@xianc78 @PurpCat @beardalaxy @sun In 1983, rms announced the GNU OS and then did it - starting by improving the Church of Emacs, writing GCC and other free software, finding existing free software that would do, or could be cleaned up and finding programmers to assist him and write other GNU packages. In early 1989, it was evident that the last piece needed was a free kernel, so GNU developers looked around and found Mach and asked the University that developed it to free it. Finally in early 1991 the University freed mach and GNU developers started working on it (it used a experimental microkernel design, so it took until 1993 before it could be reliably booted and used).
In late 1991, Linus announced his intention to write an OS, except "not big an professional like GNU" and he started writing a kernel with the help of GNU software, using a quick and dirty monolithic design like Unix kernel's used. He only ever got around to writing a kernel - he and the other Linux developers didn't write the remaining needed parts of an OS, as everything else needed was already there - as GNU had already written it - "what good fortune" those developers thought and proceeded to think `I'll name the whole system "Linux" and I'll consider myself a "Linux user", even though I know that Linux is only a kernel` (the ultimate cognitive dissonance).
Development didn't go very well from 1991 to early 1992, as GNU developers has no interest in supporting a proprietary kernel - but Linus finally released Linux as free software under a free license under the GPLv2-ambigious mid 1992, earning the goodwill of GNU developers and so GNU developers assisted Linux developers, doing the difficult job of porting glibc and many other GNU libraries to work with Linux - forming the GNU/Linux system and accelerating development.
Unfortunately, many Linux developers proceeded to shit on that goodwill with malice - calling the whole system as "Linux", adding proprietary software into Linux in 1996 (making Linux proprietary software again) and as in yet another insult, Linus announced that the license was GPLv2-only and not GPLv2-or-later (as he didn't like how the GPLv3 effectively defended the users freedom) and that he wouldn't enforce his license for freedom (so far he has only ever enforced his license against freedom by denying compatibility with GPLv3+ software).
GNU/Hurd was "completed" years ago - you can boot right up into the Church of Emacs.
He didn't even have a chance of writing a functional kernel without GNU.
In 1992 he determined that if his project was going to get anywhere, he needed to integrate it with GNU and he deemed the only way he could get the GNU developers to trust him not to make things proprietary and so do most of the work for him, was to adopt a free GNU license and so he did so, deeming that he'll use it as "tit-for-tat license", rather than a license that defends the users freedom.
Of course he burnt that trust in 1996, by allowing the first of many proprietary software programs into his kernel, but by that stage the GNU/Linux system was pretty much ready for general usage, thus support for Linux was not dropped and when the proprietary finally compounded to a ridiculous level in 2008, Jeff released a free version of Linux; https://web.archive.org/web/20140203134408/http://lists.autistici.org/message/20080221.002845.467ba592.en.html resulting in GNU/Linux-libre (later maintained by lxo and jxself to make GNU/GNU Linux-libre.