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. 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.