GNU bug report logs - #61105
cp/mv: want a fatal --no-clobber

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Mike Frysinger <vapier <at> gentoo.org>
To: bug-coreutils <at> gnu.org
Subject: cp/mv: want a fatal --no-clobber
Date: Fri, 27 Jan 2023 15:52:09 -0500

[Message part 1 (text/plain, inline)]

i've been under the mistaken assumption that the -n/--no-clobber option exits
non-zero when the target exists, but someone pointed out to me recently that
it silently ignores existing files.  can we get a setting to control this ?

basically i've been writing things like:
if ! cp -n foo bar; then
  ... error out because bar already exists, or otherwise failed ...
fi

when really i need to write:
if [ -e bar ] || ! cp foo bar; then
  ... error out ...
fi
-mike

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Message #8 received at 61105 <at> debbugs.gnu.org (full text, mbox):

From: Pádraig Brady <P <at> draigBrady.com>
To: Mike Frysinger <vapier <at> gentoo.org>, 61105 <at> debbugs.gnu.org
Subject: Re: bug#61105: cp/mv: want a fatal --no-clobber
Date: Fri, 27 Jan 2023 23:59:47 +0000

On 27/01/2023 20:52, Mike Frysinger wrote:
> i've been under the mistaken assumption that the -n/--no-clobber option exits
> non-zero when the target exists, but someone pointed out to me recently that
> it silently ignores existing files.  can we get a setting to control this ?

Yes --no-clobber={skip (default), fail} would have some value.
Especially for handling multiple files.

> 
> basically i've been writing things like:
> if ! cp -n foo bar; then
>    ... error out because bar already exists, or otherwise failed ...
> fi
> 
> when really i need to write:
> if [ -e bar ] || ! cp foo bar; then
>    ... error out ...
> fi

The above is racy if the file is created between the test and the cp.

I suppose you could leverage the shell to create the file atomically like:

cp-n-fail() {
  (set -C && cat < "$1" > "$2")
}

cp-a-n-fail() {
  cp-n-fail "$1" "$2" &&
  cp -a --attributes-only "$1" "$2"
}

But yes hack, only handles single files, doesn't handle sparseness, ...

cheers,
Pádraig

Message #11 received at 61105 <at> debbugs.gnu.org (full text, mbox):

From: Mike Frysinger <vapier <at> gentoo.org>
To: Pádraig Brady <P <at> draigBrady.com>
Cc: 61105 <at> debbugs.gnu.org
Subject: Re: bug#61105: cp/mv: want a fatal --no-clobber
Date: Fri, 27 Jan 2023 19:22:58 -0500

[Message part 1 (text/plain, inline)]

On 27 Jan 2023 23:59, Pádraig Brady wrote:
> On 27/01/2023 20:52, Mike Frysinger wrote:
> > basically i've been writing things like:
> > if ! cp -n foo bar; then
> >    ... error out because bar already exists, or otherwise failed ...
> > fi
> > 
> > when really i need to write:
> > if [ -e bar ] || ! cp foo bar; then
> >    ... error out ...
> > fi
> 
> The above is racy if the file is created between the test and the cp.

yes, that's why i want to only use cp :)

although if i was super concerned about TOCTOU races, i wouldn't be using
shell in the first place
-mike

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Message #14 received at 61105 <at> debbugs.gnu.org (full text, mbox):

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Mike Frysinger <vapier <at> gentoo.org>, 61105 <at> debbugs.gnu.org
Subject: Re: bug#61105: cp/mv: want a fatal --no-clobber
Date: Mon, 30 Jan 2023 12:12:28 -0800

Better, I think, would be to change cp -n to be compatible with FreeBSD, 
where 'cp -n A B' exits with status 1 if B exists. This matches what 
FreeBSD 'cp -i A B' does when you say "no". This would be more useful 
than what GNU cp does, and the FreeBSD cp -i behavior conforms to POSIX 
whereas GNU cp -i (I think accidentally) does not. This would be simpler 
than adding a fatal --no-clobber, and would encourage portability a bit.

The reason it's a POSIX conformance issue is that POSIX says exit status 
zero means "All files were copied successfully." which "cp -n A B" does 
not do when B exists.

Similarly for 'mv' (though FreeBSD mv -i does not conform to POSIX here, 
unfortunately).

