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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.
The comm utility shall conform to the Base Definitions volume of POSIX.1‐2008, 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.1‐2008, 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
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).
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.
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
"%s\n", <line in file1>
If the −2 option is not selected, lines contained only in file2 are written using the
"%s%s\n", <lead>, <line in file2>
where the string <lead> is as follows:
<tab> The −1 option is not selected.
The −1 option is selected.
If the −3 option is not selected, lines contained in both files shall be written using the
"%s%s\n", <lead>, <line in both>
where the string <lead> is as follows:
Neither the −1 nor the −2 option is selected.
<tab> Exactly one of the −1 and −2 options is selected.
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.
The standard error shall be used only for diagnostic messages.
The following exit values shall be returned:
0 All input files were successfully output as specified.
>0 An error occurred.
CONSEQUENCES OF ERRORS
The following sections are informative.
If the input files are not properly presorted, the output of comm might not be useful.
If a file named xcu contains a sorted list of the utilities in this volume of
POSIX.1‐2008, 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.1‐2008 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.1‐2008.
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