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$(( <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`

.

## Discussion

The line

read -p "Enter a number: "

in the second example should read

read -p "Enter a number: " number

Fixed, thx

Should mention that

`$(())`

form doesn't accept quoted variable names.