What is Unix? I've known Unix long enough to know that the trademark and industry usage want the name rendered in all capital letters (UNIX), although many publications (mine included) don't like it that way. Having written about Unix for a couple of decades, I've come to take for granted that everyone knows what Unix is. Certainly, no one would ask me what Unix is. I get jabbed all the time to define Linux (kernel) whenever I use the term, and I've been asked why I don't refer to Intel Macs as PCs (proprietary platform). But nobody has ever noticed me refer to Unix and written to say, "What do you mean, Unix?" I wish someone had. Figuring that the economy would make Unix vendors the ready pan of market analysts praying to get something right, I had in mind to write a sort of You Don't Know Unix column. The trouble with an in-your-face headline like that, is that it turns embarrassing when the author has to admit he couldn't meet his own challenge. The simple question I asked myself one recent morning became the deep thought into late the following afternoon. I had all kinds of clever things to say in defence of Unix, but none of it was relevant to IT, and IT deserves a relevant look at Unix as something other than a culture, a history, or a meaningless banner over all operating systems in the "not Windows" category. The fact is Unix matters to IT and for a reason that may not occur even to those shops that already have it. Unix matters for a reason that escapes analysts' notice. I missed it, too. It's that little circle with the R in it. IBM, Sun, Fujitsu, HP, and Apple sell proprietary enterprise operating systems branded AIX, Solaris, HP-UX, and Mac OS X Leopard. These are very different. Mac OS X Leopard is very, very different. IBM, Sun, HP, Fujitsu and Apple also sell Unix. All of these vendors' Unix implementations are a precise match for the others, and vendors sign a contract guaranteeing IT that applications written for one Unix can run on all Unixes, and that a network of any size and reach can mix Unixes at will with guaranteed interoperability across vendors. One (large) set of documentation covers all Unix implementations for architects, developers and administrators. Non-branded Unix docs make no mention of the brand on the server machine, and they have no need to. All Unix, every Unix, works as described in that one set of manuals. Unix is not a core of source code common to all of the proprietary OSes I've described. If Unix were software, it would have died out during battles to own it. Unix is a registered trademark of The Open Group, which keeps the Single Unix Specification 03 (Unix 03, or just Unix). The specification — which the proprietary operating systems of IBM, Sun, Fujitsu, HP and Apple adhere to — definitively and inclusively describes Unix from the microscopic level (the C language and system data structures) to the command line. Any skills, staff, source code, infrastructure and solutions you invest in to Unix are portable across IBM, Sun, Fujitsu, HP, Apple and generic 32 and 64-bit x86 hardware. The Unix 03 spec is open, meaning fully publicly available in its final form. Unix 03 is drawn from several contributors and provides a single approach to key modernisations, including mixed execution of 32-bit and 64-bit code, and incorporating internationalised text. The spec doesn't get into the buried plumbing, only what's visible to users, admins and developers. For that matter, the spec doesn't care how a vendor implements it. IBM and HP have closed source implementations, while Sun and Apple have opened theirs. Nothing prevents Microsoft, Red Hat, Novell, or anyone from attaining the Unix trademark. Yes, Microsoft could conceivably slap the Unix trademark on Windows, but for a few million lines of code. The reason that only five vendors ply the trademark is that the Unix validation is the easy part. Or, rather, the cheap part. (Validation is hardly easy; ask Apple.) The dotted line that a vendor signs to use the trademark is the contract with Unix customers, software vendors, and competitors — the ecosystem — guaranteeing full interoperability across vendors. The trademark puts legal teeth in the Unix spec. Though Unix's smooth interoperability — and the freedom that independent software vendors enjoy to have one database or CRM code base cover so many different platforms — is a product of cooperation among Unix vendors, IT operations, universities and professional organisations. The Open Group didn't make that happen; it's always been the case. The trademark merely provides IT organisations that need to be sure, without the need for digging, that Unix means something, and it does. It means that Unix enterprise solutions work, and work together, without regard for the brand on the hardware.