Creating a subprocess

    • Deno is capable of spawning a subprocess via Deno.run.
    • permission is required to spawn a subprocess.
    • Spawned subprocesses do not run in a security sandbox.
    • Communicate with the subprocess via the , stdout and streams.
    • Use a specific shell by providing its path/name and its string input switch, e.g. Deno.run({cmd: ["bash", "-c", "ls -la"]});
    1. // define command used to create the subprocess
    2. const cmd = ["cmd", "/c", "echo hello"];

    The --allow-run permission is required for creation of a subprocess. Be aware that subprocesses are not run in a Deno sandbox and therefore have the same permissions as if you were to run the command from the command line yourself.

    1. /**
    2.  * subprocess.ts
    3.  */
    4. const fileNames = Deno.args;
    6. const p = Deno.run({
    7.   cmd: [
    8.     "deno",
    9.     "run",
    10.     "--allow-read",
    11.     "https://deno.land/std@$STD_VERSION/examples/cat.ts",
    12.     ...fileNames,
    13.   ],
    15. });
    17. const { code } = await p.status();
    19. // Reading the outputs closes their pipes
    20. const rawOutput = await p.output();
    21. const rawError = await p.stderrOutput();
    23. if (code === 0) {
    24.   await Deno.stdout.write(rawOutput);
    25. } else {
    26.   const errorString = new TextDecoder().decode(rawError);
    27.   console.log(errorString);
    28. }
    30. Deno.exit(code);

    When you run it:

    1. /**
    2.  * subprocess_piping_to_file.ts
    3.  */
    5. import {
    6.   readableStreamFromReader,
    7.   writableStreamFromWriter,
    8. } from "https://deno.land/std@$STD_VERSION/streams/conversion.ts";
    10. // create the file to attach the process to
    11. const file = await Deno.open("./process_output.txt", {
    12.   read: true,
    13.   write: true,
    14.   create: true,
    15. });
    16. const fileWriter = await writableStreamFromWriter(file);
    18. // start the process
    19. const process = Deno.run({
    20.   cmd: ["yes"],
    21.   stdout: "piped",
    22.   stderr: "piped",
    23. });
    25. // example of combining stdout and stderr while sending to a file
    26. const stdout = readableStreamFromReader(process.stdout);
    27. const stderr = readableStreamFromReader(process.stderr);
    28. const joined = mergeReadableStreams(stdout, stderr);
    29. // returns a promise that resolves when the process is killed/closed
    30. joined.pipeTo(fileWriter).then(() => console.log("pipe join done"));
    32. // manually stop process "yes" will never end on its own
    33. setTimeout(async () => {
    34. }, 100);

    Run it: