Determine which Version of UNIX is installed on your Machine

Although it is common practice to talk about the "UNIX operating system" or a "UNIX machine", there is really no such thing as UNIX. Instead, there is a family of related operating systems, with many common features, all of which trace their ancestry back to a single source.

UNIX resulted from research at AT&T Bell Laboratories in the 1970s. The name is a pun: At the time, an operating system named MULTICS was in use at AT&T, and the inventors of UNIX made up their name to contrast their system with MULTICS. The early versions of the operating system were developed at Bell Labs through the early 1970s. Later, the UNIX code was released to various universities for further development. The most significant developments were made by the Computer Science laboratories at the University of California at Berkeley. At the same time, the commercial versions of UNIX continued to develop at AT&T. In the 1980s, the two strands of UNIX development

1. AT&T and
2. Berkeley

continued in parallel.

The Berkeley strand got a major boost from Sun Microsystems, which used the Berkeley code as the basis for its SunOS operating system. The AT&T strand was picked up by companies such as IBM and HP. The picture was further complicated by a series of "alliance wars" in the late 1980s. In these battles, various groups of companies (for example, AT&T and Sun versus IBM, HP, DEC, and others) formed associations with the goal of producing so-called UNIX standards.
The AT&T and Sun alliance produced UNIX System V, while the IBM/HP/DEC alliance (the Open Software Foundation) produced a system called OSF/1, which exists mostly as a set of standards without an implementation.
The Berkeley Software Distribution was an operating system based on Unix which was used for research, developed and distributed by the Computer Systems Research Group at the University of California, Berkeley.

From the system administration point of view, the difference between two different UNIX versions might mean something as trivial as a favorite set of options or that one command works on one machine but not another. On the other hand, the difference might be so significant that an entire technique for managing some resource (such as printing) must be done completely differently on two different platforms. Knowing something about how the chaotic state of UNIX development came to be and understanding a bit about the family tree of the different UNIX systems are necessary pieces of background knowledge for a UNIX administrator.

When trying to classify a particular version of UNIX, the most important issue is to decide if the system is primarily a Berkeley-derived operating system (generically called BSD) or a System V-based operating system. The most commonly encountered BSD system is SunOS version 4.3. Berkeley's BSD version 4.4 is also fairly common.
The other systems include:

1. Sun's Solaris system (also called SunOS version 5)
2. HP's HP-UX
3. IBM's AIX
4. Linux

Solaris, HP-UX, and AIX are System V UNIX with a range of BSD compatibility options.
Linux, although similar to System V, is in a class of its own. The differences among various UNIX versions can be irritating to a system administrator.
For a programmer, the problem is much more serious. Writing code that will run reliably on a wide range of UNIX platforms is tricky.