PROLOG

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of this interface
       may differ (consult the corresponding Linux manual page for details of Linux behavior), or the  interface
       may not be implemented on Linux.

NAME

       comm — select or reject lines common to two files

SYNOPSIS

       comm [−123] file1 file2

DESCRIPTION

       The  comm  utility shall read file1 and file2, which should be ordered in the current collating sequence,
       and produce three text columns as output: lines only in file1, lines only in file2,  and  lines  in  both
       files.

       If the lines in both files are not ordered according to the collating sequence of the current locale, the
       results are unspecified.

OPTIONS

       The comm utility shall conform to the Base Definitions volume  of  POSIX.1‐2008,  Section  12.2,  Utility
       Syntax Guidelines.

       The following options shall be supported:

       −1        Suppress the output column of lines unique to file1.

       −2        Suppress the output column of lines unique to file2.

       −3        Suppress the output column of lines duplicated in file1 and file2.

OPERANDS

       The following operands shall be supported:

       file1     A pathname of the first file to be compared. If file1 is '−', the standard input shall be used.

       file2     A  pathname  of  the  second  file to be compared. If file2 is '−', the standard input shall be
                 used.

       If both file1 and file2 refer to standard input or to the same FIFO special, block special, or  character
       special file, the results are undefined.

STDIN

       The standard input shall be used only if one of the file1 or file2 operands refers to standard input. See
       the INPUT FILES section.

INPUT FILES

       The input files shall be text files.

ENVIRONMENT VARIABLES

       The following environment variables shall affect the execution of comm:

       LANG      Provide a default value for the internationalization variables that are unset or null. (See the
                 Base  Definitions  volume  of POSIX.1‐2008, Section 8.2, Internationalization Variables for the
                 precedence  of  internationalization  variables  used  to  determine  the  values   of   locale
                 categories.)

       LC_ALL    If  set  to a non-empty string value, override the values of all the other internationalization
                 variables.

       LC_COLLATE
                 Determine the locale for the collating sequence comm expects to have been used when  the  input
                 files were sorted.

       LC_CTYPE  Determine  the  locale  for the interpretation of sequences of bytes of text data as characters
                 (for example, single-byte as opposed to multi-byte characters in arguments and input files).

       LC_MESSAGES
                 Determine the locale that should be used to  affect  the  format  and  contents  of  diagnostic
                 messages written to standard error.

       NLSPATH   Determine the location of message catalogs for the processing of LC_MESSAGES.

ASYNCHRONOUS EVENTS

       Default.

STDOUT

       The  comm  utility  shall produce output depending on the options selected. If the −1, −2, and −3 options
       are all selected, comm shall write nothing to standard output.

       If the −1 option is not selected, lines contained only in file1 shall be written using the format:

           "%s\n", <line in file1>

       If the −2 option is not selected, lines contained only in file2 are written using the format:

           "%s%s\n", <lead>, <line in file2>

       where the string <lead> is as follows:

       <tab>     The −1 option is not selected.

       null string
                 The −1 option is selected.

       If the −3 option is not selected, lines contained in both files shall be written using the format:

           "%s%s\n", <lead>, <line in both>

       where the string <lead> is as follows:

       <tab><tab>
                 Neither the −1 nor the −2 option is selected.

       <tab>     Exactly one of the −1 and −2 options is selected.

       null string
                 Both the −1 and −2 options are selected.

       If the input files were ordered according to the collating sequence of  the  current  locale,  the  lines
       written shall be in the collating sequence of the original lines.

STDERR

       The standard error shall be used only for diagnostic messages.

OUTPUT FILES

       None.

EXTENDED DESCRIPTION

       None.

EXIT STATUS

       The following exit values shall be returned:

        0    All input files were successfully output as specified.

       >0    An error occurred.

CONSEQUENCES OF ERRORS

       Default.

       The following sections are informative.

APPLICATION USAGE

       If the input files are not properly presorted, the output of comm might not be useful.

EXAMPLES

       If  a file named xcu contains a sorted list of the utilities in this volume of POSIX.1‐2008, a file named
       xpg3 contains a sorted list of the utilities specified in the X/Open Portability Guide, Issue  3,  and  a
       file  named  svid89  contains  a  sorted list of the utilities in the System V Interface Definition Third
       Edition:

           comm −23 xcu xpg3 | comm −23 − svid89

       would print a list of utilities in this volume of POSIX.1‐2008 not  specified  by  either  of  the  other
       documents:

           comm −12 xcu xpg3 | comm −12 − svid89

       would print a list of utilities specified by all three documents, and:

           comm −12 xpg3 svid89 | comm −23 − xcu

       would  print a list of utilities specified by both XPG3 and the SVID, but not specified in this volume of
       POSIX.1‐2008.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       cmp, diff, sort, uniq

       The Base Definitions volume of POSIX.1‐2008, Chapter 8,  Environment  Variables,  Section  12.2,  Utility
       Syntax Guidelines

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition,
       Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open  Group  Base
       Specifications  Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc
       and The Open Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In  the  event
       of  any  discrepancy between this version and the original IEEE and The Open Group Standard, the original
       IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at
       http://www.unix.org/online.html .

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