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COMM(1P) POSIX Programmer's Manual COMM(1P)

PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (con-
sult 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 IEEE Std 1003.1-2001, 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 IEEE Std 1003.1-2001, Section 8.2, Internationalization Variables for the precedence of international-
ization 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 stan-
dard 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 IEEE Std 1003.1-2001, 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 con-
tains 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 IEEE Std 1003.1-2001 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
IEEE Std 1003.1-2001.

RATIONALE
None.

FUTURE DIRECTIONS
None.