1973–1979

This period became the most significant in the development of UNIX. Ritchie and Thompson had developed the C programming language between 1969 and 1973 and now rewrote the UNIX kernel in the high-level C language. The operating system could be compiled to run on different computers. Within months, UNIX could be ported to new hardware. Modifications to the operating system were easy. Again, Thompson resonated with members of the academic community who were already using UNIX in many of their system-design courses. UNIX, written in a general-purpose language featuring modern commands, began to take off in the areas of word processing and programming.

By this point, UNIX was at version 6. This was the first release of UNIX to be picked up by a commercial firm, Whitesmiths, Inc., which created a commercial copy of version 6 called Idris.

In 1975, Thompson visited Berkeley while on sabbatical and installed version 6 on a PDP-11/70. It was at this time that two graduate students, Bill Joy and Chuck Haley, got involved with version 6 and later played an important role in the development of the UNIX system at Berkeley. The first project they worked on was the development of the UNIX ex editor.

Joy and Haley began to take interest in the internal operations of UNIX— specifically, the kernel. Joy put together a distribution of UNIX called the Berkeley Software Distribution (BSD). He included enhancements such as the C shell (a C-like interface to UNIX) and the vi editor. 1BSD was released in 1975. By the second release of BSD in 1978, Joy had added virtual memory support, which allowed programs to run even if they required more physical memory than was available at the time. This second edition of BSD had a strong influence on the release of Bell Labs’ version 7 of UNIX, which was released in 1979 and was the last of the “clean” versions of UNIX (produced solely by Bell Labs).Version 7 gave rise to a number of UNIX ports to other platforms, and for the first time, both industry and academia supplied enhancements, which were incorporated into future releases.

In the late 1970s, the United States Department of Defense’s Advanced Research Projects Agency (DARPA) decided to base its universal computing environment on Berkeley’s version of UNIX. In the 4.1 release of BSD, DARPA provided some important performance tune-ups. The fast file system, which provided a way to improve the file system’s performance and prevent file fragmentation, was added in release 4.2.