Thangavel PrasanthTP

OS

4 min read

Operating System (OS)

Windows

Linux vs Unix

  • Unix is the original operating system from the 1970s (AT&T Bell Labs). It’s a proprietary, certified OS family (e.g. Solaris, HP-UX, AIX).
  • Linux is a free, open-source kernel created by Linus Torvalds in 1991 that is Unix-like — it was inspired by Unix and follows similar design principles (POSIX standards), but it does not share Unix source code. It’s a reimplementation from scratch.

Think of it like: Unix is the original blueprint, Linux is a free recreation built from the ground up following that same blueprint.

macOS

macOS is built on Unix, not Linux. Specifically:

  • macOS sits on top of Darwin, an open-source Unix-based kernel
  • Darwin is derived from BSD (Berkeley Software Distribution), which is a direct descendant of the original AT&T Unix
  • macOS is POSIX-certified — meaning it’s officially a “real” Unix, not just Unix-like
AT&T Unix (1970s)
├── BSD (direct descendant)
│   └── Darwin
│       └── macOS (certified Unix)
└── Inspired (but no shared code)
    └── Linux (Unix-like)
        ├── Ubuntu, Debian, Fedora...
        └── Alpine (what Docker uses)

That’s also why the macOS terminal feels so natural for Linux users — they share the same Unix philosophy and many of the same commands (ls, cd, grep, chmod, etc.).

Why Apple chose Unix (BSD) over Linux

Historical context: When Steve Jobs returned to Apple in 1997, he brought NeXTSTEP — the OS from his company NeXT, which was already built on top of BSD Unix (Mach kernel + BSD). macOS is literally the evolution of NeXTSTEP. Apple didn’t sit down and choose between Linux and Unix — they inherited BSD through the NeXT acquisition. At that point, Linux was only ~6 years old and far less mature.

But even if they had the choice, BSD had clear advantages:

  1. Licensing (the biggest reason)
    • BSD license: “Do whatever you want, just give credit” — allows proprietary/closed-source modifications
    • Linux (GPL): Any modifications to the kernel must be open-sourced and shared back
    • Apple wanted to build proprietary features on top of the kernel without being forced to release their source code. BSD’s permissive license made this possible
  2. Maturity and stability — BSD had ~20+ years of production use in academia and enterprise. Its networking stack was considered superior (the original TCP/IP implementation came from BSD)
  3. Code quality and coherence — BSD is a complete, cohesive OS (kernel + userland designed together). Linux is just a kernel — you need to assemble userland tools from various GNU projects
  4. POSIX certification — Building on BSD made it easier for Apple to get official Unix certification, which mattered for enterprise credibility

Sony also chose BSD for the PlayStation OS, and Netflix runs on FreeBSD — for the same licensing reasons.

But who maintains Darwin, and why not just use Linux?

Darwin is maintained primarily by Apple engineers. While it’s technically open source, in practice almost nobody outside Apple contributes to it — the community is tiny compared to Linux.

Apple’s competitive advantage isn’t in the kernel — it’s in everything on top of it:

  • The kernel is a commodity — it manages memory, processes, I/O, etc. Whether it’s Darwin or Linux, it does largely the same job
  • What makes macOS macOS is the proprietary stack above the kernel: Aqua UI, Metal graphics, Core Animation, Swift/Cocoa frameworks, Continuity with iPhone, etc.
  • With BSD, Apple can keep all of that closed. With Linux (GPL), there’s a risk that the copyleft requirement could “infect upward” — if proprietary code links too closely to the kernel, it may legally need to be open-sourced. This is a grey area that companies like Google (with Android) have to carefully navigate (e.g. Oracle v. Google)

The trade-off: Less community help maintaining the kernel, but total legal freedom above it. For a company with Apple’s resources, that trade-off is worth it — they don’t need the Linux community’s contributions to the kernel, but they do need the legal freedom to build proprietary software on top without licensing obligations bleeding into their closed-source products.

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