macOS (formerly Mac OS X)

The modern Mac operating system is now termed macOS, after an entire generation of Mac OS X and its many variants (10.0, 10.1, 10.2, and the then-current 10.4) having initially been given feline code names — Cheetah, Puma, Jaguar, Panther, Tiger, Leopard — later replaced by grander place-based monikers like El Capitan, Sierra, etc.

It is essentially a current-day evolution of NeXTSTEP with the Aqua interface wrapped around it — that is to say, a BSD-like UNIX system running atop the Mach microkernel.

macOS (formerly Mac OS X) layers a number of foundational technologies:

• UI: Aqua, AppKit (Cocoa), SwiftUI (modern additions)
• Frameworks: Cocoa / Carbon (legacy), Core Foundation, Core Graphics, Core Audio, Core Animation
• Runtime: Objective-C runtime plus (later) Swift runtime
• Kernel: XNU hybrid (Mach + BSD + I/O Kit)
• Heritage: Direct lineage from NeXTSTEP (Objective-C, Display PostScript lineage, IPC model)

The kernel (XNU) blends the Mach microkernel (scheduling, VM, IPC primitives) with BSD services (POSIX processes, networking, VFS) and an object-oriented device driver framework (I/O Kit, C++ based). Many user‑space abstractions (launchd, Grand Central Dispatch, sandboxing) build atop these primitives.

The best (still) available overview of its internals is Amit Singh’s article What Is Mac OS X?, which I recommend heartily to anyone coming from mainstream UNIX systems like Linux and BSD — it explains much of the rationale and heritage of Mac OS X in a clear, well written fashion, and draws appropriate comparisons along the way.

Amit also published a must‑have book on Mac OS X and made available a vastly extended version of its initial chapter covering the system’s precursors and history.

O’Reilly also has a handy book for people coming to Mac OS X from other UNIXes: “Mac OS X for Unix Geeks”:

• MacJanitor, MacSniffer, Pseudo, xFiles
• Mac OS X Labs

• GSM and GPRS scripts for the Sony Ericsson T68i
• GSM/GPRS Dial-up

• Boot Keys
• My Hints For Newcomers To The Mac

• Fink
• sendmail HOW-TO (pre-Panther)
• Mac OS X Ports (not as interesting as Fink, but occasionally useful)
• DarwinPorts
• Marc Liyanage – Ports
• Common Problems and Troubleshooting

• OmniWeb
• Fugu (SSH/sftp)
• HenWen (snort front-end)
• NetNewsWire (RSS reader)

• How Mac OS X executes applications (on otool and .dylibs)

• Menu Items list (applications that use menu bar add-ons extensively)
• Tomcat and Cocoon (Tomcat 3.2.1 and Cocoon 1.8.2 as an installer package)
• Java and Tomcat on Mac OS X
• tun/tap driver (useful for running QEMU)
• Mac OS Through The Years — turns out I remember pre‑6.0 versions, even though I spent many years on 6.0.4 and 7.1…

• Classic Mac OS
• Big Sur
• Cheetah, Puma, Jaguar, Panther, Tiger, Leopard, Snow Leopard, Lion, Mountain Lion, Mavericks