Unix is the topic of this site. Do not use this tag except for questions about the historical UNIX product.
Unix in general is the topic of this site. Do not use this tag unless your question is about the historical UNIX product from AT&T. If your question is about a particular unix variant (e.g. linux, freebsd, solaris, …) or a particular application, use the corresponding tag. Otherwise stick to one or more tags reflecting what you are trying to do.
Unix is an operating system that was initially developed at AT&T Bell Labs as a simpler Multics. Since then, there have been many operating systems based on Unix, at least reproducing the interfaces if not the design and sometimes the code.
The original Unix code is proprietary software, originally owned by AT&T and licensed by several companies. This spurned several groups into developing independent code bases with similar design. The best known are bsd and linux.
UNIX® is a registered trade mark owned by The Open Group. Only certified products may use the brand.
In informal usage, “unix” or “Unix” or “Un*x” or “*nix” can mean any Unix-like system, whether it is derived from the original code or not, and whether it has the brand or not.
The POSIX and Single UNIX standards codify many interfaces for programmers and command-line users.
- The Single UNIX® Specification, Version 2 (a superset of the original POSIX, which is IEEE 1003 and ISO/IEC 9945)
- The Open Group Base Specifications Issue 6, which is also the Single UNIX Specification version 3 and POSIX:2001
- The Open Group Base Specifications Issue 7, which is also the Single UNIX Specification version 4 and POSIX:2008
System administration is not so standardized and varies greatly from one variant to the next. (See the Rosetta Stone for Unix for an overview.)
Derived from the original Unix code:
- aix AIX (IBM)
- solaris, opensolaris Solaris (historically Sun, now Oracle)
- sco SCO OpenServer
- hp-ux HP/UX (Hewlett-Packard)
Derived from BSD:
- linux Linux, with many distributions
- minix Minix
POSIX emulation layers on non-unix operating systems:
(And many more that have not (yet) been discussed on this site.)