Creating a Subprocess

Concepts

• Deno is capable of spawning a subprocess via Deno.Command.
• --allow-run permission is required to spawn a subprocess.
• Spawned subprocesses do not run in a security sandbox.
• Communicate with the subprocess via the stdin, stdout and stderr streams.

Simple example

This example is the equivalent of running 'echo hello' from the command line.

/**
 * subprocess_simple.ts
 */

// define command used to create the subprocess
const command = new Deno.Command(Deno.execPath(), {
  args: [
    "eval",
    "console.log('hello'); console.error('world')",
  ],
});

// create subprocess and collect output
const { code, stdout, stderr } = await command.output();

console.assert(code === 0);
console.assert("world\n" === new TextDecoder().decode(stderr));
console.log(new TextDecoder().decode(stdout));

Run it:

$ deno run --allow-run --allow-read ./subprocess_simple.ts
hello

Security

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.

Communicating with subprocesses

By default when you use Deno.Command() the subprocess inherits stdin, stdout and stderr of the parent process. If you want to communicate with started a subprocess you must use the "piped" option.

Piping to files

This example is the equivalent of running yes &> ./process_output in bash.

/**
 * subprocess_piping_to_file.ts
 */

import {
  mergeReadableStreams,
} from "https://deno.land/std@0.219.0/streams/merge_readable_streams.ts";

// create the file to attach the process to
const file = await Deno.open("./process_output.txt", {
  read: true,
  write: true,
  create: true,
});

// start the process
const command = new Deno.Command("yes", {
  stdout: "piped",
  stderr: "piped",
});

const process = command.spawn();

// example of combining stdout and stderr while sending to a file
const joined = mergeReadableStreams(
  process.stdout,
  process.stderr,
);

// returns a promise that resolves when the process is killed/closed
joined.pipeTo(file.writable).then(() => console.log("pipe join done"));

// manually stop process "yes" will never end on its own
setTimeout(() => {
  process.kill();
}, 100);

Run it:

$ deno run --allow-run --allow-read --allow-write ./subprocess_piping_to_file.ts