Is it POSIX?

You occasionally hear the phrase, “Is it Posix?” referring to whether some OS feature or program is POSIX-compliant. The first time I heard it was in college. At that time, I had never even heard of POSIX before.

Later I came across some Stack Overflow answers here and here that I thought were really helpful in explaining what POSIX is and what it means for software to be POSIX-compliant. The first one is especially thorough.

You’re probably already writing POSIX-compliant software whether you realize it or not; if you’re considering portability and how to make your software usable across systems. “Does the shell have cd (like pretty much every shell except Windows command prompt)?” “Is this system call available in all Unix-like operating systems?” Well there’s a phrase for that.

So, is it POSIX?