Command:   comm - print lines common to two sorted files
Syntax:    comm [-123] file1 file2
Flags:     -1  Suppress column 1 (lines present only in file1)
           -2  Suppress column 2 (lines present only in file2)
           -3  Suppress column 3 (lines present in both files)
Examples:  comm file1 file2         # Print all three columns
           comm -12 file1 file2     # Print only lines  common  to  both
                                      files

     Two sorted files are read and compared.  A three column listing  is
produced.   Files only in file1 are in column 1; files only in file2 are
in column 2; files common to both files are in column 3.  The file  name
- means stdin.