On September 27, 1983, Richard Stallman announced the GNU Project – his plan to create a complete, free Unix-compatible operating system.
GNU provided the essential tools, libraries, and the legal/ethical framework (via the Free Software Foundation and copyleft) that, paired with the Linux kernel, became the GNU/Linux systems used around the world.
Its insistence on the four software freedoms (run, study, modify, share) helped create and define today’s FOSS movement.
"I consider that the golden rule requires that if I like a program I must share it with other people who like it. I cannot in good conscience sign a nondisclosure agreement or a software license agreement."