From Peter Seebach’s Standards and specs: Not by UNIX alone (IBM developerWorks: 8 March 2006):
In the past 20 years, developers for “the same” desktop platform (“whatever Microsoft ships”) have been told that the API to target is (in this order):
* and most recently .NET.
Of course, that list is from last year, and now the “stable” target that you should be developing for, if you have an eye for the future, is Vista.
It hasn’t been quite as bad in the Macintosh world, where the number of major API changes has been limited: classic single-tasking Mac OS, classic multitasking Mac OS (System 7), Carbon (System 8/9 and preview of OS X), and Cocoa (OS X), but even there, the cost of migration has been significant. At least OS X finally offers a stable UNIX API for the back-end part of programs, allowing developers to ignore the API creep except in GUI code.
By contrast, twenty-year-old UNIX utilities still compile and run. A new desktop computing API will come and everyone will have to rewrite for it, but mountains will erode away before read() and write() stop working. This is the reason that all the hassle of formal UNIX standards has had so little effect on practical UNIX software development; the core API is simple, clean, and well-designed, and there is no need to change it significantly.
… UNIX users have been switching hardware platforms since the 1970s; it’s no big deal. …
Just as there are many varieties of UNIX, there are many UNIX standards:
* Probably the oldest standard that people still refer to is AT&T’s 1985 System V Interface Definition (SVID). This standard shows up, for instance, in man pages describing the standards compliance of functions that have been in the C library “forever.”
* Meanwhile, X/Open (now the Open Group) was developing “portability guides” with names like XPG2, XPG3, and so on. XPG1 was actually released in 1995. The XPG guides are largely subsumed into newer specs, but once again, are still referred to sometimes in documentation.
* The IEEE’s POSIX standard showed up in 1990 with updates in 1992 and 1993 and a second edition in 1996. It’s still a viable standard, although it has suffered from poor accessibility. POSIX specs have names like 1003.x; for instance, 1003.1 and 1003.2, which refer to different parts of the standard, or 1003.1-1988 and 1003.1-1990, which refer to two versions of the standard.
* The fairly ominous sounding “Spec 1170” (also known as “UNIX 98” or “Single Unix Specification”) is probably the most complete specification; it is produced by the Open Group, and is effectively a descendant of the XPG series. In practice, this is “the” UNIX standard these days, although it’s a little large; this has had an impact on conformance testing.
* The Linux Standards Base is not strictly a UNIX standard, but it’s a standardization effort relevant to a very large number of developers working with code designed to run “on UNIX.” …
You can look at OS specifications in two very different ways: one is from the point of view of a developer trying to port an application, and the other is from the point of view of the user trying to interact with the system.
UNIX conveniently blurs this distinction. The primary user interface is also one of the primary development environments; therefore, UNIX specifications often cover not only the C language API, but also the shell environment and many of the core utilities shell programmers rely on. …
From the perspective of a developer who’s seen many Unix-like systems, Linux is probably mostly sort of similar to System V. The heavy focus on GNU utilities gives a sort of surreal combination of Berkeley and System V features, but if you have to guess whether Linux does something the Berkeley way or the System V way, go with System V. This is especially true of system startup; nearly all Linux systems use the System V /etc/inittab and /etc/rc.d structure, or something very close to it. …