A lot of this dates back from the old “tty” days (or at least crt terminals that were still alphanumeric). Even the concept of putting a job in the background with & is really a feature of trying to multiplex multiple tasks on the same dumb terminal. This is indeed less important with windowing things like the DMDs or modern windowing workstatiosn when you can just get another window. The Berkeley job control was an interesting hack. For us at BRL the problem was I absolutely detested the C shell syntax. The Korn shell hadn’t escaped from AT&T yet, so, I spent time figuring out how that really worked in the C shell (not really well documented), mostly by inspection, and then reimplemented it in the Bourne Shell (we were using the System V source code version for that). I still couldn’t get traction at BRL for using the Bourne shell because by that time, tcsh had come out with command line editing. So back to the shell sources I went. By this time, 5R2 had come out so I grabbed the shell source form that. I was very delighted to find that the macros that made C look sorta like Algol had been unwound and the thing was back to straight C. I reworked emacs-ish command line editing into the shell. Subsequently, I had a nice conversation with David Korn at USENIX, being probably at that point the two most familiar with Bourne shell job control internals. I also sat down with the guys writing either bash or the pdksh (can’t remember which) and explained all how this work. As a result my name ended up in the Linux manpages which was pretty much all I found for a while when I googled myself.
Years later, I had left the BRL, spent three years as a Rutgers administrator and was working for a small startup in Virginia. There was a MIPS workstation there. I was slogging along using ed (my employees always were amazed that if there was no emacs on the system, I just used ed, having never learned vi). Not thinking about it, I attempted to retrieve a backgrounded job by typing “fg.” To my surprise the shell printed “Job control not enabled.” Hmm, I say. That sounds like my error message. “set -J” I type. “Job control enabled.” Hey! This is my shell. Turns out Doug Gwyn put my mods into his “System V on BSD” distribution tape and it had made its way into the Mach code base and so every Mach-derived system ended up with it. Certainly, I found it convenient.
There have been other schemes other than job control to multiplex terminals. IBM in the AIX that ran on the 370/PS2/i860 had a device called the High Function Terminal that allowed you to swap screens on the console. When we implemented the i860 (which was an add in card for the micro channel), we called our console the Low Function Terminal.
Then after spending some time on MIT’s ITS and TOPS20, I got intrigued by the fact on those systems you could have a “shell” that persisted across logins and could be detached and reattached on another device at another time. I set about making such an implementation. Not particularly efficient, it essentially grabbed one of the BSD ptys and spawned the shell there and then a small alternative login shell forwarded the real tty to that. You could then detach it leaving the shell running on the PTY and reattach it elsewhere. Much like ITS, on. login it reminded you that you had a detached shell running and offered to reattach it rather than spawning a new one (complete with the ITS-ish: space for yes, rubout for no). It never really caught on.
Oh well, pardon my ramblings.
-Ron