By the mid-1980s, Unix had fragmented into a sprawling ecosystem of incompatible variants — BSD, System V, SunOS, AIX, HP-UX — each subtly different in ways calculated to ruin portability. The IEEE responded with POSIX (Portable Operating System Interface), a family of standards specifying how a conforming Unix-like system should behave. The name was suggested by Stallman. The standard itself runs to thousands of pages and has been continuously revised ever since. POSIX compliance became a selling point; actual POSIX compliance — every edge case, every signal behaviour, every obscure ioctl — is vanishingly rare. Linux is “mostly POSIX.” macOS is “mostly POSIX.” The BSDs are “mostly POSIX.” The word “mostly” is doing considerable heavy lifting in each of those sentences.
