Today I needed to find matching lines in a number of text files. My first thought was, what is the opposite of diff? the answer is comm. To compare two text files and output lines that appear in both use

comm -1 -2 <file 1> <file 2>

To get matching lines between 4 files I redirected the output to tempory files and then comm’d them.

comm -1 -2 <file 1> <file 2> > tmp1
comm -1 -2 <file 3> <file 4> > tmp2
comm -1 -2 tmp1 tmp2

You can pipe into comm by using ‘-’ instead of a filename so you could also compare 4 files with

comm -1 -2 <file 1> <file 2> | comm -1 -2 - <file 3> | comm -1 -2 - <file 4>