Comparing two lists

[Rolf Nielsen]

Hello all,

I have two text files, quite extensive ones. They have some lines in 
common and some lines are unique to one of the files. The lines that do 
exist in both files are not necessarily in the same location. Now I need 
to compare the files and output a list of lines that exist in both 
files. Is there a simple way to do this? diff? awk? sed? cmp? Or a 
combination of two or more of them?

TIA,

Rolf