bug#21094: cp: add option to sort when copying

[Pádraig Brady]

On 20/07/15 01:34, Pádraig Brady wrote:
> On 19/07/15 07:09, Steve Russell wrote:
>> When performing a recursive copy of a large number of files it would be 
>> helpful to have an option to copy the files in each directory in sorted 
>> order of the file names. There are a couple of use cases for this.
>>  
>> The first is to be able to easily observe the progress of the copy using the 
>> -v option. Currently, the "random" order of the copy means I can not tell 
>> how far the copy has progressed. On occasions I have seen cp -v display a 
>> top level directory starting with 'z' and assumed the copy was almost done 
>> only to see it start copying files from another directory earlier in the 
>> alphabet.
>>  
>> The second use case is sometimes I would like to start working with the 
>> destination files before the copy has completed. For example if I see the 
>> destination has the top level directory "foo", an ordered copy means I can 
>> work with directories that are alphabetically before "foo" knowing that they 
>> will not change.
>>
>> Another minor use case is some mp3 players ignore the file names and 
>> metadata and always play tracks in the order they appear in the directory. 
>> Usually the filesystems used by these players will order directories in the 
>> same order files are added. For content like eBooks where track order is 
>> critical, a sorted copy would be a useful workaround for the limitations of 
>> these players.
> 
> Thanks for the suggestion.
> 
> The implementation would be quite simple given that we already
> sort directory entries by inode number to improve performance
> (though note that that performance enhancement may not be
> effective going forward with newer file systems and SSDs).
> 
> The implementation would leverage gnulib's SAVEDIR_SORT_NAME functionality.
> 
> The interface would probably mirror tar's --sort option.
> 
> I'm 55:45 for exposing this option to cp, install and maybe mv.

Upon consideration I'm down to 50:50.

For the progress use case, one can use rsync,
or perhaps an explicit progress option in cp.

For the mp3 order case, one can use tar --sort
to do the transfer.

thanks,
Pádraig.