UNIX was developed by researchers who needed a set of modern computing tools to help them with their projects. The system allowed a group of people working together on a project to share selected data and programs while keeping other information private.
Universities and colleges played a major role in furthering the popularity of UNIX through the "four-year effect." When the UNIX operating system became widely available in 1975, Bell Labs offered it to educational institutions at nominal cost. The schools, in turn, used it in their computer science programs, ensuring that computer science students became familiar with it. Because UNIX was such an advanced development system, the students became acclimated to a sophisticated programming environment. As these students graduated and went into industry, they expected to work in a similarly advanced environment. As more of these students worked their way up in the commercial world, the UNIX operating system found its way into industry.
BSD (Berkeley) UNIX
In addition to introducing students to UNIX, the Computer Systems Research Group (CSRG) at the University of California at Berkeley made significant additions and changes to the operating system. They made so many popular changes that one of the versions of the system is called the Berkeley Software Distribution (BSD) of the UNIX system (or just Berkeley UNIX). The other major version is UNIX System V, which descended from versions developed and maintained by AT&T and UNIX System Laboratories. Mac OS X inherits much more strongly from the BSD branch of the tree.