摘自：http://www.apuebook.com/index.html
- What is the difference between "UNIX System" and "UNIX system?"
When we speak of the UNIX System itself as an entity, we capitalize the ‘S’ in System. When we are speaking about a possible implementation of the UNIX System or a particular UNIX system, we use a lower-case ‘s’.
The Unix operating system was developed at AT&T Bell Laboratories. When AT&T registered the name as a trademark, they capitalized the name to UNIX. Today the trademark is owned by The Open Group. For a system to call itself UNIX, it needs to meet certain criteria, pass conformance tests, and license the name.
While Mac OS X resembles a UNIX system and provides a UNIX-compatible environment, it has not yet been formally certified as a UNIX system. On the other hand, if it looks like a duck, walks like a duck, and quacks like a duck, then it’s probably a duck.
While FreeBSD resembles a UNIX system and provides a UNIX-compatible environment, it has not yet been formally certified as a UNIX system. On the other hand, if it looks like a duck, walks like a duck, and quacks like a duck, then it’s probably a duck.
While Linux resembles a UNIX system and provides a UNIX-compatible environment, it has not yet been formally certified as a UNIX system. On the other hand, if it looks like a duck, walks like a duck, and quacks like a duck, then it’s probably a duck.
Yes, Solaris has been formally certified to be a UNIX system.
简单地用中文总结一下：Mac OS X FreeBSD Linux 都是类unix系统，但是没有正式地说法；Solaris确实是真正的unix系统