BSD and Linux are both Unix-like systems (or Unices), systems that descend from or are inspired by Unix, the operating system that was first developed in the 1960s by Bell Labs, part of AT&T, the American phone company.
POSIX (Portable Operating System Interface ) is a standard that defines the minimal set of functionalities, commands, and interfaces, as well as the directory structure, that a system must implement to be considered a Unix system.
About Unix and POSIX alone, entire books can be written. ...
