Variants of the Unix Operating System
Last revision September 1, 2011
Strictly speaking, the term Unix Operating System refers only to those operating systems that have passed the Unix standards test of the X-Open Foundation, which owns the Unix trademark. X-Open acquired the trademark and standards from AT&T, the original creator of Unix. In practice, Unix refers to any operating system that provides the programs, commands, and programming interfaces normally found on official Unix.
There are two main branches of Unix, which diverged from the same original AT&T source code from the 1970s and 1980s. System V gave rise to mostly commercial implementations such as AIX (IBM), HP/UX (Hewlett-Packard), and Solaris (Sun Microsystems - now Oracle). The University of California created the "Berkeley Unix" line that gave rise to open-source versions such as FreeBSD, NetBSD, and Darwin (the underpinnings of Mac OS X). Berkeley Unix also contributed greatly to Solaris.
There is also one important Unix work-alike - Linux - that looks like Unix to most users and programmers, but has a totally separate origin and shares no source code with official Unix.
To the normal user, the biggest difference among these variants of Unix is the type of shell, or command interpreter, you normally use when connected from a command line login, or the look and feel of the graphical desktop when connected via an X Window session. The general user programs and utilities are pretty much standardized among Unix versions. For the system administrator or programmer who needs to use system routines, there are some important differences.
The pages in this Using Unix section are geared towards the syntax and capabilities of Berkeley Unix derivatives, but are generally applicable to the commercial Unix variants and Linux. It is always a good idea to first check the documentation for your system to verify what I describe here.
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