Do you know about `comm`? Given two sorted files it will show you in three columns.

    * unique lines to file1
    * unique lines to file2
    * lines common to both files

You can pass it various options to suppress any of the three columns.

I once had an interview at Facebook where one of the problems was easily solved by `comm`. I found it funny that the interviewer, or anyone who reviewed the interview question, had never heard of it. I was a good sport about it though, I ended up writing a janky Perl script that roughly implemented `comm` to solve the problem, which (modulo Perl) was what they wanted me to do.