Unix command to find lines common in two files

Question

I\'m sure I once found a unix command which could print the common lines from two or more files, does anyone know its name? It was much simpler than diff.

Answer 1

To easily apply the comm command to unsorted files, use Bash's process substitution:

$ bash --version
GNU bash, version 3.2.51(1)-release
Copyright (C) 2007 Free Software Foundation, Inc.
$ cat > abc
123
567
132
$ cat > def
132
777
321

So the files abc and def have one line in common, the one with "132". Using comm on unsorted files:

$ comm abc def
123
    132
567
132
    777
    321
$ comm -12 abc def # No output! The common line is not found
$

The last line produced no output, the common line was not discovered.

Now use comm on sorted files, sorting the files with process substitution:

$ comm <( sort abc ) <( sort def )
123
            132
    321
567
    777
$ comm -12 <( sort abc ) <( sort def )
132

Now we got the 132 line!