Ok, that's better than Excel, as I have Ubuntu running in a vm if I need it.

So, the joining/merging gave me an idea to speed up another part of this puzzle.

Does the first command generate a CSV with row IDs from 0 to 13? And the join replaces existing rows in that file with the numbered rows from the source file?

If so, then that definitely going to be the big time saver. I don't need to use the command line to generate the original CSV as I've actually already got another one I can use for this, with IDs from 0 to 255.

If I can join/merge the second csv into this one then that will be perfect. Replacing any existing lines in the target with the corresponding ones from the source.