<code>uniq</code> 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.
The command is a kind of filter program. Typically it is used after <code>sort</code>. It can also output only the duplicate lines (with the <code>-d</code> option), or add the number of occurrences of each line (with the <code>-c</code> option). For example, the following command lists the unique lines in a file, sorted by the number of times each occurs:
Using <code>uniq</code> like this is common when building pipelines in shell scripts.
First appearing in Version 3 Unix, <code>uniq</code> 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 <code>uniq</code> 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 command has also been ported to the IBM i operating system.