Date: Sun, 11 Oct 1992 15:29:00 +0000
Version: 2.0
The following is offered with no guarantee as to accuracy or completeness. I have done what I can in the time available and it still is very much work in progress. I hope to keep improving this summary. Comments welcome: lew@bnr.ca. Acknowledgements at the end.
First a short definition. By Unix we mean an operating system typically written in C, with a hierarchical file system, integration of file and device I/O, whose system call interface includes services such as fork(), pipe(), and whose user interface includes tools such as cc, troff, grep, awk, and a choice of shell. Note that UNIX is a registered trademark of USL (AT&T), but will be used here in its generic sense.
Most Unices (the more common plural form) are derived more or less directly from AT&T code (some code from the first C version is presumably still left in most), but there are also clones (i.e. Unix-compatible systems with no AT&T code).
In addition, there are also Unix-like environments (e.g. VOS) sitting on top of other OSs, and OSs inspired from Unix (yes, even DOS!). These are not covered here. Little on real-time Unices yet (although more is planned).
Unix comes in an incredible variety of flavors. This is to a large extent due to availability of sources and the ease of porting and modifying Unix. Typically, a vendor of Unix will start with one basic flavor (see below), take ideas/code from the other major flavor, add and change many things, etc. This results in yet another new Unix flavor. Today, there are literally hundreds of Unices available, the closest thing to standard Unix being (by definition) System V.
This answer was put together mostly from information on the net and email. Some specific sources are also mentioned in the appropriate sections.