Re: about [ and [[ in bash

> Has been a while since I had to ask a question on this list!
>
> I was having trouble understanding how the compound command [[ and the
> builtin [ commands were different from each other and why/when would it be
> better to choose one over the other.

In order to answer your question, I joined #bash at irc.freenode.net
and used [ and [[ command of its bot. (It is used to keep a FAQ about
bash and shell scripting). I could use help [ and help [[ at shell
prompt, but this doesn't provide links for further reading :)

Look:

<dark> [
<greybot> [ is NOT part of the `if' syntax. It's a normal, ordinary
COMMAND. The `if' just checks its exit status. See ''help test'',
http://mywiki.wooledge.org/BashGuide/Practices/BashTests and
http://partmaps.org/era/unix/award-example-backticks.html
<dark> [[
<greybot> [[ is a bash keyword similar to (but more powerful than) the
[ command. See http://mywiki.wooledge.org/BashFAQ/031 and
http://mywiki.wooledge.org/BashGuide/Practices/BashTests Unless you
are scripting for POSIX sh we recommend [[.

[ is from the original unix shell. It's portable between shells, and
acessible if you are using /bin/sh as your shell, for example. It's
equivalent to test, but you have to put a ] in the end. so:

if test -e /; then echo a; fi

is the same as

if [ -e / ]; then echo a; fi

on my system, it's a shell builtin. but here it is available as a
regular program too, at /usr/bin/[, so this has the same result as

if /usr/bin/[ -e / ]; then echo a; fi

this would spawn another process, pass -e, /, ] as parameters, and
watch for the return value: 0 means true, nonzero means false. (if you
know C, the parameters would be inserted in argc/argv arguments of
main, and the return value would be the int it returns). that's how if
works (for non-builtins).

[[ is different from test, and isn't available at all shells. it is
more powerful though. you don't need to quote variables, for example.
some advise that you use [[ when you expect to use bash only (or ksh,
..), and [ if you need a portable shell script.

(ps: also note this is not strictly linux-related: linux works at a
lower level. this was actually a shell question. ^^)

-- 
Elias Gabriel Amaral da Silva <[email protected]>
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