this post was submitted on 17 Aug 2023
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Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. Linux is typically packaged in a Linux distribution (or distro for short).
Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the GNU Project. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses the name GNU/Linux to emphasize the importance of GNU software, causing some controversy.
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I'll give them a little credit: OS X is not quite built on a verbatim copy, it's cobbled from a few open source and licensed parts, and a not-insignificant amount of in-house development some of which is contributed back upstream.
NextStep started out as more or less the 4.3BSD userland hosted on the Mach 2.5 kernel instead of the monolithic traditional Unix style kernel the BSDs are built on, with a DisplayPostScript based UI (large parts licensed from Adobe) layered on top.
After Apple bought Next (or Next bought Apple with Apple's money, because Apple's management at the time was staggeringly dysfunctional and almost all the management after the dust settled ended up being Next people), they made major changes. NextStep/OpenStep tended to perform not-that-well because of additional overhead passing things in and out of the microkernel, a problem many microkernel based Unix-likes had, so they updated to the OSFMK 7.3 Mach variant, the BSD code to versions from FreeBSD, then hybridized it by pushing some pieces that traditional Microkernels ran in user space into kernel space for performance reasons, resulting in the XNU kernel that essentially every modern Apple product runs.
They also completely replaced the GUI layer with something custom and proprietary - the original plan for what became OS X was to use the Display Post Script system + a hosted classic environment, but 1. Many third party developers revolted against needing to make a ground-up new port of their software in a totally different environment and 2. the Adobe licensing costs were higher than the price of a normal PC, which was kind of OK for Next competing in the workstation market, but not OK for Apple selling consumer machines.
Apple publishes the open-source parts including most of the kernel (lately an increasing portion of drivers and platform support stuff are distributed as object files not under the open license) on a regular basis, formerly under the name "Darwin" which could be built as a pretty typical BSD-like OS, but in a way that's sufficiently community hostile to prevent anyone from really building successful derivative projects or contributing back to it. I think the most recent attempt was called "PureDarwin" and last I checked they've been stalled for about 2 years.
The engineer in charge of kernel stuff for the NeXTStep/OpenStep/Rhapsody/OS X family from inception in the late 80s to 2006 was Avie Tevanian, one of the original developers of Mach.
One who does use a lot of FreeBSD parts where it's not entirely clear how much they contribute back is Sony. The CellOS and OrbisOS that the PS3 and PS4 used are close relatives of FreeBSD, and it's possible they hid their contributions via contractors or consultants to not expose internal plans...or they just leeched, it's not really clear.