The first thing I see wrong with this is that you ignore the possibility that one user name may be contained within another. For instance - jon and jonathan.
The second thing I see wrong is that you don't consider that a user's primary group need not be specified in /etc/group if it is present in /etc/passwd. For instance - bronto may belong to group 123 but not appear in group 123's line in /etc/group. At least on some *nix - which would mean 'robin' didn't end up in all groups as 'bronto'.
The last thing is, your documentation is lacking. I know what -i.save does but would everyone who used it? What if someone made a typo - would they know how to undo it?
Yep, every single byte you wrote is plain right. Thanks for taking the time to read and comment this node.
I sometimes need to solve problems, like the one above, that require me a one-liner solution. When it happens, I often save the one liner in a file for future use, along with a very short comment to remind me what I wrote it for.
I Often won't use it again as-is, but at least I have a good starting point for the next time.
Basically, I did the same thing, publishing this one here instead of just saving it locally. So... have fun! :-)
In theory, there is no difference between theory and practice. In practice, there is.