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File: coreutils.info,  Node: comm invocation,  Next: ptx invocation,  Prev: uniq invocation,  Up: Operating on sorted files

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 *note 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 not be empty.