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bash-hackers-wiki/docs/syntax/expansion/arith.md

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# Arithmetic expansion
$(( <EXPRESSION> ))
$[ <EXPRESSION> ]
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The [arithmetic expression](../../syntax/arith_expr.md) `<EXPRESSION>` is
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evaluated and expands to the result. The output of the arithmetic
expansion is guaranteed to be one word and a digit in Bash.
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Please **do not use the second form `$[ ... ]`**! It's deprecated. The
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preferred and standardized form is `$(( ... ))`!
Example
``` bash
function printSum {
typeset -A args
typeset name
for name in first second; do
[[ -t 0 ]] && printf 'Enter %s positive integer: ' "$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]}))
}
```
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**Note** that in Bash you don't need the arithmetic expansion to check
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for the boolean value of an arithmetic expression. This can be done
using the [arithmetic evaluation compound
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command](../../syntax/ccmd/arithmetic_eval.md):
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``` bash
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:
``` bash
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.
```
## Bugs and Portability considerations
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- The original Bourne shell doesn't have arithmetic expansions. You
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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
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fix any code you find that's still using `expr`.
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## See also
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- [arithmetic expressions](../../syntax/arith_expr.md)
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- [arithmetic evaluation compound
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command](../../syntax/ccmd/arithmetic_eval.md)
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- [Introduction to expansion and
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substitution](../../syntax/expansion/intro.md)
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- [POSIX
definition](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_04)
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