Difference between `[[ $a -lt 2 ]]` and `(( $a < 2 ))`

Question

Is there any significant difference between using [[ $a -lt 2 ]] and (( $a < 2 ))?

For example, is one of them faster or more POSIX compliant than the other?

Related (but not duplicate) [Is test or [ or [ more portable both between bash shells and between other shells — Chris Davies, Oct 04 '16 at 21:10
(( $a < 2 )) is a valid command in bash, distinct from the expression $(( $a < 2 )). — chepner, Oct 04 '16 at 21:12

score 5 · Accepted Answer · edited Oct 04 '16 at 22:13

5

Neither is POSIX-compatible. In a POSIX shell, you can use the command [ "$a" -lt 2 ] or the expression $(( a < 2 )).

In bash, the former is simply the conditional command supporting a superset of the conditional expressions that [ supports, and the latter is a standalone command that exits with status 0 if the enclosed arithmetic expression is non-zero, or 1 otherwise. Other than readability, there is no significant difference between the two when used properly.

edited Oct 04 '16 at 22:13

Jeff Schaller

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answered Oct 04 '16 at 21:12

chepner

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(( )) and $(( )) would perhaps make it easier to mix other arithmetic in, if that's ever needed? – ilkkachu Oct 04 '16 at 21:22
Is [ available in any shell? If there would be a shell where [ is not built-in then $(( )) could be significant faster. – rudimeier Oct 04 '16 at 22:30
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@rudimeier In theory [ can be external but in practice it's always built in. – Gilles 'SO- stop being evil' Oct 05 '16 at 00:12

Difference between `[[ $a -lt 2 ]]` and `(( $a < 2 ))`

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