How can I show lines in common (reverse diff)?

On *nix, you can use comm. The answer to the question is:

comm -1 -2 file1.sorted file2.sorted 
# where file1 and file2 are sorted and piped into *.sorted

Here’s the full usage of comm:

comm [-1] [-2] [-3 ] file1 file2
-1 Suppress the output column of lines unique to file1.
-2 Suppress the output column of lines unique to file2.
-3 Suppress the output column of lines duplicated in file1 and file2. 

Also note that it is important to sort the files before using comm, as mentioned in the man pages.