GNU bug report logs - #9742
touch option for existence?

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Message #8 received at 9742 <at> debbugs.gnu.org (full text, mbox):

From: Pádraig Brady <P <at> draigBrady.com>
To: Morty <morty+coreutils <at> frakir.org>
Cc: 9742 <at> debbugs.gnu.org
Subject: Re: bug#9742: touch option for existence?
Date: Thu, 13 Oct 2011 10:05:05 +0100

On 10/13/2011 02:45 AM, Morty wrote:
> [This is more of a feature request than a bug request.  But I don't
> see where I can make feature requests.]
> 
> It would be nice if touch had an option to only "touch" if the file
> doesn't already exist.  Sort of like -c, but the other way around.
> This is useful because often, the reason one is using "touch" in a
> script is because one wants to make sure that a file exists before
> doing an operation that expects the file to already exist.  But if the
> file already exists, then touch has a side effect of changing the
> mtime or the atime.  [On many systems, one can work on a file without
> changing the atime thanks to mount options such as noatime or
> relatime.]
> 
> It's easy enough to wrap touch in an if:
> 
>   if [ ! -e $file ]; then touch $file; fi
> 
> But it would be nicer if this common case were built in.  I would
> suggest -e, for exists.  Thanks!

Note the above is easier to express in shell like:

  [ -e "$file" ] || touch "$file"

But that is racy.  If you were using touch for locking purposes
then adding -e (must create), would allow one to add O_EXCL to the open(),
so you could then do things like:

  until touch -e "$lock_file"; do sleep .1; done

However that functionality is already supported like:

  until mkdir "$lock_dir"; do sleep .1; done

So I'm not sure this is warranted.
I'm 60:40 against currently.

cheers,
Pádraig.

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