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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 seemingly 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); that analysis is based on the source code as preserved in the reconstruction, but other recollections (closer to the time) differ. Thus Steve Bourne wrote in The Unix System V Environment
A cast-off PDP 7 with a 340 display was available but the PDP 7 provided only an assembler and a link editor. One user at a time could use the computer, each user having exclusive use of the machine. This environment was crude and parts of a single user UNIX system were soon forthcoming. The space travel program was rewritten for the PDP 7 and an assembler and rudimentary operating system kernel were written and cross assembled for the PDP 7 on the GECOS system. This early system did not provide time-sharing. indeed, much like the modern personal computers, the PDP 7 hardware was simple and provided no support for such activities. An assembler and a command interpreter were soon available. This file system provided a name structure that was a directed graph. A single directory was used for all subdirectories and links made through this directory.
Cross assembling meant using two computer systems and carrying paper tapes of programs from one to the other each time a change was made. The system was soon bootstrapped onto the PDP 7. The process creation primitive, fork, and process images were added to the system during this rewrite. Essential utilities, such as file copy, edit, remove, and print were soon available. This system supported two people working at the same time and the term UNIX was coined by Brian Kernighan in 1970.
Thus it appears that the system on the PDP-7 was initially single-user, but that was a short phase during the initial development, and eventually Unix on the PDP-7 supported two users simultaneously; this also matches Ritchie’s recollections in the paper linked above:
Processes (independently executing entities) existed very early in PDP-7 Unix. There were in fact precisely two of them, one for each of the two terminals attached to the machine.
Presumably the system was still limited to running a single process in memory at any given time, even if two users could log in in parallel. Before Unix moved to the PDP-11, its developers had even managed to build two B interpreters (including one with virtual memory) and used that to write a few utilities!
Diomidis Spinellis’ Unix history repo contains source code for most historical Unix releases, including PDP-7 Unix.
Unix was almost certainly never a single-user system. PDP-7 Unix supported at least two users (likely Ken and Dennis).
As evidence, I quote Doug McIlroy's paper on the history of research Unix:
Doug (M. Douglas) McIlroy exercised the right of a department head to muscle in on the original two-user PDP-7 system.
(The quote is taken from section 1.1, People)
And, now I've poked around a little more, here's some other evidence, from Dennis Ritchie's paper The Evolution of the Unix Time-sharing System; in the section on Process Control:
Processes (independently executing entities) existed very early in PDP-7 Unix. There were in fact precisely two of them, one for each of the two terminals attached to the machine.
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. Apparently, the PDP-7 running Unix could support two terminals. So Unix has always been a multi-user operating system, though of course hardware dependent - if you ran it on a system with only one console then it was effectively a single-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.
That one sentence in my comment sure did blow up.
The PDP machines mentioned in the other answers were commercially available early on, and indeed used by Bell Labs (AT&T) in the development of what later became known as UNIX. But AT&T at that time was a big company, and had started the development of its own computer - the 3B series. These machines were the in-house solution, whereas the PDP series came from DEC.
As for the phone switch, that would be the 4ESS and later 5ESS. These had a 3B20D (duplex) as their Administrative Node. Duplex doesn't mean "two users" here, but two CPU cores running in lockstep. A failure in one half would not take the switch down.
AT&T at this time was barred from doing any other business in the US but providing Common Carrier phone services. So it made perfect sense to use UNIX for its primary business. Of course, Bell Labs was very much an like academic environment, and it may not always have been directly focused on the larger AT&T business interests.
There were a number of companies which shipped UNIX-derivatives configured for single-user operation, even if UNIX itself was thoroughly multi-tasking. Broadly speaking these booted straight into a shell rather than having init or equivalent which forked multiple processes.
I came across one in the UK which had a range of machines which could either boot CP/M on a Z80 or a single-user UNIX on a 68K and was typically sold as an accounting system, and there was the Burroughs (formerly Convergent) B20 which was shipped with BTOS ("B-Twenty Operating System").
Considering OP, I think it might be worth considering that many computers were shipped with no provision for attaching supplementary terminals or Telnet sessions, and as such were effectively single-user even if still capable of multitasking.
My copy of The Bell System Technical Journal v57#6 part 2, July-August 1978 (see it here and elsewhere - an issue entirely concerned with the "Unix Time-Sharing System" doesn't mention using Unix as a phone switch OS though it talks about many other applications.
Some of those applications include being a support system for a phone switch ("No. 4 ESS Diagnostic Environment") where an application running on Unix talked to a 1A processor - using special software added to that processor's system (4ESS); also being the operator's interface in a larger system ("The Network Operations Center System") where real-time data was collected into a database (by a separate system) and then a (multi-user) Unix application was used to interrogate that database.
This issue also describes a separate operating system build by AT&T ("The MERT Operating System") - which was an early hypervisor capable of running Unix environment as a "supervisor process" alongside other supervisor processes that provided real-time capabilities. No specific installations of MERT are described.
The whole issue is well worth reading for any Retro-computing enthusiast interested in Unix.
