Uniq

is a utility command on Unix, Plan 9, Inferno, and Unix-like operating systems which, when fed a text file or standard input, outputs the text with adjacent identical lines collapsed to one, unique line of text.

Overview
The command is a kind of filter program. Typically it is used after. It can also output only the duplicate lines (with the  option), or add the number of occurrences of each line (with the   option). For example, the following command lists the unique lines in a file, sorted by the number of times each occurs:

Using  like this is common when building pipelines in shell scripts.

History
First appearing in Version 3 Unix,  is now available for a number of different Unix and Unix-like operating systems. It is part of the X/Open Portability Guide since issue 2 of 1987. It was inherited into the first version of POSIX and the Single Unix Specification.

The version bundled in GNU coreutils was written by Richard Stallman and David MacKenzie.

A  command is also part of ASCII's MSX-DOS2 Tools for MSX-DOS version 2.

The command is available as a separate package for Microsoft Windows as part of the GnuWin32 project and the UnxUtils collection of native Win32 ports of common GNU Unix-like utilities.

The uniq command has also been ported to the IBM i operating system.