System z is IBM's mainframe architecture (the hardware).

z/OS is the flagship operating system. It is at its heart a batch-oriented system, but it does have a Unix subsystem (called USS for "Unix System Services", formerly known as MVS OpenEdition or OMVS) that is in fact certified against a rather old version of the Single Unix Specification.

So in a way, z/OS is in fact a Unix system, but that is kind of like saying that Windows is a Unix due to Cygwin or Microsoft's Services for Unix. Mostly, it is very non-Unix-like.

Interestingly, IBM also offers a number of other operating systems for System z, including VM (although that system apparently does little but host virtual machines), TPF (a "real-time" transaction processing system) and zVSE (formerly known as DOS/360, another, smaller batch-oriented system). And, of course, Linux is available from a number of distributions. Oh, and just for good measure, Open Solaris was ported to run inside VM, although I am not sure what became of that or if anyone is actually using it.

What is using it like? Command line? Would it be familiar-ish to someone with Unix / DOS experience or totally foreign?

All command line, and totally foreign on a level that makes switching from Windows to Linux look trivial by comparison.

(which coincidentally, is why I want to explore it)

Here's the official IBM "for dummies" book: https://www.redbooks.ibm.com/redbooks/pdfs/sg246366.pdf

It uses the 3270 terminal (xterm has a 3270 mode I've used), interface is predominantly 'ispf', it's essentially menu driven; you can browse/search the filesystem, edit files, submit jobs (everything is a job ie jcl ie job control language; compile, run programs etc) browse job spools (stdout from a job).

Google will show you images of ispf, browsing the jcl manuals will give you nightmares.

My first job was writing JCL decks - it's not that bad.

It is not terrifying per se, just ... very different.

I have only had brief contact with JCL, but I remember having to figure out how to get a command line into JCL that was more than 80 characters in length. It took me two days to figure that out, and none of the old-timers in the team had ever had to do that. When I did find the solution, it was surprisingly simple, but finding it took me a lot of time.

Wait, is it a UNIX system or have a UNIX subsystem? Those are different. I know it has the subsystem. I'm trying to recall when someone said it was a UNIX. Stuff I read on it is more MVS than UNIX. Example with some data:

https://en.wikipedia.org/wiki/Z/OS

It has a Unix subsystem. It is a lineal descendant of OSes which predate Unix by quite a bit, and has grown by accretion since then.

It's POSIX certified. Linux isn't. That should tell people quite a bit about POSIX, but it never seems to...

POSIX isn't actually about operating systems. POSIX (1003.1) is about source-level compatibility for 2 languages: C99 and shell scripts.

If your C programs use only C99+POSIX facilities, and your shell programs use only POSIX features and utilities, and they work on a platform, then that platform is eligible to be POSIX certified. POSIX doesn't care about the kernel or syscalls or anything like that. It cares about libc, libm, libl, and a couple of file paths.

Actually to a certain extent, I think one could call POSIX the actual C runtime library.

As C runtime + POSIX calls (I know it didn't exist back then) is what defined C when it was still UNIX only, but ANSI didn't want to make the language standard that big.

Linux is just the kernel. Distributions should certify. And I do not see anyone going fot it. There is at least one distribution posix certified: Inspur K-UX.

The equivalence in linux land should be LSB, which many distributions certify to.

Lol at the last line. No doubt.

OMVS mapped a hierarchical filesystem onto the dataset based mainframe filesystem. It's one hell of a kludge, imo. It's kind of like how cygwin attempts to emulate stuff windows doesn't have, like fork. Mainframe has the concept of a started task, which is kinda similar, but not quite

It's an architecture called z/Architecture, used for z Systems CPUs such as the z13 CPUs in IBM mainframes (the z13, same name). The 'z' in 'IBM z Systems' stands for 'zero downtime'.

[1] https://en.wikipedia.org/wiki/Z/Architecture

[2] https://en.wikipedia.org/wiki/IBM_z13_(microprocessor)

I thought it came from a play on the naming convention. I don't have the source but it went something like this:

System/360 - The system for the 60's

System/370 - The system for the 70's

System/390 - The system for the 90's

System/Z - The system for the 2000's, which sounds like a Z to some at the end.

System/360 was meant to be a good all-around system, not optimized specifically for business or science, as previous mainframes were, but good for both. 360 degrees in a circle, you know.

Notice how it skips System/380. IBM was going to replace the mainframe line with its Future Systems project, which would have single-level store (everything is RAM, everything persists, page cache takes care of moving stuff in and out of physical disk drives) and be so tightly-integrated nobody would be able to clone it, as the plug-compatible vendors had been able to clone parts of the System/360 and /370 systems. The only real result of this was the AS/400 midrange systems, now the i Series, I think.

Re 360

I remember reading that it converged two seperate segments into one. 360 degrees makes sense.

Re System/380

Always wondered why they skipped 380. Thanks for the enlightening details on that. The anti-cloning angle is amusing.

>The 'z' in 'IBM z Systems' stands for 'zero downtime'.

Maybe also meant to imply, the ultimate? :) Heh. Just guessing.

"z" is the marketing term for all of IBM's mainframe products. There are several different operating systems that can run on the mainframes.

This port only applies to those running Linux on their mainframe, which isn't terribly common. Most would be running zOS (https://en.wikipedia.org/wiki/Z/OS) or TPF. Their VM hypervisor is also commonly used.

The AS/400 line uses i, not z.

The AS/400 isn't a mainframe. It sits in the same space as VAX/VMS and the HP3000 line.

Edit: The common term for these, during their heyday, was "minicomputer" or "midrange".

Sure, but I still see it as a mainframe, even though you could seat on them (I actually used to do that on a dead one).

z/OS is the rebrand of OS/360, which predates Unix and has been running Western civilization for about half a century.

It's more of a follow-on to MVS, actually; OS/360 is a couple steps back.