The original motivation for the development of Unix standards was the split between the AT&T and Berkeley lines of development that we examined in Chapter2.

The 4.x BSD Unixes were descended from the 1979 Version 7. After the release of 4.1BSD in 1980 the BSD line quickly developed a reputation as the cutting edge of Unix. Important additions included the vi visual editor, job control facilities for managing multiple foreground and background tasks from a single console, and improvements in signals (see Chapter7). By far the most important addition was to be TCP/IP networking, but though Berkeley got the contract to do it in 1980, TCP/IP was not to ship in an external release for three years.

But another version, 1981's System III, became the basis of AT&T's later development. System III reworked the Version 7 terminals interface into a cleaner and more elegant form that was completely incompatible with the Berkeley enhancements. It retained the older (non-resetting) semantics of signals (again, see Chapter7 for discussion of this point). The January 1983 release of System V Release 1 incorporated some BSD utilities (such as vi(1)).

The first attempt to bridge the gap came in February 1983 from UniForum, an influential Unix user group. Their Uniforum 1983 Draft Standard (UDS 83) described a “core Unix System” consisting of a subset of the System III kernel and libraries plus a file-locking primitive. AT&T declared support for UDS 83, but the standard was an inadequate subset of evolving practice based on 4.1BSD. The problem was exacerbated by the July 1983 release of 4.2BSD, which added many new features (including TCP/IP networking) and introduced some subtle incompatibilities with the ancestral Version 7.

The 1984 divestiture of the Bell operating companies and the beginnings of the Unix wars (see Chapter2) significantly complicated matters. Sun Microsystems was leading the workstation industry in a BSD direction; AT&T was trying to get into the computer business and use control of Unix as a strategic weapon even as it continued to license the operating system to competitors like Sun. All the vendors were making business decisions to differentiate their versions of Unix for competitive advantage.

During the Unix wars, technical standardization became something that cooperating technical people pushed for and most product managers accepted grudgingly or actively resisted. The one large and important exception was AT&T, which declared its intention to cooperate with user groups in setting standards when it announced System V Release 2 (SVr2) in January 1984. The second revision of the UniForum Draft Standard, in 1984, both tracked and influenced the API of SVr2. Later Unix standards also tended to track System V except in areas where BSD facilities were clearly functionally superior (thus, for example, modern Unix standards describe the System V terminal controls rather than the BSD interface to the same facilities).

In 1985, AT&T released the System V Interface Definition (SVID). SVID provided a more formal description of the SVr2 API, incorporating UDS 84. Later revisions SVID2 and SVID3 tracked the interfaces of System V releases 3 and 4. SVID became the basis for the POSIX standards, which ultimately tipped most of the Berkeley/AT&T disputes over system and C library calls in AT&T's favor.

But this would not become obvious for a few years yet; meanwhile, the Unix wars raged on. For example, 1985 saw the release of two competing API standards for file system sharing over networks: Sun's Network File System (NFS) and AT&T's Remote File System (RFS). Sun's NFS prevailed because Sun was willing to share not merely specifications but open-source code with others.

The lesson of this success should have been all the more pointed because on purely logical grounds RFS was the superior model. It supported better file-locking semantics and better mapping among user identities on different systems, and generally made an effort to get the finer details of Unix file system semantics precisely right, unlike NFS. The lesson was ignored, however, even when it was repeated in 1987 by the open-source X windowing system's victory over Sun's proprietary Networked Window System (NeWS).

After 1985 the main thrust of Unix standardization passed to the Institute of Electrical and Electronic Engineers (IEEE). The IEEE's 1003 committee developed a series of standards generally known as POSIX.^[144] These went beyond describing merely systems calls and C library facilities; they specified detailed semantics of a shell and a minimum command set, and also detailed bindings for various non-C programming languages. The first release in 1990 was followed by a second edition in 1996. The International Standards Organization adopted them as ISO/IEC 9945.

Despite being underspecified in a couple of key areas such as signal-handling semantics and omitting BSD sockets, the original POSIX standards became the basis of all later Unix standardization work. They are still cited as an authority, albeit indirectly through references like POSIX Programmer's Guide [Lewine]. The de facto Unix API standard is still “POSIX plus sockets”, with later standards mainly adding features and specifying conformance in unusual edge cases more closely.

The next player on the scene was X/Open (later renamed the Open Group), a consortium of Unix vendors formed in 1984. Their X/Open Portability Guides (XPGs) initially developed in parallel with the POSIX drafts, then after 1990 the XPGs incorporated and extended POSIX. Unlike POSIX, which attempted to capture a safe subset of all Unixes, the XPGs were oriented more toward common practice at the leading edge; even XPG1 in 1985, spanning SVr2 and 4.2BSD, included sockets.

XPG2 in 1987 added a terminal-handling API that was essentially System V curses(3). XPG3 in 1990 merged in the X11 API. XPG4 in 1992 mandated full compliance with the 1989 ANSI C standard. XPG2, 3, and 4 were heavily concerned with support of internationalization and described an elaborate API for handling codesets and message catalogs.

In 1993 seventy-five systems and software vendors including every major Unix company put a final end to the Unix wars when they declared backing for X/Open to develop a common definition of Unix. As part of the arrangement, X/Open acquired the rights to the Unix trademark. The merged standard became Single Unix Standard version 1. It was followed in 1997 by a version 2. In 1999 X/Open absorbed the POSIX activity.

In 2001, X/Open (now The Open Group) issued the Single Unix Standard version3. All the threads of Unix API standardization were finally gathered into one bundle. This reflected facts on the ground; the different varieties of Unix had re-converged on a common API. And, at least among old-timers who remembered the turbulence of the 1980s, there was much rejoicing.