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> ITS is also available as open source (https://github.com/PDP-10/its https://github.com/PDP-10/its) Sometimes I wonder, if you wanted to create an operating s

> ITS is also available as open source (https://github.com/PDP-10/its https://github.com/PDP-10/its) Sometimes I wonder, if you wanted to create an operating system like ITS for more modern platforms, what it would look like? ITS is written in PDP-10 assembly, what if someone wrote a compiler which read in PDP-10 assembly language and spit out C code? Could that be a first step to porting it? It is surely a lot more complicated than that. It contains self-modifying code, which would obviously break that translation strategy. A lot of hardware-specific code would have to be rewritten. 6 character filenames without nested directories might have been acceptable in the 1970s, but few could endure it today. A multi-user system with a near-total absence of security was acceptable back then, obviously not in today's very different world. ITS does have some interesting features contemporary systems don't: A process can submit commands to be run by the shell that spawned it – actually MS-DOS COMMAND.COM also had that feature (INT 2E), but I haven't seen anything else with it. A Unix shell could implement this by creating a Unix domain socket, and passing its path to subprocesses via an environment variable–but I've never seen that done. Another was that a program being debugged could call an API to run commands in its own debugger – I've never seen that in any other debugger, although I suppose you could write a GDB plugin to implement it (have a magic do-nothing function, then a GDB Python script sets a breakpoint on that function, and interprets its argument as commands for GDB.) Actually, in ITS these two features were the exact same feature, since the debugger was used as the command shell. Another was that a program had named subprocesses (by default the subprocess name was the same as the executable name, but it didn't have to be.) Compare that to most Unix shells, where it is easy to forget what you are running as background jobs 1, 2 or 3.

javert 5y ago
I wish we had more of this kind of content on Hacker News.
cycomanic 5y ago
Interesting context thanks! > A process can submit commands to be run by the shell that spawned it – actually MS-DOS COMMAND.COM also had that feature (INT 2E), but I haven't seen anything else with it. A Unix shell could implement this by creating a Unix domain socket, and passing its path to subprocesses via an environment variable–but I've never seen that done. What would you use it for? How is that superior to simply spawning a new shell process?
- skissane 5y ago
  Norton Utilities for DOS came with a program called NCD (Norton Change Directory). It was a full-screen replacement for the CD command, you could browse through the filesystem to get to the directory you wanted, then it would exit and return you to COMMAND.COM's prompt, with you already in that directory. That worked because under DOS, the current directory was system-wide, [0] not per-process, so a subprocess could change the parent process' current directory. On Unix systems, and the Windows NT family, the current directory is per-process, so that doesn't work any more. Now, you can implement the same idea on Unix–but it is more complex. You need to define a shell function in your .profile/.bash_profile/.zprofile/whatever. That shell function then executes the external change-directory program, and passes the destination directory back to the shell function somehow. The shell function then actually changes the directory. What if shells had exposed some kind of standard API to their subprocesses? Maybe something as simple as an environment variable containing the path to a Unix domain socket which accepts a simple text-based protocol. You could then implement an NCD-style command, by having it send the shell which called it a request to change its directory. That way, you would not have to install such a command by modifying your profile and restarting your shell, the command would just work the first time you ran it. Similarly, there are a lot of tools out there for supporting multiple versions of development tools concurrently (Environment Modules, venv for Python, nvm for Node, conda, etc.) Most of these tools work by manipulating your shell environment. Since, on Unix, environment variables are per-process, and a process cannot modify the environment of its parent, you have to install some shell function in your profile and restart your shell before using one of them. Once again, if shells exported some kind of "get/set shell environment" API to their subprocesses, these kind of tools would work the first time you ran them, without any need to modify your shell profile. [0] Actually, in MS-DOS and Windows 3.x/9x/Me, there is both a system-wide current drive, and a separate system-wide current directory for each drive – Windows NT family's cmd.exe simulates this by using a hidden environment variable per a drive to store its current directory, but that is a convention which only cmd.exe supports, the rest of Windows knows nothing about it.
  - formerly_proven 4y ago
    > That worked because under DOS, the current directory was system-wide, [0] not per-process, so a subprocess could change the parent process' current directory. For people who don't know DOS: DOS didn't really have multiple processes. COMMAND.COM made part of it resident (TSR), which is how it "survived" invoking other applications. At any given point, there was exactly one process running, and all the state you'd typically associate with a process was "system-wide".

javert 5y ago

I wish we had more of this kind of content on Hacker News.

cycomanic 5y ago

Interesting context thanks! > A process can submit commands to be run by the shell that spawned it – actually MS-DOS COMMAND.COM also had that feature (INT 2E), but I haven't seen anything else with it. A Unix shell could implement this by creating a Unix domain socket, and passing its path to subprocesses via an environment variable–but I've never seen that done. What would you use it for? How is that superior to simply spawning a new shell process?

skissane 5y ago
Norton Utilities for DOS came with a program called NCD (Norton Change Directory). It was a full-screen replacement for the CD command, you could browse through the filesystem to get to the directory you wanted, then it would exit and return you to COMMAND.COM's prompt, with you already in that directory. That worked because under DOS, the current directory was system-wide, [0] not per-process, so a subprocess could change the parent process' current directory. On Unix systems, and the Windows NT family, the current directory is per-process, so that doesn't work any more. Now, you can implement the same idea on Unix–but it is more complex. You need to define a shell function in your .profile/.bash_profile/.zprofile/whatever. That shell function then executes the external change-directory program, and passes the destination directory back to the shell function somehow. The shell function then actually changes the directory. What if shells had exposed some kind of standard API to their subprocesses? Maybe something as simple as an environment variable containing the path to a Unix domain socket which accepts a simple text-based protocol. You could then implement an NCD-style command, by having it send the shell which called it a request to change its directory. That way, you would not have to install such a command by modifying your profile and restarting your shell, the command would just work the first time you ran it. Similarly, there are a lot of tools out there for supporting multiple versions of development tools concurrently (Environment Modules, venv for Python, nvm for Node, conda, etc.) Most of these tools work by manipulating your shell environment. Since, on Unix, environment variables are per-process, and a process cannot modify the environment of its parent, you have to install some shell function in your profile and restart your shell before using one of them. Once again, if shells exported some kind of "get/set shell environment" API to their subprocesses, these kind of tools would work the first time you ran them, without any need to modify your shell profile. [0] Actually, in MS-DOS and Windows 3.x/9x/Me, there is both a system-wide current drive, and a separate system-wide current directory for each drive – Windows NT family's cmd.exe simulates this by using a hidden environment variable per a drive to store its current directory, but that is a convention which only cmd.exe supports, the rest of Windows knows nothing about it.
- formerly_proven 4y ago
  > That worked because under DOS, the current directory was system-wide, [0] not per-process, so a subprocess could change the parent process' current directory. For people who don't know DOS: DOS didn't really have multiple processes. COMMAND.COM made part of it resident (TSR), which is how it "survived" invoking other applications. At any given point, there was exactly one process running, and all the state you'd typically associate with a process was "system-wide".