POSIX - The Portable Operating System Interface

Does the standard of Unix exist? What is POSIX?

POSIX is the name for a collection of software standards, based on but not limited to the Unix system.

The existence of the different versions of the Unix systems has been a persistent source of frustration for users and programmers. Users who moved from different versions of the system would discover that commands that worked on the system did not work, or work differently, on the other. That's why in 1984 the group of the Institute for Electrical and Electronics Engineering (IEEE) started working for the developing of Unix standard.