#xnu

Open Terminal on a Mac and the machine suddenly starts speaking a language familiar to almost anyone who has spent time with Linux.

There is ls.

There is grep.

There is ssh.

There are Unix-style permissions, processes, pipes, signals, sockets, shells, and paths beginning at /.

You can compile C code, start an SSH session, manipulate files with command-line utilities, run servers, and use many of the same development tools you would expect to find on a Linux machine.

Then close Terminal.

The resemblance seems to disappear.

You are back in macOS, with Finder, the Dock, Spotlight, AppKit, Swift, Metal, APFS, Apple Silicon, iCloud, and an operating system whose design and software ecosystem remain unmistakably Apple's.

This creates an interesting historical question.

If Apple transformed the Mac into a Unix system around the turn of the century, why did the Mac not gradually converge with Linux?

Why did two operating systems with so much low-level common ground remain so obviously separate?

The answer begins with an important correction to the question itself.

Apple did not originally set out to turn the Mac into Unix.

Apple set out to save the Mac.

The operating system underneath the original Macintosh was getting old

The original Macintosh appeared in 1984.

Its operating system was built for a very different kind of computer from the machines that would exist a decade later. The first Macintosh had 128 kilobytes of memory. It was designed as a personal computer for one person sitting in front of one machine, running a relatively small collection of programs.