explainer Apple’s latest OS release is the newest member of the Open Group list of officially verified UNIX variants (by a wide margin).
Apple macOS 15 Sequoia arrived in mid-September and is the official UNIX™-compliant version, but that may not mean exactly what you think. For example, macOS doesn’t use any AT&T source code. As we discussed earlier last year when we announced the retirement of Unix, the meaning of the word “Unix” dates back to 1993, when Novell acquired UNIX from Bell Labs.
Shortly after Sequoia was released, news of some security software corruption surfaced, followed by the release of its first update, version 15.0.1, earlier this month. Several other events occurred following the arrival of 15.0.1. One is not important at all. After about a day, The Leg FOSS desk MacBook Air has started receiving updates.
The other is slightly more important to the wider world. Sequoia is the latest entry on the Open Group’s UNIX® Certified Products Register. In fact, the Apple Silicon and x86-64 versions have separate entries, so they occupy both first and second place. There is no special significance to this order, but if Apple continues to pay for certification, at some point the x86-64 version will fall off the list if Apple stops supporting Intel-powered kits.
Unix is just a new name for POSIX
It’s not about the code. It’s been more than 30 years since Novell acquired the original Unix from AT&T. In fact, what UNIX™ certification now means is what was once called “POSIX compatible.” This happens to be an abbreviation coined by Richard Stallman.
POSIX is essentially a set of compatibility specifications and tests that involves putting the right tools in the right places. As long as these are present, the OS can pass the test, which is why systems such as IBM’s z/OS mainframe operating system are on the list. z/OS is a distant descendant of IBM’s 24-bit MVS for the System/370 mainframe, released in 1974, and is essentially more Unix-like than the Apple II running ProDOS.
That’s why young Linus Torvalds asked: comp.os.minix Newsgroup July 3, 1991:
The POSIX standard has evolved over the years, but interestingly Apple only claims UNIX 03 in 2002. A single product, IBM AIX 7, boasts compatibility with version 4 of the UNIX® V7 branded standard (also known as POSIX.1-2008). .
Since then, this standard has continued to evolve. The version 4 specification was last revised in 2018 and there is also a 2024 version. No one seems to be paying attention anymore, which is fair enough. The world is moving away from proprietary Unix, and all important Unix-like OSes are FOSS or freeware, so you can add missing pieces without paying.
For example, POSIX resolved the differences between various archiving tools by adding a new command called pax that can handle all major formats. It is a hybrid of tar and cpiomost Linux distributions do not include this, as existing tools can process the file. is missing pax The command means the OS is not POSIX-1.2001 or later compliant, but no one cares anymore.
So what makes an OS similar to Unix?
If even the few companies that don’t need to use the original AT&T source code and continue to pay for official Unix certifications don’t bother trying to be compliant with the latest version of POSIX, then what does the OS consist of? That’s it. Like Unix?
At a higher level, Unix is about looking like Unix, acting like Unix, and allowing programs written for Unix to be ported without major changes.
The core of macOS is close enough to qualify. It uses a kernel called XNU (which, ironically, stands for “XNU is Not Unix”) and a userland derived primarily from BSD code. XNU is based on the Mach kernel [PDF]. Specifically, after Apple acquired NeXT, they updated the Mach component of the NeXTstep kernel with an enhanced version of DEC OSF/1 (later sold as Compaq Tru64). It also features a large in-kernel “Unix server” derived from BSD code. In other words, the most popular and successful microkernel OS in the industry is not actually a true microkernel.
In addition to that, the “userland” (the text mode stuff underneath the GUI, the various commands, shells, etc.) is mostly open source, and much of it comes from BSD. For example, the XNU kernel is on GitHub, as is most of macOS and iOS. It’s the GUI layer, the visible part that makes it look good, and it’s unique. These are primarily written in Objective-C and more recently in Swift.
Apple had made nearly standalone versions of the lower layers of these OSs available as a project called Darwin. There were also some distributions that tried to round it out using bits from other FOSS OSes, such as OpenDarwin and PureDarwin. One of the more interesting projects for us was NextBSD, which went in the opposite direction. The FreeBSD kernel remains intact, but has been modified to use some of Apple’s high-level code, including launchd, Apple’s next-generation init system. In other words, it’s equivalent to Cupertino’s systemd.
Apple announced its acquisition of NeXT Computer in late 1996 and released a preview of its next-generation operating system called Rhapsody in October 1997. Rhapsody was effectively NeXTstep 5. In 1999 it became Mac OS X Server 1.0, still distinctly similar to NeXTstep. It evolved into Mac OS X 1.0 in 2000.
NeXTstep (case has changed several times) became OPENSTEP, then Rhapsody, then Mac OS X Server, then Mac OS X, then just OS X with 10.8 Mountain Lion, then 10.12 Sierra Now it’s simply macOS. However, it is clear that this is the same OS as NeXTstep 0.8, as demonstrated by a young Steve Jobs in 1988.
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Boots Notes: So all C?
No, it’s not have Serenity OS is implemented in C++ and Redox OS is written in Rust, both of which are very Unix-like. At the extreme, C++ and Rust are both curly-braced languages within the broader C family, but TUNIS, the University of Toronto System, is a Unix Seventh Edition-compatible language implemented in Concurrent Euclid, a variant of Pascal. It was an OS.
