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From: ZyX <kp-pav@yandex.ru>
To: Eric Cook <llua@gmx.com>,
	"zsh-workers@zsh.org" <zsh-workers@zsh.org>
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Subject: Re: [[ is being treated as a pattern in the command/reserved word position.
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Date: Sun, 29 Mar 2015 02:19:28 +0300
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29.03.2015, 02:07, "Eric Cook" <llua@gmx.com>:
> On 03/28/2015 06:33 PM, ZyX wrote:
>>  29.03.2015, 01:25, "Bart Schaefer" <schaefer@brasslantern.com>:
>>>  On Mar 28,  5:55pm, Eric Cook wrote:
>>>  }
>>>  } % print $ZSH_PATCHLEVEL; emulate sh -c '[[ a == a ]]'
>>>  } zsh-5.0.7-362-gab40656
>>>  } zsh: command not found: [[
>>>  }
>>>  } Did that behavior change?
>>>
>>>  Aha.  That makes more sense.
>>>
>>>  The '[[' reserved word is handled a special kind of built-in alias [*]
>>>  and the change to POSIX_ALIASES handling caused that to be disabled.
>>>
>>>  This will take a bit of thought.
>>>
>>>  [*] Not literally, but the alias expansion code is where '[[' is noted
>>>  and the lexer changed into "parsing a conditional" state.
>>  I would say that this is actually an expected behaviour: `posh -c '[[ a == a ]]'` will show `posh: [[: not found` because `[[` is not in POSIX. Similar error will be shown by dash.
>
> Nor is it disallowed by POSIXand it used to work, the bug report is
> still valid.

`[[` is a *syntax extension*. This *is* going against POSIX. If `[[` in POSIX emulation mode was implemented as a shell built-in in a manner that allows implementing it as a script your concern would be valid. But implementing `[[` as a shell built-in is breaking certain expectations about how `[[` is supposed to work.

Specifically, with

    setopt SH_WORD_SPLIT
    w="a = b"
    test $w # False: a ≠ b, expected
    [[ a = b ]] # False: a ≠ b, expected
    [[ $w ]] # True: w is not empty, should be false if shell is POSIX

. It is easier to disable `[[` then keep two implementations of it.