GNU bug report logs - #11427
cp 8.16 not writing through, writing over

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From: Jim Meyering <jim <at> meyering.net>
To: karl <at> freefriends.org (Karl Berry)
Cc: 11427 <at> debbugs.gnu.org
Subject: bug#11427: cp 8.16 not writing through, writing over
Date: Mon, 07 May 2012 19:05:18 +0200

Karl Berry wrote:
> Create dangling symlink:
> $ ln -s foo bar
>
> Attempt to write over it with cp:
> $ \cp -i /etc/issue bar
> cp: not writing through dangling symlink 'bar'
>
> In the past, it would ask me if I wanted to replace bar.  (As desired.)

Hi Karl,

When I try that in an empty directory and using coreutils-6.7's cp
(which predates the change mentioned below), it does this:

    $ ln -s foo bar
    $ env cp -i /etc/issue bar                                :
    cp: cannot create regular file `bar': File exists

Maybe you want to use --backup?

> The error message makes me think that it is thinking I am writing
> "through" a dangling symlink to a directory.  But that gets
> a different error message, which seems fine:
> $ \cp -i /etc/issue bar/baz
> cp: cannot create regular file 'bar/baz': No such file or directory
>
> coreutils 8.16, compiled from original source on CentOS 5.8 (libc 2.5,
> it seems).

[quick answer: set POSIXLY_CORRECT]

That behavior change dates back to 2007.
Here's the NEWS snippet:


    * Noteworthy changes in release 6.9.90 (2007-12-01) [beta]
      ...
    ** Changes in behavior

      cp, by default, refuses to copy through a dangling destination symlink
      Set POSIXLY_CORRECT if you require the old, risk-prone behavior.


There's more detail here:

  http://git.sv.gnu.org/cgit/coreutils.git/commit/?id=2bdc48121916ab0d7bb7d

and even more on the mailing list just before I made that change.

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