Unix vs Unix-like vs POSIX

Unix

Unix today is a trademark. Historically it was an operating system from AT&T's Bell Labs (1969), then a family of operating systems descended from the original AT&T Unix. Now, to legally call your OS "Unix", you need trademark certification from The Open Group.

macOS is certified Unix.

Unix-like

Unix-like means implementing the Unix interface:

• Hierarchical file system rooted at /
• Everything is a file (devices, sockets, pipes)
• Text streams for input/output
• Pipes to compose programs
• A shell for scripting and interaction
• The standard utilities (grep, sed, awk, cat, ls...)
• Fork/exec process model
• User/group permissions

Linux is Unix-like. It's not Unix™, but it's Unix in the ways that matter.

POSIX

POSIX (Portable Operating System Interface) is the IEEE standard that defines what "Unix-like" means technically. It specifies the system calls, shell behavior, and utilities that make Unix Unix. If your OS is POSIX-compliant, code written for Unix will run on it.

How they relate

• Unix is the origin and the trademark
• POSIX is the specification that captured what Unix is
• Unix-like means "POSIX-compliant, spiritually Unix, but not branded"

The original Unix operating system died; the philosophy survived. See The world Unix made.

2025-11-28

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