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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 been introduced
during the conversion of the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 COMM(1POSIX)