Rename file without overwriting existing files

[Chris Angelico]

On Tue, Jan 31, 2017 at 12:58 AM, Peter Otten <__peter__ at web.de> wrote:
>> I looked at help(os.link) on Python
>> 3.4 and the corresponding current library documentation on the web. I
>> saw no mention of what happens when dst exists already.
>>
>> Also, creating a hard link doesn't seem to work between different file
>> systems, which may well be relevant to Steve's case.
>
> In his example above he operates inside a single directory. Can one
> directory spread across multiple file systems?

Yep. Try unionfs.

... make ourselves some scratch space ...
$ mkdir space modifier
$ dd if=/dev/zero of=space.img bs=4096 count=65536
$ mkfs space.img
$ sudo mount space.img space
$ dd if=/dev/zero of=modifier.img bs=4096 count=1024\
$ mkfs modifier.img
$ sudo mount modifier.img modifier

... put some content into the base directory ...
$ sudo -e space/demo.txt

... and now the magic:
$ unionfs modifier=RW:space=RO joiner/
$ cd joiner

At this point, you're in a directory that is the union of the two
directories. One of them is read-only, the other is read/write. It is
thus possible to view a file that you can't hard-link to a new name,
because the new name would have to be created in the 'modifier' file
system, but the old file exists on the 'space' one.

ChrisA