I heard recently that the process model in very early variants of UNIX was quite a bit different to the fork/exec model used nowadays.

How did it differ from the current state?


If you search for the seminal 1979 paper from Dennis Ritchie, entitled The Evolution of the Unix Time-Sharing System, it covers this (amongst a few other things like incredibly difficult-to-use file-system links, only being able to create directories at boot time, and why the password file has a GECOS field).

First, we'll recap the current model. A process is one type of execution unit within UNIX while a program is a runnable item that lives within a process (when it's running). That distinction is important.

A running process that wants to start a new process will call fork and this gives you two nearly identical processes running the same program (at the same point), where only one existed before.

At that point, one of them (usually the child) may choose to exec a new program to perform some other work - this exec basically replaces the program in the current process with a whole new program.

Should the original process wish to wait until the child exits, it can call wait to do so.

The old model was a little similar but it only ever had a limited number of processes, one for each of the terminals hooked up to the machine. These processes were created at boot time and there was therefore no fork. A shell ran in each of these processes, interacting with the user on the given terminal.

When the user specified a program to run, the shell would:

  1. Create a link to the file in the current directory (this has to do with the "incredibly difficult-to-use file-system links" mentioned earlier).

  2. Open the file.

  3. Remove the link.

  4. Copy a small bootstrap program to the top of memory and jump to it.

  5. This bootstrap program would read in the already-open file over the current shell code, then jump to the first location of the command (exec).

  6. After the command had done its work, it called exit. But this isn't the exit we know and love nowadays. What this exit did was simply to reload the shell program into the process in much the same way as the shell had loaded the program in the first place.

At that point, you would be back in the shell, ready to type in another command. As you may imagine, this had no support for pipeslines/filters but, interestingly, had I/O redirection from a very early stage - all the shell had to do was connect the standard handles to specific files rather than the terminal device.

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  • IIRC, though not in the initial release, pipes were trivial to add; trivial as in one person spending an afternoon coding (or something like that). – Alex Hajnal Nov 22 '18 at 7:22
  • ^^^ Citation needed. If anyone has a primary source to back up my assertion, kindly post below. – Alex Hajnal Nov 22 '18 at 7:39
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    @Alex, the paper states that: "Some time later, thanks to McIlroy’s persistence, pipes were finally installed in the operating system (a relatively simple job)". Not the bit about it taking an afternoon and presumably we had to wait until we had a proper fork (to get more than one process per terminal), but it says it was easy. – user6464 Nov 22 '18 at 8:19
  • Yea, I saw that mention in the paper. The bit about it taking an afternoon (or overnight) is I think from an oral history interview. – Alex Hajnal Nov 22 '18 at 8:28
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    Found it: "The basic redirectability of input-output made it easy to put pipes in when Doug McIlroy finally persuaded Ken Thompson to do it. In one feverish night Ken wrote and installed the pipe system call, added pipes to the shell, and modified several utilities, such as pr and ov … to be usable as filters. The next day saw an unforgettable orgy of one-liners as everybody joined in the excitement of plumbing." A Research UNIX Reader: Annotated Excerpts from the Programmer’s Manual, 1971-1986, M. Douglas McIlroy, p. 9 – Alex Hajnal Nov 22 '18 at 15:16

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