History and Evolution of Unix and the XNU Kernel

Apple has long contributed to open‑source projects, yet its UNIX‑based system remained closed until 2017 when the company released the source code of its flagship XNU kernel on GitHub; XNU (standing for "XNU is Not Unix") powers macOS, iOS and other Darwin‑based operating systems, helping developers understand kernel interactions.

The XNU source is distributed under the Apple Public Source License 2.0, a strict license that permits developers to incorporate the code into their own projects, with the full repository and details available via the original link.

UNIX originated from the collaborative MULTICS project at Bell Labs, GE, and MIT, and was created in 1969 by Ken Thompson, Dennis Ritchie, and Douglas McIlroy at AT&T's Bell Labs. Initially called "UNICS" (Uniplexed Information and Computing Service), it later became UNIX.

The UNIX trademark is owned by the Open Group, and only systems that conform to the Single UNIX Specification may use the UNIX name; otherwise they are referred to as "Unix‑like" systems, with Apple’s XNU being one such example.

UNIX development has historically been driven by research interest rather than commercial incentives, resulting in a rich family tree of variants, as illustrated by the accompanying diagram.

In the 1970s AT&T recognized UNIX's commercial potential and began licensing its source for educational use. By the 1980s, two major branches emerged: Berkeley's BSD UNIX and AT&T's System V, leading to a proliferation of Unix‑like derivatives.

Sun Microsystems built SunOS (later known as Solaris) on top of BSD UNIX, while AT&T's System V gave rise to IBM AIX and HP‑UX.

macOS, originally Mac OS X (with "X" representing the Roman numeral ten), is a graphical operating system built on a Unix kernel and later extended to run on PCs as "Mac PC".

IBM's AIX conforms to the Open Group's UNIX 98 standard, supporting both 32‑bit and 64‑bit applications on IBM Power and RS/6000 platforms, and incorporates logical volume management.

Solaris, developed by Sun, evolved from BSD UNIX and remained proprietary until Sun open‑sourced the later version (Solaris 11) under the CDDL license, creating the OpenSolaris project.

SunOS 5.0 marked the transition to System V 4, eventually being renamed Solaris 2.0; later versions dropped the "2" prefix, so SunOS 5.10 became Solaris 10.

Solaris runs on both Intel x86 and SPARC/UltraSPARC hardware, abstracting platform differences to provide a consistent user experience.

HP‑UX, derived from System V, runs on HP PA‑RISC, Intel Itanium, and historically on Apollo/Domain systems, with early versions also supporting Motorola 68000‑based HP 9000 machines.

IRIX, created by Silicon Graphics (later acquired by HPE), is a System V/BSD‑based UNIX that operates on SGI's RISC workstations and servers using 32‑ or 64‑bit MIPS processors.

Xenix, a UNIX variant licensed to Microsoft in 1979 for Intel processors, was later acquired by SCO and marketed as SCO UNIX (or SCO OpenServer), adhering to the UNIX System V SVID specifications.

A/UX, standing for Apple UNIX, was Apple’s UNIX operating system for Macintosh computers, first released in 1988; it is based on System V 2.2 and incorporates features from System V 3, System V 4, BSD 4.2, and BSD 4.3, complying with POSIX and SVID standards.

Unix history can be divided into classical, modern, and contemporary eras; Linux, which originated from Minix, is technically not a UNIX system, while mobile platforms such as iOS, Android, MeeGo, and WebOS are Linux‑based.

Linux offers many free distributions (e.g., Fedora, Ubuntu, Red Hat, Debian, SUSE) and is widely embedded in routers, switches, telephony systems, smartphones, GPS devices, and other hardware.

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