Howto rename a file

[Xavier Maillard <zedek@gnu-rox.org>]

Hello,

Given the situation I have files in a directory such as foo-1, foo-2
...

When I use an Emacs batch session, I want these files to be renamed
as bar-1, bar-2 and so on.

In certain cases, I may have other files named foo-1 that should be
renamed, following this logic, as bar-1. What I want is that if a
file bar-1 already exists, then the file should be renamed as
bar-<n> where <n> is a unique number following the maximum found
in the directory.

So foo-1 will be renamed as bar-3 in my example.

How would you do that ?

Thank you

Xavier