7.4 comm: Compare two sorted files line by line

comm writes to standard output lines that are common, and lines that are unique, to two input files; a file name of ‘-’ means standard input. Synopsis:

comm [option]… file1 file2

Before comm can be used, the input files must be sorted using the collating sequence specified by the LC_COLLATE locale. If an input file ends in a non-newline character, a newline is silently appended. The sort command with no options always outputs a file that is suitable input to comm.

With no options, comm produces three-column output. Column one contains lines unique to file1, column two contains lines unique to file2, and column three contains lines common to both files. Columns are separated by a single TAB character.

The options -1, -2, and -3 suppress printing of the corresponding columns (and separators). Also see Common options.

Unlike some other comparison utilities, comm has an exit status that does not depend on the result of the comparison. Upon normal completion comm produces an exit code of zero. If there is an error it exits with nonzero status.

If the --check-order option is given, unsorted inputs will cause a fatal error message. If the option --nocheck-order is given, unsorted inputs will never cause an error message. If neither of these options is given, wrongly sorted inputs are diagnosed only if an input file is found to contain unpairable lines. If an input file is diagnosed as being unsorted, the comm command will exit with a nonzero status (and the output should not be used).

Forcing comm to process wrongly sorted input files containing unpairable lines by specifying --nocheck-order is not guaranteed to produce any particular output. The output will probably not correspond with whatever you hoped it would be.

‘--check-order’

Fail with an error message if either input file is wrongly ordered.

‘--nocheck-order’

Do not check that both input files are in sorted order.

Other options are:

‘--output-delimiter=str’

Print str between adjacent output columns, rather than the default of a single TAB character.

The delimiter str may be empty, in which case the ASCII NUL character is used to delimit output columns.

‘--total’

Output a summary at the end.

Similar to the regular output, column one contains the total number of lines unique to file1, column two contains the total number of lines unique to file2, and column three contains the total number of lines common to both files, followed by the word ‘total’ in the additional column four.

In the following example, comm omits the regular output (-123), thus just printing the summary:

$ printf '%s\n' a b c d e     > file1
$ printf '%s\n'   b c d e f g > file2
$ comm --total -123 file1 file2
1       2       4       total

This option is a GNU extension. Portable scripts should use wc to get the totals, e.g. for the above example files:

$ comm -23 file1 file2 | wc -l    # number of lines only in file1
1
$ comm -13 file1 file2 | wc -l    # number of lines only in file2
2
$ comm -12 file1 file2 | wc -l    # number of lines common to both files
4

‘-z’ ¶

‘--zero-terminated’

Delimit items with a zero byte rather than a newline (ASCII LF). I.e., treat input as items separated by ASCII NUL and terminate output items with ASCII NUL. This option can be useful in conjunction with ‘perl -0’ or ‘find -print0’ and ‘xargs -0’ which do the same in order to reliably handle arbitrary file names (even those containing blanks or other special characters).