are backticks), and arithmetic expansion is required by POSIX, you should not worry about this, and preferably fix any code you find that's still using expr.

• arithmetic expressions
• arithmetic evaluation compound command
• Introduction to expansion and substitution
• POSIX definition