GNU bug report logs - #62572
cp --no-clobber behavior has changed

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From: Pádraig Brady <P <at> draigBrady.com>
To: Paul Eggert <eggert <at> cs.ucla.edu>, Michael Stone <mstone <at> debian.org>
Cc: 1058752 <at> bugs.debian.org, Bernhard Voelker <mail <at> bernhard-voelker.de>, 62572 <at> debbugs.gnu.org
Subject: bug#62572: Bug#1058752: bug#62572: cp --no-clobber behavior has changed
Date: Tue, 30 Jan 2024 11:18:12 +0000

On 29/01/2024 21:44, Paul Eggert wrote:
> On 1/29/24 08:11, Pádraig Brady wrote:
> 
>> Right, that's why I'm still leaning towards my proposal in the last mail.
> 
> Well, I won't insist on doing nothing; however, the proposal needs
> ironing out and now's a good time to do it before installing changes.
> 
> 
>>     - revert to previous exit success -n behavior
>>     - document -n as deprecated
>>     - provide --update=noclobber to give exit failure functionality
> 
> So --update=noclobber would differ in meaning from the deprecated-in-9.5
> --no-clobber, but would agree in meaning with 9.4 --no-clobber? That
> sounds pretty confusing for future users. (And a nit: why should one
> spelling have a hyphen but the other doesn't?)

That's a fair point; just avoid "no-clobber" entirely.
We could choose a new name so. How about --update=none-fail

>>       - BTW, it probably makes sense to print a diagnostic for each
>> skipped file here
>>         as it's exceptional behavior, for which we're exiting with
>> failure for.
> 
> Coreutils 9.4 cp -n already does that, no? So I'm not sure what's being
> proposed here.
> 
>     $ touch a b
>     $ cp -n a b; echo $?
>     cp: not replacing 'b'
>     1

Sorry I got confused between your suggestion of added diagnostics,
and FreeBSD's silent behavior here. Looks like we just keep the
existing behavior of diagnosing 'not replacing' iff exiting with failure.

>>     - the existing --update=none provides the exit success functionality
> 
> It seems to me that this proposal conflates two questions:
> 
> * What rules should cp use to decide whether to update a destination?
> 
> * When cp decides not to update a destination, what should it do? Exit
> with nonzero status? Output a diagnostic? Both? Neither?
> 
> Aren't these independent axes? If so, shouldn't they have independent
> options? For example, since we have --update=older, shouldn't there be a
> way to say "I want to copy A to B only if B is older than A, and I want
> the exit status to be zero only if A was copied to B"?

Well they're not entirely independent.
It's more appropriate to output a diagnostic if exiting failure,
and POSIX also advises along the same lines.
We also have --verbose to control diagnostics somewhat
for the non failure case.

So we now have the proposed change as:

  - revert -n to old silent success behavior
  - document -n as deprecated
  - Leave --update=none as is (will be synonymous with -n)
  - Provide --update=none-fail to diagnose and exit failure

thanks,
Pádraig.

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