Unix vs Linux: what is actually different, and what is not

Short answer: UNIX is a family of operating systems begun at Bell Labs in 1969; today the name is a trademark of The Open Group, used only by certified systems such as IBM AIX. Linux is a free kernel written from scratch in 1991. It behaves like Unix and follows the same standards, but contains no Unix code and is not certified.

So Linux is “Unix-like”, and for most practical purposes (the shell, the file layout, the commands, the way processes and permissions work) the two feel the same. The real differences are history, ownership, licensing, who builds the system and what it runs on. The sections below take them one at a time, then show where each runs today.

The short version

• UNIX (in capitals) is both a lineage of systems descended from AT&T’s original code and a certification mark. Commercial Unix systems are proprietary and usually tied to their vendor’s hardware.
• Linux is only a kernel. A Linux distribution (Debian, Ubuntu, Rocky Linux, Alpine) combines it with tools, mostly from the GNU project, into a complete operating system that anyone may use, change and redistribute.
• The BSDs (FreeBSD, OpenBSD, NetBSD) descend from Berkeley’s Unix, are free and open source, and are not certified either. macOS is built on a BSD-derived core and is the best-known certified Unix on a desktop.
• On servers, Linux is the default almost everywhere. Certified Unix systems survive mainly on IBM and HPE hardware in large enterprises.

A short history: from Bell Labs to Linux

Through the 1980s AT&T licensed its source code to universities and companies, and the result was many incompatible commercial Unixes: SunOS and later Solaris, HP-UX, IBM’s AIX and others. That fragmentation is why standards such as POSIX were written. It is also why GNU and Linux mattered: a legal dispute over BSD code in the early 1990s held the BSDs back for a while, and in that gap a free kernel plus free GNU tools gave everyone a Unix-like system with no licence fee and no lawyer involved.

What UNIX means today: the trademark and certification

“UNIX” is a registered trademark of The Open Group. A vendor may call a product UNIX only after it passes the Open Group’s test suites for the Single UNIX Specification and is listed on its register; the current marks are UNIX 03 and UNIX V7. Certification is about behaviour, not ancestry: a system written from scratch can pass, and one descended from AT&T code is not UNIX unless it is certified.

The Open Group’s register, read on 21 October 2026, lists IBM AIX 7.2 (UNIX V7, and UNIX 03 for AIX 7.1 and 7.2) and HPE HP-UX 11i v3 (UNIX 03), with IBM z/OS and older SCO products under earlier marks. Apple’s page on the register still names macOS 15 Sequoia and macOS 26 Tahoe as UNIX 03, although Apple no longer appears in the register’s main product list, so check the register itself before relying on that. Oracle Solaris 11.4 was certified UNIX V7 in 2018 and is no longer listed.

Kernels, distributions and vendor systems

A traditional Unix is one product from one company: kernel, libraries, shell, tools, documentation and support, often sold with the hardware it runs on. AIX runs on IBM’s Power servers; HP-UX runs on HPE’s Itanium-based Integrity servers, a platform in decline since Intel shipped its last Itanium chips in 2021. A BSD is one project that develops the kernel and the base system together.

Linux is organised differently. The kernel is developed by thousands of contributors under Linus Torvalds’ coordination, and distributions package it with a C library (usually GNU’s glibc, musl on Alpine), the GNU core utilities (BusyBox on Alpine), an init system (systemd on most), a package manager and their own defaults. That is why two Linux systems can differ more from each other than people expect; Debian vs Ubuntu shows how two closely related distributions still diverge.

Technically, the Linux kernel is monolithic with loadable modules: drivers and file systems run inside the kernel but can be loaded and unloaded while it runs. macOS uses XNU, a hybrid of the Mach microkernel and BSD code. The AIX and HP-UX kernels are closed source.

Licensing: who may use, change and share the code

• Linux kernel: GNU GPL version 2. Anyone may use, study, change and redistribute it, but a distributed modified version must ship its source under the same licence.
• BSDs: permissive BSD licences. The code may end up in closed products, which is how BSD code found its way into macOS and many network appliances.
• AIX, HP-UX: proprietary. You license the system, usually together with the vendor’s hardware and a support contract.
• macOS: proprietary, with its Darwin core published as open source by Apple.

POSIX: why the commands look alike

POSIX is the IEEE standard (1003.1) that defines what a Unix-like system must provide: the system calls programs use, the shell language, and utilities such as ls, cp, grep, sed, awk and find. The Single UNIX Specification builds on the same text. Linux with glibc and the GNU tools follows POSIX closely without being certified, so a POSIX shell script usually runs unchanged on Linux, the BSDs, macOS and AIX.

$ uname -s
Linux
$ getconf _POSIX_VERSION
200809

That output was checked on Debian 12 with glibc 2.36: 200809 means the POSIX.1-2008 interface. The same uname -s prints Darwin on macOS, FreeBSD on FreeBSD and AIX on AIX.

Where GNU and BSD tools differ

The commands share names, but options outside POSIX differ between the GNU tools on Linux and the BSD tools on macOS and FreeBSD. These are the differences that break scripts most often. The GNU column was checked on Debian 12 (GNU coreutils 9.1, GNU sed 4.9); the BSD column is written from the FreeBSD and macOS manual pages.

For portable scripts, stick to POSIX options, start them with #!/bin/sh only when they avoid bash features (on Debian /bin/sh is dash), and test on the system they will run on. The Linux commands cheat sheet lists the everyday commands with their GNU options.

Unix vs Linux compared

Is macOS Unix or Linux?

Unix, not Linux. macOS runs on Darwin: the XNU kernel plus a userland taken largely from FreeBSD, with Apple’s own frameworks on top, and Apple has had releases certified as UNIX 03 since 2007. That is why its Terminal feels familiar to a Linux user (the same shell, ssh, grep and file layout) and also why the GNU options in the table above fail there. Since macOS 10.15 the default shell is zsh rather than bash. Developers who want the exact Linux behaviour on a Mac usually install the GNU tools alongside, or work on a Linux server or virtual machine.

Which runs servers today?

Linux, by a wide margin. Most web servers and most cloud virtual machines run it, and containers grew up on it: Docker’s Linux containers rely on Linux kernel features (namespaces and cgroups), so Docker Desktop on a Mac or Windows PC quietly runs a small Linux virtual machine. Every system on the TOP500 list of supercomputers has run Linux since November 2017, and Android puts the Linux kernel in most of the world’s phones. Even KVM, the hypervisor behind most virtual servers, is a part of the Linux kernel; what is KVM explains how.

Certified Unix holds on where it always was strongest: AIX on IBM Power systems and z/OS on mainframes, running banking, insurance and airline back ends that have worked for decades. FreeBSD runs network and storage appliances and some very large content platforms; Netflix’s Open Connect caching servers are the best-known example.

Learning one means learning both

Because of POSIX, skills move across: the shell, pipes, permissions, ssh, grep and cron work the same way on Linux, macOS, the BSDs and AIX, with small option differences like those above. Linux is the practical place to learn them, since it is free, it is what servers run, and a fresh system is one reinstall away when an experiment goes wrong. A small Linux VPS with Debian or Ubuntu gives you root access to a real server for that.

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