The command in the

Unix Unix (, ; trademarked as UNIX) is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, a ...

family of computer

operating system An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ...

s is a utility that is used to compare two files for common and distinct lines. is specified in the

POSIX The Portable Operating System Interface (POSIX; ) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines application programming interfaces (APIs), along with comm ...

standard. It has been widely available on

Unix-like A Unix-like (sometimes referred to as UN*X, *nix or *NIX) operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Uni ...

operating systems since the mid to late 1980s.

History

Written by Lee E. McMahon, first appeared in

Version 4 Unix Research Unix refers to the early versions of the Unix operating system for PDP-7, DEC PDP-7, PDP-11, VAX and Interdata 7/32 and 8/32 computers, developed in the Bell Labs Computing Sciences Research Center (CSRC). The term ''Research Unix'' first ...

. The version of bundled in

GNU GNU ( ) is an extensive collection of free software (394 packages ), which can be used as an operating system or can be used in parts with other operating systems. The use of the completed GNU tools led to the family of operating systems popu ...

coreutils The GNU Core Utilities or coreutils is a collection of GNU software that implements many standard, Unix-based shell commands. The utilities generally provide POSIX compliant interface when the environment variable is set, but otherwise offers ...

was written by

Richard Stallman Richard Matthew Stallman ( ; born March 16, 1953), also known by his initials, rms, is an American free software movement activist and programmer. He campaigns for software to be distributed in such a manner that its users have the freedom to ...

and David MacKenzie.

Usage

reads two files as input, regarded as lines of text. outputs one file, which contains three columns. The first two columns contain lines unique to the first and second file, respectively. The last column contains lines common to both. This functionally is similar to . Columns are typically distinguished with the character. If the input files contain lines beginning with the separator character, the output columns can become ambiguous. For efficiency, standard implementations of expect both input files to be sequenced in the same line

collation Collation is the assembly of written information into a standard order. Many systems of collation are based on numerical order or alphabetical order, or extensions and combinations thereof. Collation is a fundamental element of most office fi ...

order, sorted lexically. The

sort (Unix) In computing, sort is a standard command line program of Unix and Unix-like operating systems, that prints the lines of its input or concatenation of all files listed in its argument list in sorted order. Sorting is done based on one or more so ...

command can be used for this purpose. The 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.

Return code

Unlike , the return code from has no logical significance concerning the relationship of the two files. A return code of 0 indicates success, a return code >0 indicates an error occurred during processing.

Example

$ cat foo apple banana eggplant $ cat bar apple banana banana zucchini $ comm foo bar apple banana banana eggplant zucchini 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).

Comparison to diff

In general terms, is a more powerful utility than . The simpler is best suited for use in scripts. The primary distinction between and is that discards information about the order of the lines prior to sorting. A minor difference between and is that 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.

Other options

has

command-line option A command-line interface (CLI) is a means of interacting with software via command (computing), commands each formatted as a line of text. Command-line interfaces emerged in the mid-1960s, on computer terminals, as an interactive and more user ...

s to suppress any of the three columns. This is useful for scripting. There is also an option to read one file (but not both) from standard input.

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 which does not impose any line length limits if system memory suffices. Other implementations read lines with the function . This function requires a fixed buffer. For these implementations, the buffer is often sized according to the

macro .

References

External links

* * * {{Core Utilities commands Free file comparison tools Comm Unix SUS2008 utilities Plan 9 commands Inferno (operating system) commands