Re: desired diff feature: show common lines

    > I would sometimes like see the common lines of two
files only. I haven't 

As long as there is any difference at all, I think you could
use, say.
diff -C 9999999 file1 file2
and then filter out the changed lines with grep -v [!+-] or
whatever.

Of course, if the files are sorted, comm is better.

Best,
karl