Often people compare the differences between two files using diff. But if you needed to do a comparison of similarities between files, comm is the command to use.
Code Listing 1: comm usage syntax
comm [option] file1 file2
Note: both file1 and file2 need to be sorted before using comm as it does a line by line comparison.
comm outputs three columns: lines that are unique to file1, lines that are unique to file2, and lines that are common to both files. The options '-1', '-2', '-3' will suppress the corresponding columns.
Running comm -2 file1 file2 with the following files, would output five.
Code Listing 2: comm output
File1: one two three File2: one five three
From http://www.gentoo.org/news/en/gwn/20040823-newsletter.xml