Kdtyjb

I've been told that Unix started out as a Phone Switch OS and was not a multi-user OS at some time in its infancy.

As I'm always willing to learn new things, are there any greybeards around here older than me that remember this?

_{Bonus points if you can find any documentation to back it up. Scans of old manuals count as my quick googling didn't turn up anything on-line...}

Going from “AT&T made phone switches” to the idea that Unix was intended to drive phone switches is quite a leap. The creators of Unix described its creation and development in some detail, e.g. in The Evolution of the Unix Time-sharing System:

What we wanted to preserve was not just a good environment in which to do programming, but a system around which a fellowship could form. We knew from experience that the essence of communal computing, as supplied by remote-access, time-shared machines, is not just to type programs into a terminal instead of a keypunch, but to encourage close communication.

So Unix was really a scratch-your-own-itch development at first, and intended to be multi-process and multi-user from the beginning. Of course the costs involved in developing it (mostly buying hardware) had to be justified, which quickly led to the development of text-processing software and various other developments later on.

You can run V1 Unix on an emulated PDP-11 on a modern computer; the default configuration supports 8 logins. A partial reconstruction of the original PDP-7 Unix is also available. The latter supported multiple users and processes, but only one of each at a time (ten process slots were available but only one process was swapped in at a time).

Diomidis Spinellis’ Unix history repo contains source code for most historical Unix releases, including PDP-7 Unix.

From this page, Wikipedia and elsewhere, it is pretty clear that the first Unix system was on a PDP-7. As far as I can tell, the PDP-7 (unlike the PDP-11 where Unix became more than just a one-off system) was not a multi-user system. On the other hand, one could argue that Unix wasn't officially "Unix" on that very first PDP-7. So from very early on, though possibly not the first system, Unix has been a multi-user system. I doubt there is much extant documentation form the pre-PDP-11 era, particularly since little was released publicly until a few years after Unix was first written. Or as this PDP-11 history page describes it:

These are the original Bell Laboratories releases of Unix; the first 4 were only internal to Bell, the Fifth saw limited distribution outside it, and the Sixth took over the world.

On the other hand, plenty of multi-user operating systems - Unix, Linux, MP/M, MP/M-86 - and I am sure many other mini-computer & micro-computer operating systems - have been used as single-user systems over the years by power users as an easy way to get a multi-tasking system for increased productivity, especially prior to functional versions of Microsoft Windows.