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Re: shopt can't set extglob in a sub-shell?


From: Dan Douglas
Subject: Re: shopt can't set extglob in a sub-shell?
Date: Sun, 26 Feb 2012 05:06:15 -0600
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On Saturday, February 25, 2012 09:42:29 PM Davide Baldini wrote:

> Description:
>       A 'test.sh` script file composed exclusively of the following text
>       fails execution:
>               #!/bin/bash
>               (
>                       shopt -s extglob
>                       echo !(x)
>               )
>       giving the output:
>               $ ./test.sh
>               ./test.sh: line 4: syntax error near unexpected token `('
>               ./test.sh: line 4: `    echo !(x)'
>       Moving the shopt line above the sub-shell parenthesis makes the script
>       work.
> 
>       The debian man pages give no explanation.
> 
>       Thank you.

Non-eval workaround if you're desperate:

#!/usr/bin/env bash
(
        shopt -s extglob
        declare -a a='( !(x) )'
        echo "address@hidden"
)

You may be aware extglob is special and affects parsing in other ways. Quoting 
Greg's wiki (http://mywiki.wooledge.org/glob):

> Likewise, you cannot put shopt -s extglob inside a function that uses
> extended globs, because the function as a whole must be parsed when it's
> defined; the shopt command won't take effect until the function is called, at
> which point it's too late.

This appears to be a similar situation. Since parentheses are "metacharacters" 
they act strongly as word boundaries without a special exception for extglobs.

I just tested a bunch of permutations. I was a bit surprised to see this one 
fail:

f()
        if [[ $FUNCNAME != ${FUNCNAME[1]} ]]; then
                trap 'shopt -u extglob' RETURN
                shopt -s extglob
                f
        else
                f()(
                        shopt -s extglob
                        echo !(x)
                )
                f
        fi

f

I was thinking there might be a general solution via the RETURN trap where you 
could just set "trace" on functions where you want it, but looks like even 
"redefinitions" break recursively, so you're stuck. Fortunately, there aren't a 
lot of good reasons to have extglob disabled to begin with (if any).
-- 
Dan Douglas

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