All operating systems (OSs) in this category support POSIX standards fully. The main standard defining what constitutes a Unix OS is POSIX, an acronym for: Portable Operating System Interface for UniX. Much like TRON, POSIX is not a body of computer code that is compiled and run on some processor. Rather, it is a set of standards (IEEE 1003.1): interfaces, design guidelines, software design specifications, defining (for creating) the computer code that will become language interfaces between an OS kernel and its programs, to give compatibility when moving programs between compatible systems. POSIX is made mostly of features from BSD Unix and Unix System V.

Much like Open Source software, all POSIX standards are copyrighted (by the Institute of Electrical and Electronics Engineers, Inc., IEEE; new versions have joint copyright by IEEE and Open Group), but available for use by software developers anywhere in the world for free. Thus the OS architecture based on POSIX is an open architecture that invites and welcomes cloning and interoperability.

Unix is a highly developed, mature, very stable, complex, and very powerful family of OSs for computers for running data processing and telephone systems. It provides multi-tasking, and multi-user abilities that let multiple programs run on one computer simultaneously, and let multiple users use one computer simultaneously; Unix systems can be servers, clients, and/or both, at once, all at the same time, as needed.

On this page, links are arranged in three groups and levels: 1) Top group: issues spanning multiple unrelated Unix OSs. 2) Middle group: types or classes of OS, or OSs for which there are more than one instance of an OS of this name/type, an OS family. 3) Bottom group: specific Unix OSs.