COMM(1) General Commands Manual COMM(1)

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 XBD V1_chap07(7)) and any lines from the input files collate equally but are not identical, comm shall treat them as different lines and shall expect them to be ordered according to a further byte-by-byte comparison using the collating sequence for the POSIX locale; 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 XBD V1_chap12(7).

The following options shall be supported:

Suppress the output column of lines unique to file1 .
Suppress the output column of lines unique to file2 .
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:

Provide a default value for the internationalization variables that are unset or null. (See XBD for the precedence of internationalization variables used to determine the values of locale categories.)
If set to a non-empty string value, override the values of all the other internationalization variables.

Determine the locale for the collating sequence comm expects to have been used when the input files were sorted.
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.
[ XSI] Determine the location of messages objects and message catalogs.

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:

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 format:

"%s%s\n", <lead>, <line in both>

where the string < lead > is as follows:

Neither the -1 nor the -2 option is selected.
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 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, then lines written that collate equally but are not identical shall 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:

All input files were successfully output as specified.
An error occurred.

Default.

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, since 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 lines that collate equally but are not identical are not ordered in the way that comm expects. If the input does not come from utilities (such as ls(1) and sort(1)) which provide this ordering, the problem can be avoided by pre-sorting the input files using sort(1).

If a file named contains a sorted list of the utilities in this volume of POSIX.1-2024, a file named contains a sorted list of the utilities specified in the X/Open Portability Guide, Issue 3, and a file named 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.1-2024 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.1-2024.

None.

None.

cmp(1), diff(1), sort(1), uniq(1)

XBD V1_chap07(7), V1_chap08(7), V1_chap12(7)

First released in Issue 2.

The normative text is reworded to avoid use of the term "must" for application requirements.

POSIX.1-2008, Technical Corrigendum 2, XCU/TC2-2008/0076 [963], XCU/TC2-2008/0077 [663], and XCU/TC2-2008/0078 [963] are applied.

Austin Group Defect 1070 is applied, changing the requirements when any lines from the input files collate equally but are not identical.

Austin Group Defect 1122 is applied, changing the description of NLSPATH .

June 14, 2024 posix.fail