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Re: best way to test for empty dir?


From: Andreas Schwab
Subject: Re: best way to test for empty dir?
Date: Tue, 15 Dec 2009 15:23:33 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/23.1 (gnu/linux)

"Matias A. Fonzo" <address@hidden> writes:

> On Tue, 15 Dec 2009 10:37:36 +0100
> Andreas Schwab <address@hidden> wrote:
>
>> "Matias A. Fonzo" <address@hidden> writes:
>> 
>> > On Mon, 14 Dec 2009 12:21:12 +0000
>> > Marc Herbert <address@hidden> wrote:
>> >
>> >> Matias A. Fonzo a écrit :
>> >> > On Fri, 11 Dec 2009 16:16:13 +0000
>> >> > Marc Herbert <address@hidden> wrote:
>> >> 
>> >> >> In case anyone is interested my winner (so far) is:
>> >> >>
>> >> >> exists()
>> >> >> {
>> >> >>     [ -e "$1" -o -L "$1" ]
>> >> >> }
>> >> >>
>> >> > 
>> >> 
>> >> > The -L is redundant.
>> >> 
>> >> Not for me. I need -L because I want to consider broken symlinks just
>> >> like anything else. A broken symlink would be a bug in my code and I want 
>> >> to
>> >> detect it ASAP.
>> >> 
>> >> 
>> >> > Because, if the symlink is not broken, the regular file "exists" ( -e ).
>> >> 
>> >> Please forget about correct symlinks. The -L is here for *broken*
>> >> symlinks.
>> >> 
>> >
>> > The [ -L "foo" -a ! -e "foo" ] is a specific case to check dangling 
>> > symlinks.
>> 
>> Combine that with the existence check and you have exactly the
>> expression above.
>> 
>
> Not quite.
>
> Here an interesting quote from the Greg's FAQ:
>
> "The -e test (like all other tests besides -L or -h) follows the symbolic 
> link, and therefore it checks on the thing pointed to, not on the link 
> itself. The -L test does not follow the symlink, so it's checking on the link 
> itself. Together, they can indicate the presence of a dangling symlink."
>
> You can see, creating a dangling symlink:
>
> $ ln -sf x y
> $ sh -c '[ -e "y" ] && echo true || echo false'
> false
> $ sh -c '[ -L "a" ] && echo true || echo false'
> true

Combine the two tests and you have exactly the expression above.

Andreas.

-- 
Andreas Schwab, address@hidden
GPG Key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."




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