{{Short description |Shell command for comparing files}} {{Other uses}} {{for|the Portuguese Order of Merit|ComM}} {{more footnotes|date=January 2013}} {{lowercase title}} {{Infobox software | name = comm | logo = | screenshot = Comm-example.png | screenshot size = | caption = Example usage of <code>comm</code> command | author = Lee E. McMahon | developer = AT&T Bell Laboratories, Richard Stallman, David MacKenzie | released = {{Start date and age|1973|11}} | latest release version = | latest release date = | programming language = C | operating system = Unix, Unix-like, Plan 9, Inferno | platform = Cross-platform | genre = Command | license = coreutils: GPLv3+<br />Plan 9: MIT License | website = }} <code>'''comm'''</code> is a shell command for comparing two files for common and distinct lines. It reads the files as lines of text and outputs text as three columns. The first two columns contain lines unique to the first and second file, respectively. The last column contains lines common to both. Columns are typically separated with the tab character. If the input text contains lines beginning with the separator character, the output columns can become ambiguous.
For efficiency, standard implementations of {{code|comm}} expect both input files to be sequenced in the same line collation order, sorted lexically. The <code>sort</code> command can be used for this purpose. The {{code|comm}} algorithm makes use of the collating sequence of the current locale. If the lines in the files are not both collated in accordance with the current locale, the result is undefined.
The command is specified in the POSIX standard. It has been widely available on Unix-like operating systems since the mid to late 1980s. Originally implemented by Lee E. McMahon, the command first appeared in Version 4 Unix.<ref name="reader">{{cite tech report | first1 = M. D. | last1 = McIlroy | authorlink1 = Doug McIlroy | year = 1987 | url = https://www.cs.dartmouth.edu/~doug/reader.pdf | title = A Research Unix reader: annotated excerpts from the Programmer's Manual, 1971–1986 |series=CSTR | number = 139 | institution = Bell Labs}}</ref> The version in GNU coreutils was written by Richard Stallman and David MacKenzie.<ref>{{Cite web|url=https://linux.die.net/man/1/comm|title=Comm(1): Compare two sorted files line by line - Linux man page|access-date=2019-01-26|archive-date=2019-01-26|archive-url=https://web.archive.org/web/20190126065722/https://linux.die.net/man/1/comm|url-status=live}}</ref>
==Example== <syntaxhighlight lang="console"> $ cat foo apple banana eggplant $ cat bar apple banana banana zucchini $ comm foo bar apple banana banana eggplant zucchini </syntaxhighlight>
This shows that both files have one banana, but only '''bar''' has a second banana.
In more detail, the output file has the appearance that follows. Note that the column is interpreted by the number of leading tab characters. \t represents a tab character and \n represents a newline (Escape character#Programming and data formats).
{|class="wikitable" style="text-align:center" ! style="min-width:20px" | !! style="min-width:20px" | 0 !! style="min-width:20px" | 1 !! style="min-width:20px" | 2 !! style="min-width:20px" | 3 !! style="min-width:20px" | 4 !! style="min-width:20px" | 5 !! style="min-width:20px" | 6 !! style="min-width:20px" | 7 !! style="min-width:20px" | 8 !! style="min-width:20px" | 9 |- ! 0 | \t || \t || a || p || p || l || e || \n |- ! 1 | \t || \t || b || a || n || a || n || a || \n |- ! 2 | \t || b || a || n || a || n || a || \n |- ! 3 | e || g || g || p || l || a || n || t || \n |- ! 4 | \t || z || u || c || c || h || i || n || i || \n |}
==Limits== Up to a full line must be buffered from each input file during line comparison, before the next output line is written.
Some implementations read lines with the function {{code|readlinebuffer()}} which does not impose any line length limits if system memory suffices.
Other implementations read lines with the function <code>fgets()</code>. This function requires a fixed buffer. For these implementations, the buffer is often sized according to the POSIX macro {{code|LINE_MAX}}.
==Comparison to diff== Although also a file comparison command, <code>diff</code> reports significantly different information than {{code |comm}}. In general, {{code|diff}} is more powerful than {{code|comm}}. The simpler {{code|comm}} is best suited for use in scripts.
The primary distinction between {{code|comm}} and {{code|diff}} is that {{code|comm}} discards information about the order of the lines prior to sorting.
A minor difference between {{code|comm}} and {{code|diff}} is that {{code|comm}} will not try to indicate that a line has changed between the two files; lines are either shown in the "from file #1", "from file #2", or "in both" columns. This can be useful if one wishes two lines to be considered different even if they only have subtle differences.
Unlike for {{code|diff}}, the exit code of {{code|comm}} does not indicate whether the files match. As is typical, 0 indicates success, and other positive values indicate an error.
==See also== * {{Annotated link |Comparison of file comparison tools}} * {{Annotated link |List of POSIX commands}} * {{Annotated link |cmp (Unix)}} * {{Annotated link |cut (Unix)}}
==References== {{Reflist}}
==External links== {{Wikibooks|Guide to Unix|Commands}} * {{man|cu|comm|SUS|select or reject lines common to two files}} * {{man|1|comm|Plan 9}} * {{man|1|comm|Inferno}}
{{Unix commands}} {{Plan 9 commands}} {{Core Utilities commands}}
Category:Free file comparison tools Comm Category:Unix SUS2008 utilities Category:Plan 9 commands Category:Inferno (operating system) commands