Re: { mv test test; } should not fail

[Eric Blake]

On 02/03/2011 04:42 PM, Krzysztof Żelechowski wrote:
> Dear all:
> 
> When I say { mv test test; }, I mean that the file currently named "test" 
> should be named "test" in the future.  After { mv test test; } executes, my 
> wish is successfully fulfilled, so why does the command fail?!
> 
> (This breaks scripts that rename "lib" to "%{_lib}" on openSUSE, where 
> %{_lib} may be "lib" or "lib64", depending on context.)

Here's what POSIX requires on the matter:
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/mv.html

If the source_file operand and destination path name the same existing
file, then the destination path shall not be removed, and one of the
following shall occur:

   1.

      No change is made to source_file, no error occurs, and no
diagnostic is issued.
   2.

      No change is made to source_file, a diagnostic is issued to
standard error identifying the two names, and the exit status is affected.
   3.

      If the source_file operand and destination path name distinct
directory entries, then the source_file operand is removed, no error
occurs, and no diagnostic is issued.


So you're asking us to change from behavior 2 to behavior 1.  Which will
solve things for you if you used fixed coreutils, but not if you use
some other POSIX-compliant mv.  Since POSIX already says you have to be
prepared for failure, the best you can do is to instead rewrite your
openSUSE scripts to not fail in the first place by doing a preparatory
check, and rewriting 'mv lib %{_lib}' into 'test %{_lib} != lib && mv
lib %{_lib}', so that you will work regardless of whether coreutils
changes behavior, or even whether you are using coreutils' mv.

-- 
Eric Blake   address@hidden    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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