Running an external program and wait for input?

Learning
1

Hey!

I’m writing a small TUI in Notty where it displays a list of grepped files.

I’d like to be able to allow the user to edit their files in $EDITOR and then return to the program, but I can’t figure out how that works with Notty.

So the idea is, you enter a file name, it opens $EDITOR and waits for you to complete and then you reenter the program, reloading the state.

How would you set that up in OCaml?

2

Basically you spawn $EDITOR with the filename as an argument and block until the spawn returns. For example here’s (.mli, .ml) an API that does that.

I would really advise you to use Unix.create_process_*, and then Unix.wait_pid on the result. But if you are fond of escaping problems you might even try to use Sys.command with an appropriate dose of Filename.quote_command.

(And I guess there may be a few reset to peform on your notty rendering)

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3

Excellent, in my main loop with notty, I did

    let pid = Unix.create_process "vi" [||] Unix.stdin Unix.stdout Unix.stderr in
    let _ = Unix.waitpid [] pid in
    main t (x, y)

to test and it worked wonderfully.

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4

Note that create_process won’t work when $EDITOR is the prefix for a shell command, rather than just a path. VS Code, for example, has to be set as EDITOR="code --wait", otherwise it’ll return immediately.

So, to match common usage, you’d have to go with Sys.command, which is admittedly a bit more awkward to get right without quotation bugs/vulnerabilities.

5

If you go with Sys.command, do not forget to take a look at Filename.quote_command which will take care of much of the difficulty around quoting.

Cheers,
Nicolas

6

You don’t have to. If you assume $EDITOR is reasonably specified (doesn’t expect shell expansions) you can also simply parse $EDITOR (which is what the API I linked to does with this function).

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7

So the Unix.create_process solution basically works, except that when I resize the window it crashes with this error:

Fatal error: exception Unix.Unix_error(Unix.EINTR, "wait", "")
Raised by primitive operation at Dune__exe__Main.main in file "bin/main.ml", line 69, characters 12-24
Called from Dune__exe__Main in file "bin/main.ml", line 75, characters 9-42

The logic I have for my launching my program is

  | `Key (`ASCII 'e', []) | `Key (`Enter, []) ->
    (* Editor might be set with extra args, in that case we need to separate them from the editor itself. *)
    let[@warning "-8"] (editor :: args) =
      String.split_on_char ' ' (Sys.getenv "EDITOR")
    in
    let selected_file, _ = get_selected_file files y in
    let full_path_file = execution_directory ^ "/" ^ selected_file in
    let full_args = Array.append (Array.of_list args) [| full_path_file |] in
    Term.cursor t (Some (0, 0));
    let _ =
      Unix.create_process_env
        editor
        full_args
        (Unix.environment ())
        Unix.stdin
        Unix.stdout
        Unix.stderr
    in
    let _ = Unix.wait () in
    Term.cursor t None;
    main t ((x, y), scroll)

Currently my method of extracting arguments might be a bit naive, I just split on space.

I do also have this Notty case

  | `Resize _ -> main t (pos, scroll)

Maybe that’s what is causing the issue?

8

Window resizes trigger the SIGWINCH signal, which interrupts your system call, you need to retry it (example). See the discussion here in the ocamlunix course.

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