GNU bug report logs - #69532
mv's new -x option should be made orthogonal to -t/-T/default

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Message #53 received at 69532 <at> debbugs.gnu.org (full text, mbox):

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Pádraig Brady <P <at> draigBrady.com>
Cc: Petr Malat <oss <at> malat.biz>, util-linux <util-linux <at> vger.kernel.org>,
 Dominique Martinet <asmadeus <at> codewreck.org>, 69532 <at> debbugs.gnu.org,
 Karel Zak <kzak <at> redhat.com>, Rob Landley <rob <at> landley.net>
Subject: Re: bug#69532: mv's new -x option should be made orthogonal to
 -t/-T/default
Date: Sat, 16 Mar 2024 23:10:50 -0700

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

On 2024-03-05 06:16, Pádraig Brady wrote:
> I think I'll remove the as yet unreleased mv --swap from coreutils, 
> given that
> util-linux is as widely available as coreutils on GNU/Linux platforms.

Although removing that "mv --swap" implementation was a win, I don't 
think we can simply delegate this to util-linux's exch command. 
Exchanging files via a renameat-like call is not limited to the Linux 
kernel; it's also possible on macOS via renameatx_np with RENAME_SWAP, 
and there have been noises about adding similar things to other 
operating systems.

I just now added support for macOS renameatx_np to Gnulib, so coreutils 
does not need to worry about the macOS details; it can simply use 
renameatu with the Linux flags. See:

https://git.savannah.gnu.org/cgit/gnulib.git/commit/?id=af32ee824ee18255839f9812b8ed61aa5257a82b

Even with Linux it's dicey. People may have older util-linux installed 
and so lack the 'exch' utility; this is true for both Fedora 39 and 
Ubuntu 23.10, the current releases. Ubuntu is also odd in that it 
doesn't install all the util-linux utilities as part of the util-linux 
package, so it's not clear what they will do with 'exch'.

So I propose that we implement the idea in coreutils in a better way, 
that interacts more nicely with -t, -T, etc. Also, I suggest using the 
Linuxish name "--exchange" instead of the macOSish name "--swap", and 
(for now at least) not giving the option a single-letter equivalent as I 
expect it to be useful from scripts, not interactively.

After looking at various ways to do it I came up with the attached 
proposed patch. This should work on both GNU/Linux and macOS, if your OS 
is recent enough and the file system supports atomic exchange.

[0001-mv-new-option-exchange.patch (text/x-patch, attachment)]

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