I've never heard of "HP True 64".
I have heard of "HP Tru64" but they quit making that long ago.
In fact the first line of this article:
https://en.wikipedia.org/wiki/Tru64_UNIX
Quote:
Tru64 UNIX is a discontinued 64-bit UNIX operating system for the Alpha instruction set architecture (ISA), currently owned by Hewlett-Packard (HP).
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HP does make a proprietary UNIX called HP-UX that was far more widely used than Tru64 ever was. You can learn more about it from their site:
http://www8.hp.com/us/en/products/servers/hp-ux.html
HP-UX runs only on Itanium hardware these days. Originally it ran on their own RISC series of chips but they quit making those at the end of 2009. It is a good platform for high availability but you pay for that (much more than the $99 you think you saw). I've worked at several jobs where we ran large Oracle DBs on HP-UX.
Solaris is another proprietary UNIX you didn't list. That was made by Sun Microsystems but they got bought by Oracle a few years back. At one point Oracle announced they wouldn't continue to build their DB and other products for HP-UX on Itanium (one guesses this was because they owned Solaris and also had forked RHEL into their own OEL Linux) but lost a court case:
http://www.informationweek.com/appli.../d-id/1106118?
I haven't really followed it since then so don't know if they still support it on HP-UX. I did find a sublink at the above HP link that talks about "Oracle Alliance" but didn't delve into it.
There have been various other proprietary UNIX flavors over the years but most of them have gone by the wayside. Even HP, IBM and Oracle are pushing Linux solutions these days for most uses. Many of us who started out as UNIX Admins have spent a lot of the last few years porting from UNIX to Linux. In fact here in Atlanta the local UNIX user's group disbanded last year after 30 years.
At my current job our main ERP systems moved to Linux off HP-UX 3 years ago and all of our other systems but our payroll solution had moved off HP-UX by the end of 2014. We just moved payroll to a cloud solution so will be retiring that last HP-UX hardware/software any day now.
Unless you have a specific position in mind that requires it you probably shouldn't spend much effort on learning one of the proprietary UNIX versions.