After a while, the legal restrictions posed on AT&T subsided, and it started to “smell money” and believe it can do better selling UNIX commercially. It created the AT&T System V system, touted it was better than AT&T UNIX and the BSDs, and sold it to vendors. System V was sold under a very restrictive licence, that forced them to hold the source code for themselves. Even cooperation between two different vendors was not allowed.
Gradually, vendors licensed the System V source code and ported it to their own architectures. This caused an explosion of proprietary UNIX systems. Sun Microsystems and other vendors took the BSD source code, diverged from it and distributed it without full access to the code to all customers. A similar thing happened with other software distributed under similar licences.
To answer this threat, a new phenomenon sprang into existence: the “free software” movement, the GNU project and the copyleft licences, all led by one dynamic personality: Richard M. Stallman.
Richard Stallman (aka RMS) published the GNU Manifesto in 1984, which coined the term “free software”, and explained the rationale behind it. The Manifesto was also a creed for the the GNU project which aimed to be a complete UNIX-compatible replacement for UNIX systems, while being completely original work. The software of the GNU project was released as free software, under the terms of the GNU General Public License (or GPL for short).
Gradually, the GNU project created more and more C code to replace the UNIX and BSD utilities. It was already installable and usable on various flavours of UNIX, and became a fully independent system once the Linux kernel was written.
The GPL licence is a free software licence that has many fine points. The most important concepts in it are:
Copyleft - making sure that derived work that are distributed to the outside includes the source and is distributed under the same licence. Note that this does not apply to modifications done for internal or private use.
The incentive to restrict a software this way rather than following the more traditional public domain or public-domain-like licences (as used by such software as the TeX typesetting system), was to make sure that the core GNU system would always remain free as well.
Encouraged by Stallman’s growing momentum behind the Free Software Foundation and the GNU project, Berkeley University changed the licence of the parts that they have originated, to a a free software licence which is now called “The Original BSD License”, which qualified as free software, but as opposed to the GPL was public-domain-like. [3] To add to this effort, some UCB students decided to rewrite the remaining parts that were licensed to AT&T under the BSD licence. This task was eventually completed that resulted in a BSD system that was entirely under the BSD licence.
However, AT&T did not stand by, and pressed charges against UCB and some other organisations, for claiming they actually own parts of the BSD operating system. This brought uncertainty into the BSD world, which would not be resolved until the 1990s, when the law-suit was decided mostly in favour of UCB. As a result of this uncertainty, the status of some spin-offs of BSD (such as 386BSD, and its derived operating systems such as FreeBSD or NetBSD) was in a legal limbo.
[3] The original BSD licence also has an advertising clause, that makes it incompatible with the GPL, and a problem in general. Later versions of the license removed this clause, and use of the original BSD licence is no longer recommended by the FSF, although some FOSS packages are still distributed under it.