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PROLOG | NAME | SYNOPSIS | DESCRIPTION | OPTIONS | OPERANDS | STDIN | INPUT FILES | ENVIRONMENT VARIABLES | ASYNCHRONOUS EVENTS | STDOUT | STDERR | OUTPUT FILES | EXTENDED DESCRIPTION | EXIT STATUS | CONSEQUENCES OF ERRORS | APPLICATION USAGE | EXAMPLES | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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COMM(1P) POSIX Programmer's Manual COMM(1P)
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.
comm — select or reject lines common to two files
comm [-123] file1 file2
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.
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.
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.
The standard input shall be used only if one of the file1 or file2
operands refers to standard input. See the INPUT FILES section.
The input files shall be text files.
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.
Default.
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.
The standard error shall be used only for diagnostic messages.
None.
None.
The following exit values shall be returned:
0 All input files were successfully output as specified.
>0 An error occurred.
Default.
The following sections are informative.
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.
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.
None.
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.
cmp(1p), diff(1p), sort(1p), uniq(1p)
The Base Definitions volume of POSIX.1‐2017, Section 7.3.2,
LC_COLLATE, Chapter 8, Environment Variables, Section 12.2,
Utility Syntax Guidelines
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(1P)
Pages that refer to this page: cmp(1p), diff(1p), join(1p), sort(1p), uniq(1p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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