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

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Jim Meyering <jim <at> meyering.net>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#9742: closed (touch option for existence?)
Date: Thu, 13 Oct 2011 13:39:02 +0000

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Your message dated Thu, 13 Oct 2011 15:37:42 +0200
with message-id <8739exc8rd.fsf <at> rho.meyering.net>
and subject line Re: bug#9742: touch option for existence?
has caused the debbugs.gnu.org bug report #9742,
regarding touch option for existence?
to be marked as done.

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From: Morty <morty+coreutils <at> frakir.org>
To: bug-coreutils <at> gnu.org
Subject: touch option for existence?
Date: Wed, 12 Oct 2011 21:45:25 -0400

[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!

- Morty

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From: Jim Meyering <jim <at> meyering.net>
To: Morty <morty+coreutils <at> frakir.org>
Cc: Pádraig Brady <P <at> draigBrady.com>,
	9742-close <at> debbugs.gnu.org
Subject: Re: bug#9742: touch option for existence?
Date: Thu, 13 Oct 2011 15:37:42 +0200

tags 9742 + notabug
thanks

Morty wrote:
> On Thu, Oct 13, 2011 at 10:05:05AM +0100, Pádraig Brady wrote:
>
>> Note the above is easier to express in shell like:
>>
>>   [ -e "$file" ] || touch "$file"
>
> Thanks, I'm familiar with it.  :)
>
>> But that is racy.  If you were using touch for locking purposes
>
> I'm using touch to make sure the file exists before an operation that
> will yield an error if it doesn't exist.  For example:

Thanks for the suggestion.

However, when there's such an easy and portable way to do what you want,

  touch_if_absent() { test -e "$1" || touch "$1"; }

it is counterproductive (and contrary to the "Unix way") to encumber
a tool like "touch" with an option to provide that functionality.
Besides, then you'd have to wait until a release including your
option makes it into whatever distribution you use (or build from source).
With the above, you have something that is portable and works now.

> resolv=/etc/resolv.conf
> touch $resolv
> ci -l -t-$resolv -m"check-in existing" $resolv
> grep -q domain $resolv || echo domain $domain >> $resolv
> ci -l -m"add domain $domain" $resolv
>
> The intent of code like the above is to make sure a certain file is in
> revision control and contains a certain setting.  If the file already
> is in revision control and has the setting, the code should do
> nothing.  But touch changes the mtime.  The problem could be fixed by
> slinging around ifs, but that's inelegant.

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