How do you define UNIX system

何謂 UNIX 系統：

• SUS 是 POSIX.1 的超集(superset)，定義了比 POSIX.1 更多的擴增函式介面(interfaces)，完整的系統介面設定稱之為 X/Open System Interface (XSI)。
• 只有 XSI-conforming(一致) 的系統實作才能被稱之為 UNIX 系統。
• SUS 是由 Open Group 定義的。
  

Unix-Based 常見標準:

• IEEE POSIX - (Institute of Electrical and Electronics Engineers) Portable Operating System Interface
• SUS - The Single UNIX Specification, a superset of the POSIX.1, "SUS mandatory"
• XSI - X/Open System Interface (_XOPEN_UNIX)
• XSI-Conforming - Only XSI-conforming implementations can be called UNIX systems.
• FIPS - Fedral Information Processing Standard

下面這篇文章就是在說明 UNIX 的定義：

The Open Group owns the UNIX trademark and uses the Single UNIX
Specification to define the interfaces an implementation must support to
call itself a UNIX system. Implementations must file conformance
statements, pass test suites that verify conformance, and license the right
to use the UNIX trademark.

SUS 的版本沿革如下：

SUS v1 1994 (Spec 1170)
SUS v2 1997
SUS v3 2001 (equal to IEEE Standard 1003.1-2001 = ISO/IEC 9945:2002 (approved by ISO at 2002)
SUS v3 divied into 4 parts:

1. Base Definitions
2. System Interface
3. Shell and Utilities
4. Rationale

ISO/IEC 9945:2003
SUS v3 2004 Edition.

因此:
Solaris -> UNIX system
Linux -> NOT UNIX system, through Duck Test, we can treat it is...
FreeBSD -> NOT UNIX system, through Duck Test, we can treat it is...

在 Computer Science 裡有一個 Duck Typing 跟 Duck Test 有關，無聊的話可以當小說看。