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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.
If the collating sequence of the current locale does not have a total ordering of all
characters (see the Base Definitions volume of POSIX.1‐2017, Section 7.3.2, LC_COLLATE)
and any lines from the input files collate equally but are not identical, comm should
treat them as different lines but may treat them as being the same. If it treats them as
different, comm should expect them to be ordered according to a further byte-by-byte
comparison using the collating sequence for the POSIX locale and if they are not ordered
in this way, the output of comm can identify such lines as being both unique to file1 and
unique to file2 instead of being in both files.
OPTIONS
The comm utility shall conform to the Base Definitions volume of POSIX.1‐2017, 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.1‐2017, 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 current locale. If the input
files contained any lines that collated equally but were not identical and within each
file those lines were ordered according to a further byte-by-byte comparison using the
collating sequence for the POSIX locale, and comm treated them as different lines, then
lines written that collate equally but are not identical should be ordered according to a
further byte-by-byte comparison using the collating sequence for the POSIX locale.
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.
When using comm to process pathnames, it is recommended that LC_ALL, or at least LC_CTYPE
and LC_COLLATE, are set to POSIX or C in the environment, since pathnames can contain byte
sequences that do not form valid characters in some locales, in which case the utility's
behavior would be undefined. In the POSIX locale each byte is a valid single-byte
character, and therefore this problem is avoided.
If the collating sequence of the current locale does not have a total ordering of all
characters, this can affect the behavior of comm in the following ways:
* If comm treats lines as being the same only if they are identical, some lines can be
misleadingly identified as being both unique to file1 and unique to file2.
* If comm treats lines as being the same if they collate equally and a line from file1
collates equally with a line from file2 but is not identical to it, one of the lines
is misleadingly identified as being in both files and the other is not written to the
output at all.
Such problems can be avoided by forcing the use of the POSIX locale; for example, the
following identifies lines in both file1 and file2:
LC_ALL=POSIX sort file1 > file1.posix
LC_ALL=POSIX sort file2 > file2.posix
LC_ALL=POSIX comm -12 file1.posix file2.posix | sort
The final sort re-sorts the output of comm according to the collating sequence of the
original locale. Doing this might be difficult if more than one column is output and
leading <blank>s cannot be ignored.
EXAMPLES
If a file named xcu contains a sorted list of the utilities in this volume of
POSIX.1‐2017, 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.1‐2017 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.1‐2017.
RATIONALE
None.
FUTURE DIRECTIONS
A future version of this standard may require that if any lines from the input files
collate equally but are not identical, then comm treats them as different lines and
expects them to be ordered according to a further byte-by-byte comparison using the
collating sequence for the POSIX locale.
A future version of this standard may require that if the input files contained any lines
that collated equally but were not identical and within each file those lines were ordered
according to a further byte-by-byte comparison using the collating sequence for the POSIX
locale, then lines written that collate equally but are not identical are ordered
according to a further byte-by-byte comparison using the collating sequence for the POSIX
locale.
SEE ALSO
cmp, diff, sort, uniq
The Base Definitions volume of POSIX.1‐2017, Section 7.3.2, LC_COLLATE, 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
1003.1-2017, Standard for Information Technology -- Portable Operating System Interface
(POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by
the Institute of Electrical and Electronics Engineers, Inc and The Open Group. 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.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page are most likely to have
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