The comm command in the Unix family of computer operating systems is a utility that is used to compare two files for common and distinct lines. comm is specified in the POSIX standard. It has been widely available on Unix-like operating systems since the mid to late 1980s.

comm reads two files as input, regarded as lines of text. comm 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 diff.

Columns are typically distinguished with the <tab> character. If the input files contain 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 (Unix) 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.