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.TH COMM "1P" 2013 "IEEE/The Open Group" "POSIX Programmer's Manual"
.SH 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.

.SH NAME
comm
\(em select or reject lines common to two files
.SH SYNOPSIS
.LP
.nf
comm \fB[\fR\(mi123\fB] \fIfile1 file2\fR
.fi
.SH DESCRIPTION
The
.IR comm
utility shall read
.IR file1
and
.IR file2 ,
which should be ordered in the current collating sequence, and produce
three text columns as output: lines only in
.IR file1 ,
lines only in
.IR file2 ,
and lines in both files.
.P
If the lines in both files are not ordered according to the collating
sequence of the current locale, the results are unspecified.
.SH OPTIONS
The
.IR comm
utility shall conform to the Base Definitions volume of POSIX.1\(hy2008,
.IR "Section 12.2" ", " "Utility Syntax Guidelines".
.P
The following options shall be supported:
.IP "\fB\(mi1\fP" 10
Suppress the output column of lines unique to
.IR file1 .
.IP "\fB\(mi2\fP" 10
Suppress the output column of lines unique to
.IR file2 .
.IP "\fB\(mi3\fP" 10
Suppress the output column of lines duplicated in
.IR file1
and
.IR file2 .
.SH OPERANDS
The following operands shall be supported:
.IP "\fIfile1\fR" 10
A pathname of the first file to be compared. If
.IR file1
is
.BR '\(mi' ,
the standard input shall be used.
.IP "\fIfile2\fR" 10
A pathname of the second file to be compared. If
.IR file2
is
.BR '\(mi' ,
the standard input shall be used.
.P
If both
.IR file1
and
.IR file2
refer to standard input or to the same FIFO special, block special, or
character special file, the results are undefined.
.SH STDIN
The standard input shall be used only if one of the
.IR file1
or
.IR file2
operands refers to standard input. See the INPUT FILES section.
.SH "INPUT FILES"
The input files shall be text files.
.SH "ENVIRONMENT VARIABLES"
The following environment variables shall affect the execution of
.IR comm :
.IP "\fILANG\fP" 10
Provide a default value for the internationalization variables that are
unset or null. (See the Base Definitions volume of POSIX.1\(hy2008,
.IR "Section 8.2" ", " "Internationalization Variables"
for the precedence of internationalization variables used to determine
the values of locale categories.)
.IP "\fILC_ALL\fP" 10
If set to a non-empty string value, override the values of all the
other internationalization variables.
.IP "\fILC_COLLATE\fP" 10
.br
Determine the locale for the collating sequence
.IR comm
expects to have been used when the input files were sorted.
.IP "\fILC_CTYPE\fP" 10
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).
.IP "\fILC_MESSAGES\fP" 10
.br
Determine the locale that should be used to affect the format and
contents of diagnostic messages written to standard error.
.IP "\fINLSPATH\fP" 10
Determine the location of message catalogs for the processing of
.IR LC_MESSAGES .
.SH "ASYNCHRONOUS EVENTS"
Default.
.SH STDOUT
The
.IR comm
utility shall produce output depending on the options selected. If the
.BR \(mi1 ,
.BR \(mi2 ,
and
.BR \(mi3
options are all selected,
.IR comm
shall write nothing to standard output.
.P
If the
.BR \(mi1
option is not selected, lines contained only in
.IR file1
shall be written using the format:
.sp
.RS 4
.nf
\fB
"%s\en", <\fIline in file1\fR>
.fi \fR
.P
.RE
.P
If the
.BR \(mi2
option is not selected, lines contained only in
.IR file2
are written using the format:
.sp
.RS 4
.nf
\fB
"%s%s\en", <\fIlead\fR>, <\fIline in file2\fR>
.fi \fR
.P
.RE
.P
where the string <\fIlead\fP> is as follows:
.IP <tab> 10
The
.BR \(mi1
option is not selected.
.IP "null\ string" 10
The
.BR \(mi1
option is selected.
.P
If the
.BR \(mi3
option is not selected, lines contained in both files shall be written
using the format:
.sp
.RS 4
.nf
\fB
"%s%s\en", <\fIlead\fR>, <\fIline in both\fR>
.fi \fR
.P
.RE
.P
where the string <\fIlead\fP> is as follows:
.IP <tab><tab> 10
Neither the
.BR \(mi1
nor the
.BR \(mi2
option is selected.
.IP <tab> 10
Exactly one of the
.BR \(mi1
and
.BR \(mi2
options is selected.
.IP "null\ string" 10
Both the
.BR \(mi1
and
.BR \(mi2
options are selected.
.P
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.
.SH STDERR
The standard error shall be used only for diagnostic messages.
.SH "OUTPUT FILES"
None.
.SH "EXTENDED DESCRIPTION"
None.
.SH "EXIT STATUS"
The following exit values shall be returned:
.IP "\00" 6
All input files were successfully output as specified.
.IP >0 6
An error occurred.
.SH "CONSEQUENCES OF ERRORS"
Default.
.LP
.IR "The following sections are informative."
.SH "APPLICATION USAGE"
If the input files are not properly presorted, the output of
.IR comm
might not be useful.
.SH EXAMPLES
If a file named
.BR xcu
contains a sorted list of the utilities in this volume of POSIX.1\(hy2008, a file named
.BR xpg3
contains a sorted list of the utilities specified in the X/Open
Portability Guide, Issue 3, and a file named
.BR svid89
contains a sorted list of the utilities in the System V Interface
Definition Third Edition:
.sp
.RS 4
.nf
\fB
comm \(mi23 xcu xpg3 | comm \(mi23 \(mi svid89
.fi \fR
.P
.RE
.P
would print a list of utilities in this volume of POSIX.1\(hy2008 not specified by either of the
other documents:
.sp
.RS 4
.nf
\fB
comm \(mi12 xcu xpg3 | comm \(mi12 \(mi svid89
.fi \fR
.P
.RE
.P
would print a list of utilities specified by all three documents, and:
.sp
.RS 4
.nf
\fB
comm \(mi12 xpg3 svid89 | comm \(mi23 \(mi xcu
.fi \fR
.P
.RE
.P
would print a list of utilities specified by both XPG3 and the SVID,
but not specified in this volume of POSIX.1\(hy2008.
.SH RATIONALE
None.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIcmp\fR\^",
.IR "\fIdiff\fR\^",
.IR "\fIsort\fR\^",
.IR "\fIuniq\fR\^"
.P
The Base Definitions volume of POSIX.1\(hy2008,
.IR "Chapter 8" ", " "Environment Variables",
.IR "Section 12.2" ", " "Utility Syntax Guidelines"
.SH 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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