One beginning.
Several different paths.

Unix begins

After Bell Labs withdrew from Multics, Ken Thompson, Dennis Ritchie, and colleagues wanted a usable, interactive computing environment. Thompson began building what became Unix on a little-used PDP-7. The early system brought together files, processes, a shell, and small utilities.

This was collaborative work: Ritchie’s account also names Doug McIlroy and Joe Ossanna among the central group. The familiar Unix design emerged through experimentation, rather than arriving fully formed in 1969.

C makes the system easier to move

Unix’s kernel was rewritten largely in C, the language developed by Ritchie. Moving beyond an assembly-language implementation made it easier to adapt the system to other computers. Hardware-specific work remained, but much more of the software could travel.

Source licensing helped Unix spread through universities and research institutions. Students could study and extend a working system, then carry its ideas into their own projects.

Reading: Ritchie’s first-person history · The Open Group timeline

Berkeley extends Unix

UC Berkeley acquired a Unix source license in 1974. The first Berkeley Software Distribution, or BSD, was a late-1970s collection of additions to Version 6 Unix—not an independently written replacement. Berkeley’s work grew into substantial operating-system releases.

Tools such as vi and the C shell became associated with Berkeley. The 1983 release of 4.2BSD helped spread TCP/IP networking, sockets, and a new filesystem. BSD’s influence reached well beyond systems bearing its name.

AT&T’s System V branch

AT&T released System V, another major line of Unix development. Vendor implementations varied, and the Unix world did not divide into perfectly isolated branches: later systems mixed technologies from different sources.

“Unix” increasingly described a family of systems as well as a particular historical codebase. That diversity made agreement on interfaces more important.

Reading: FreeBSD Foundation’s timeline · The Open Group history

GNU is a system project, not a kernel name

Richard Stallman announced GNU in September 1983; work began in January 1984. The aim was a free, Unix-compatible operating system, built without requiring proprietary Unix source.

GNU produced compilers, libraries, shells, and utilities. Those components later became essential to many Linux distributions, but GNU and Linux were separate projects with different starting points.

Mach investigates the kernel

Carnegie Mellon began the Mach project, studying memory management, interprocess communication, threads, and kernel structure. Unix compatibility was one of its goals. Early Mach incorporated BSD; later work explored a microkernel with more services outside the kernel.

Mach is not short for Macintosh. It is a research technology whose descendants became part of NeXT’s—and later Apple’s—operating systems.

POSIX gives software a common contract

The first POSIX.1 standard defined common operating-system interfaces. POSIX later also standardized shell and utility behavior. Instead of requiring the same internal code, standards describe what programs can expect a system to do.

That helps explain why paths, processes, permissions, and shell operations feel familiar across implementations. It does not promise identical extensions, every command option, or automatic binary compatibility.

NeXT combines operating-system and application technology

Steve Jobs founded NeXT in 1985 after leaving Apple. The NeXT computer was introduced in 1988. NeXTSTEP, released in 1989, brought together Mach, BSD facilities, a graphical interface, and an object-oriented application environment.

Later, the OpenStep API specification and NeXT’s OPENSTEP software continued this application-framework direction. OpenStep interfaces and the OPENSTEP operating-system product are related names, not interchangeable definitions.

Linux supplies an independent kernel

In 1991, Linus Torvalds began Linux while a student in Helsinki. Linux was independently developed as a Unix-like kernel, not forked from the original Unix source tree. Releasing it under the GNU GPL in 1992 helped it combine with the wider free-software ecosystem.

A kernel manages resources; it is not a complete desktop or server system. Distributions assemble Linux with libraries, command-line tools, installers, package managers, and applications. GNU contributed many central pieces, hence the name GNU/Linux for those combinations.

Debian, Fedora, Ubuntu, and Arch are examples of distributions, not separate kernel ancestries. Linux-based systems can also use alternatives to GNU components; “Linux” alone does not tell you the whole software stack.

BSD continues outside Berkeley

FreeBSD and NetBSD began in 1993, drawing on the Net/2 and 386BSD line. Following the Unix source dispute and settlement, 4.4BSD-Lite provided a further redistributable source base; FreeBSD rebuilt on it for FreeBSD 2.0. OpenBSD later branched from NetBSD.

These are ongoing operating-system projects with BSD lineage, rather than Linux distributions. Shared ancestry does not make their kernels or development policies identical.

Reading: GNU’s own account · CMU’s Mach overview · NeXT at the Computer History Museum · Linux’s original release notes · FreeBSD’s history

Apple acquires NeXT

Apple announced its plan to acquire NeXT in late 1996; the acquisition was completed in 1997. Jobs returned to Apple, and NeXT technology supplied a foundation for Apple’s next-generation operating system.

The path ran through Apple’s Rhapsody development and Mac OS X Server before the consumer Mac OS X release. This was more than putting a new interface on classic Mac OS: the underlying system architecture changed.

Darwin 1.0 exposes the core

Apple released Darwin 1.0 as open source in April 2000. Darwin is the operating-system foundation shared by macOS and other Apple operating systems, including the kernel and Unix-oriented system components. It is not the whole Mac desktop, nor simply another name for BSD.

Its kernel, XNU, combines Mach-derived technology, BSD components, and Apple’s I/O Kit driver framework. Apple describes XNU as a hybrid kernel. Darwin is broader than XNU; macOS adds graphical frameworks, applications, and other Apple software beyond Darwin.

Mac OS X arrives on the desktop

Mac OS X 10.0 arrived on March 24, 2001. Aqua was the visible interface; underneath were Unix-style processes, permissions, a hierarchical filesystem, shells, utilities, and networking facilities.

The NeXT inheritance and continued BSD contributions explain much of the familiar terminal environment. Later versions were called OS X and then macOS. The product includes substantially more than its open-source foundation.

Reading: Apple’s OS X lineage overview · Darwin 1.0 announcement · Apple’s XNU source · Mac OS X launch announcement

Mac OS X receives UNIX certification

The Open Group registered Mac OS X 10.5 Leopard under UNIX 03 in 2007 for its specified configuration. Certification concerns conformance to a specification, rather than proof of a particular source-code ancestry. Later macOS registrations also apply to named versions and hardware configurations.

Unix ancestry

A historical relationship through code and technical development.

Unix-like design

Unix-style behavior and interfaces, including independently implemented systems such as Linux.

UNIX certification

A registered product’s verified conformance to The Open Group’s UNIX requirements.

POSIX compatibility and UNIX certification are related, but not synonymous. Neither BSD ancestry nor a familiar shell automatically confers the UNIX brand.

Different systems, transferable knowledge

macOS uses Darwin and XNU; Linux distributions use the Linux kernel; BSD projects maintain their own systems. Their packaging, services, graphical environments, and development histories differ.

What travels is knowledge of interfaces and conventions: paths, permissions, processes, shells, pipes, and remote access. That common ground is useful even when command options differ.

Reading: The Open Group’s Leopard registration · UNIX and POSIX standards