To counter the move to limit software use and redistribution, Richard Stallman in 1984 started developing a free, UNIX-compatible operating system called GNU(which is a recursive acronym for "GNU's Not Unix!"). To Stallman, "free" refers to freedom of use, not price. The free-software movement does not object to trading a copy for an amount of money but holds that users are entitled to four certain freedoms: (1) to freely run the program, (2) to study and change the source code, and to give or sell copies either (3) with or (4) without changes. In 1985, Stallman published the GNU Manifesto, which argues that all software should be free. He also formed the Free Software Foundation (FSF) with the goal of encouraging the use and developement of free software.
The FSF uses the copyrights on its programs to implement "copyleft," a form of licensing invented by Stallman. Copylefting a work gives anyone that prossesses a copy of the work the four essential freedoms that make the work free, with the condition that redistribution must preserve these freedoms. The GNU General Publish License (GPL) is a common license under which free software is released. Foudamentally, the GPL requires that the source code be distributed with any binaries and that all copies (including modified versions) be released under the same GPL license. The Creative Commons "Attribution Sharealike" license is also a copyleft license; "sharealike" is another way of stating the idea of copyleft.
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