GNU bug report logs - #20094
cp: add --dry-run option

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

From: 積丹尼 Dan Jacobson <jidanni <at> jidanni.org>
To: bug-coreutils <at> gnu.org
Subject: cp --dry-run
Date: Thu, 12 Mar 2015 10:27:11 +0800

Proposal:
add a new "cp --dry-run"
Reason: we want to know what the verbose output of e.g.,
$ cp -i -av /mnt/usb/thumb/ tmp/androidBackup/SDCard
will be without actually running the command first.
We are about to commit to a megabyte copy and we want to be sure where
all those files are exactly going even after understanding all the
documentation, and without needing to do partial "wet" runs etc. etc. etc. etc.

Also add a "mv --dry-run".

Please also document both would normally be used with -v.

In fact cpio (mainly -p) needs a --dry-run too.

Sorry if I have proposed this before, but it's an idea you just can't beat!

Message #8 received at 20094 <at> debbugs.gnu.org (full text, mbox):

From: Pádraig Brady <P <at> draigBrady.com>
To: 積丹尼 Dan Jacobson <jidanni <at> jidanni.org>, 
 20094 <at> debbugs.gnu.org
Subject: Re: bug#20094: cp --dry-run
Date: Thu, 12 Mar 2015 09:17:54 +0000

On 12/03/15 02:27, 積丹尼 Dan Jacobson wrote:
> Proposal:
> add a new "cp --dry-run"
> Reason: we want to know what the verbose output of e.g.,
> $ cp -i -av /mnt/usb/thumb/ tmp/androidBackup/SDCard
> will be without actually running the command first.
> We are about to commit to a megabyte copy and we want to be sure where
> all those files are exactly going even after understanding all the
> documentation, and without needing to do partial "wet" runs etc. etc. etc. etc.
> 
> Also add a "mv --dry-run".
> 
> Please also document both would normally be used with -v.
> 
> In fact cpio (mainly -p) needs a --dry-run too.
> 
> Sorry if I have proposed this before, but it's an idea you just can't beat!

I never needed such an option myself.
It couldn't check the space or permissions of the dest,
so wouldn't be that useful. Perhaps cp --attributes-only
would help for your use case?

thanks,
Pádraig

Message #11 received at 20094 <at> debbugs.gnu.org (full text, mbox):

From: Bob Proulx <bob <at> proulx.com>
To: 積丹尼 Dan Jacobson <jidanni <at> jidanni.org>,
 20094 <at> debbugs.gnu.org
Subject: Re: bug#20094: cp --dry-run
Date: Thu, 12 Mar 2015 11:55:00 -0600

積丹尼 Dan Jacobson wrote:
> Proposal:
> add a new "cp --dry-run"
> Reason: we want to know what the verbose output of e.g.,
> $ cp -i -av /mnt/usb/thumb/ tmp/androidBackup/SDCard
> will be without actually running the command first.
> We are about to commit to a megabyte copy and we want to be sure where
> all those files are exactly going even after understanding all the
> documentation, and without needing to do partial "wet" runs
> etc. etc. etc. etc.

For this functionality I suggest using rsync which already implements
this capability.

  rsync -n -av /mnt/usb/thumb/ tmp/androidBackup/SDCard/thumb/

I see you using cp -i but if you are going to be copying megabytes and
many files I assert that cp -i isn't a good practical way to ensure a
good result.  But rsync --ignore-existing and --update options are
available for data merging.

Bob

Message #14 received at 20094 <at> debbugs.gnu.org (full text, mbox):

From: 積丹尼 Dan Jacobson <jidanni <at> jidanni.org>
To: Bob Proulx <bob <at> proulx.com>
Cc: P <at> draigBrady.com, 20094 <at> debbugs.gnu.org
Subject: Re: bug#20094: cp --dry-run
Date: Sat, 14 Mar 2015 00:58:02 +0800

I am mainly talking about "Where will it copy/move these things anyway? I'd
like to be sure before I commit. From reading the man page I have a good
idea, but I'd like to see what the path names will look like first
without actually doing anything."

And as a bonus: "I'm pretty much sure I can successfully cp/mv entire
complicated tree A onto entire complicated tree B, but it would be great
to know before I get started if deep inside B there is something that
will cause some tiny part of the whole operation to fail."

So no, cp --attributes-only can't help. And yes, indeed I see I am
hoping that these rsync features will get added to cp and mv, and cpio
too. (Or maybe just on the man pages mention: for "--dry-run"
capabilities try rsync -n !.

Message #17 received at 20094 <at> debbugs.gnu.org (full text, mbox):

From: Bernhard Voelker <mail <at> bernhard-voelker.de>
To: 積丹尼 Dan Jacobson <jidanni <at> jidanni.org>, 
 Bob Proulx <bob <at> proulx.com>
Cc: P <at> draigBrady.com, 20094 <at> debbugs.gnu.org
Subject: Re: bug#20094: cp --dry-run
Date: Sun, 15 Mar 2015 23:36:33 +0100

On 03/13/2015 05:58 PM, 積丹尼 Dan Jacobson wrote:
> I'm pretty much sure I can successfully cp/mv entire
> complicated tree A onto entire complicated tree B, but it would be great
> to know before I get started if deep inside B there is something that
> will cause some tiny part of the whole operation to fail.

I don't think a dry-run could ever predict a failure in a reliable way.
As an abstract example let's assume that such a clever copy program would
calculate the needed space in the destination upfront, and would be able
to issue an error like "the command would fail because of low disk space".
But in the case the space is sufficient (at the time of calculation), it
could never predict external influences, e.g. another program filling the
disk at the same time.  The same applies to permission issues, etc.: even
if the determination of an issue is accurate, it would still be racy in
every case.

Have a nice day,
Berny

Message #20 received at 20094 <at> debbugs.gnu.org (full text, mbox):

From: Assaf Gordon <assafgordon <at> gmail.com>
To: 20094 <at> debbugs.gnu.org
Subject: Re: bug#20094: cp --dry-run
Date: Mon, 22 Oct 2018 19:57:05 -0600

tags 20094 wontfix
severity 20094 wishlist
retitle 20094 cp: add --dry-run option
close 20094
stop

(triaging old bugs)

On 15/03/15 04:36 PM, Bernhard Voelker wrote:
> I don't think a dry-run could ever predict a failure in a reliable way.

[...]

Given the above (and mention of "rsync --dry-run" previously
in the same thread), I'm closing this as "wontfix".

Discussion can continue by replying to this thread.

-assaf

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