POSIX specifies the user and software interfaces to the OS in some 15 different documents. The standard user command line and scripting interface is the [Korn shell]?. Other user level programs/services include awk, echo, ed, and numerous (hundreds) others. Required program level services include basic I/O (file, terminal, and network) services.
Since the IEEE has been charging very high rates for POSIX documentation and not allowing on-line publication of the standards, there has been a tendency toward the "[Single Unix]?" standard, which is open, accepts imput from anyone, and is freely available on the internet.