$ diff a.txt b.txt
then create a script to filter only the lines you want. Or man diff, it probably already does that for you with some switch.
"Really, I'm not out to destroy Microsoft. That will just be a completely unintentional side effect." - Linus Torvalds
"When I do this, some people think that it's because I want my ego to be fed, right? Of course, I'm not asking you to call it "Stallmanix"!" - Richard Stallman