comm 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 comm expect both input files to be sequenced in the same line collation order, sorted lexically. The sort command can be used for this purpose. The 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. The version in GNU coreutils was written by Richard Stallman and David MacKenzie.

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).

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 readlinebuffer() which does not impose any line length limits if system memory suffices.

Other implementations read lines with the function fgets(). This function requires a fixed buffer. For these implementations, the buffer is often sized according to the POSIX macro LINE_MAX.