Message #17 received at 61105 <at> debbugs.gnu.org (full text, mbox):

From: Pádraig Brady <P <at> draigBrady.com>
To: Paul Eggert <eggert <at> cs.ucla.edu>, Mike Frysinger <vapier <at> gentoo.org>,
 61105 <at> debbugs.gnu.org
Subject: Re: bug#61105: cp/mv: want a fatal --no-clobber
Date: Mon, 30 Jan 2023 21:35:19 +0000

On 30/01/2023 20:12, Paul Eggert wrote:
> Better, I think, would be to change cp -n to be compatible with FreeBSD,
> where 'cp -n A B' exits with status 1 if B exists. This matches what
> FreeBSD 'cp -i A B' does when you say "no". This would be more useful
> than what GNU cp does, and the FreeBSD cp -i behavior conforms to POSIX
> whereas GNU cp -i (I think accidentally) does not. This would be simpler
> than adding a fatal --no-clobber, and would encourage portability a bit.
> 
> The reason it's a POSIX conformance issue is that POSIX says exit status
> zero means "All files were copied successfully." which "cp -n A B" does
> not do when B exists.
> 
> Similarly for 'mv' (though FreeBSD mv -i does not conform to POSIX here,
> unfortunately).

This is a good observation.
Also the current "skip" functionality of coreutils cp -n
is already catered for with the --update option.

cheers,
Pádraig

Message #22 received at 61105-done <at> debbugs.gnu.org (full text, mbox):

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Pádraig Brady <P <at> draigBrady.com>,
 Mike Frysinger <vapier <at> gentoo.org>
Cc: 61105-done <at> debbugs.gnu.org
Subject: Re: bug#61105: cp/mv: want a fatal --no-clobber
Date: Tue, 31 Jan 2023 08:55:48 -0800

[Message part 1 (text/plain, inline)]

On 2023-01-30 13:35, Pádraig Brady wrote:
> This is a good observation.
> Also the current "skip" functionality of coreutils cp -n
> is already catered for with the --update option.

For consistency, there too the exit status should reflect whether the cp 
action was done.

I installed the attached patch to implement this; comments welcome. In 
the meantime I'm boldly closing the bug report.

[0001-cp-ln-mv-when-skipping-exit-with-nonzero-status.patch (text/x-patch, attachment)]

Message #25 received at 61105 <at> debbugs.gnu.org (full text, mbox):

From: Pádraig Brady <P <at> draigBrady.com>
To: Paul Eggert <eggert <at> cs.ucla.edu>, Mike Frysinger <vapier <at> gentoo.org>
Cc: 61105 <at> debbugs.gnu.org
Subject: Re: bug#61105: cp/mv: want a fatal --no-clobber
Date: Tue, 31 Jan 2023 17:11:00 +0000

On 31/01/2023 16:55, Paul Eggert wrote:
> On 2023-01-30 13:35, Pádraig Brady wrote:
>> This is a good observation.
>> Also the current "skip" functionality of coreutils cp -n
>> is already catered for with the --update option.
> 
> For consistency, there too the exit status should reflect whether the cp
> action was done.
> 
> I installed the attached patch to implement this; comments welcome. In
> the meantime I'm boldly closing the bug report.

I'm not sure I agree with making this change for --update.
Note POSIX doesn't specify -u (or -n TBH), so it's a
bit of a stretch to take its specification of
"All files were copied successfully" in this case,
when it wasn't considering these cases.

To me --update is less useful if it fails in this case.

thanks,
Pádraig

Message #28 received at 61105 <at> debbugs.gnu.org (full text, mbox):

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Pádraig Brady <P <at> draigBrady.com>,
 Mike Frysinger <vapier <at> gentoo.org>
Cc: 61105 <at> debbugs.gnu.org
Subject: Re: bug#61105: cp/mv: want a fatal --no-clobber
Date: Tue, 31 Jan 2023 09:27:55 -0800

[Message part 1 (text/plain, inline)]

On 2023-01-31 09:11, Pádraig Brady wrote:
> To me --update is less useful if it fails in this case.

Fair enough, I installed the attached to revert the --update part of the 
change, so that --update will work as before.

[0001-cp-mv-skipping-due-to-u-is-success-not-failure.patch (text/x-patch, attachment)]

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