Over at Abort Retry Fail there’s a very nice and comprehensive history and discussion of BSD Unix. I’m pretty familiar with most of this history but the article mentions some things that were new to me.
A good case can be made that it was BSD Unix that jump started the free software movement, although credit should be also given to AT&T or at least The Computing Science Research Center that embraced the idea as much as the AT&T lawyers would permit.
Because of their controlled monopoly status, AT&T couldn’t offer Unix commercially so they restricted distribution—at least initially—to academic institutions. Berkeley didn’t have those restrictions and were more interested in providing Unix to the world at large but couldn’t legally provide it to anyone who didn’t have an AT&T Unix source license.
This resulted in a mostly successful effort the remove all AT&T code from the Berkeley distribution. By the time of BSD 4.0 there were only a very few files that contained ATT&T IP. At this point the lawyers got involved and the famous UC Berkeley/ATT&T lawsuit was launched. In the end, BSD was essentially open sourced and we all benefited from distributions like NetBSD and FreeBSD. Eventually, of course, Linux became the standard “Unix” and, SCO notwithstanding, was completely free of prior IP.
If you’re at all interested in Unix history, this is a great article. It’s a bit long but there’s a lot of history to cover. It’s definitely worth your time.