The Open Group publishes a document called the Single UNIX Specification that defines how a UNIX system should behave. There have been two versions of this document. Its content derives heavily from the POSIX specification, but the Single UNIX Specification has as of 2001 become more popular among operating system developers than the POSIX specification, largely because the Open Group makes its document available at a much lower price.
Additional Resources
- Text of the Single UNIX Specification, Version 2 (http://www.opengroup.org/onlinepubs/007908799/)