There are several versions of Unix. Until a few years ago, there were two main versions: the line of Unix releases that started at AT&T (the latest is System V Release 4), and another from the University of California at Berkeley (the last version was 4.4BSD). Some past and present commercial versions include SunOS, Solaris, SCO Unix, AIX, HP/UX, and ULTRIX. Freely available versions include Linux, NetBSD, and FreeBSD (FreeBSD is based on 4.4BSD-Lite).
Many Unix versions, including System V Release 4, merge earlier AT&T releases with BSD features. The POSIX standard for Unix-like operating systems defines a single interface to Unix. Although advanced features differ among systems, you should be able to use this introductory handbook on any system.
When we write "Unix" in this book, we mean "Unix and its versions" unless we specifically mention a particular version.
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