--- name: "comm" section: "1p" platform: "linux" package: "man-pages" version: "6.19-1" release: "Arch Linux 2026-11-01" platforms: ["linux"] url: "https://new.carta.tech/comm/1p" --- # comm(1p) select or reject lines common to two files ## 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. ## 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. 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. ## OPTIONS 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*. ## 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 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*. ## 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", ``` If the **-2** option is not selected, lines contained only in *file2* are written using the format: ``` "%s%s\n", , ``` where the string \<*lead*\> is as follows: - \ 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", , ``` 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. - 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. ## 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. 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 \s cannot be ignored. ## EXAMPLES 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. ## RATIONALE None. ## FUTURE DIRECTIONS 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. ## SEE ALSO *cmp*, *diff*, *sort*, *uniq* The Base Definitions volume of POSIX.1‐2017, *Section 7.3.2*, *LC\_COLLATE*, *Chapter 8*, *Environment Variables*, *Section 12.2*, *Utility Syntax Guidelines* ## COPYRIGHT 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 .