I had two big directories and I wanted to know the common files in each directory, it was like a reverse diff.
On Linux you can use the command comm, the basic details of the command below.
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.
From looking at the command above you can easily see the differences in just one of the files/directories.
You can read the main man page for teh command but basically these are the main options to use. If you are new to Linux then I would recommend knowing this one in your toolkit along with diff.
I am always interested in your thoughts so if you have any comments or feedback then please feel free to add any comments, or you can mail me here.