spawn-async

A cross-platform version of Node's child_process.spawn as an async function that returns a promise. Supports Node 12 LTS and up.

Usage:

import spawnAsync from '@expo/spawn-async';

(async function () {
  let resultPromise = spawnAsync('echo', ['hello', 'world']);
  let spawnedChildProcess = resultPromise.child;
  try {
    let {
      pid,
      output: [stdout, stderr],
      stdout,
      stderr,
      status,
      signal,
    } = await resultPromise;
  } catch (e) {
    console.error(e.stack);
    // The error object also has the same properties as the result object
  }
})();

API

spawnAsync takes the same arguments as child_process.spawn. Its options are the same as those of child_process.spawn plus:

• ignoreStdio: whether to ignore waiting for the child process's stdio streams to close before resolving the result promise. When ignoring stdio, the returned values for stdout and stderr will be empty strings. The default value of this option is false.
• maxBuffer: the maximum bytes retained from stdout and stderr (independently). Output is collected with a sliding window. When set explicitly, exceeding it rejects the promise with an error whose code is ERR_CHILD_PROCESS_STDIO_MAXBUFFER and whose stdout/stderr carry the truncated tail. When omitted, the default is buffer.constants.MAX_STRING_LENGTH (~512 MiB).

It returns a promise whose result is an object with these properties:

• pid: the process ID of the spawned child process
• output: an array with stdout and stderr's output
• stdout: a string of what the child process wrote to stdout
• stderr: a string of what the child process wrote to stderr
• status: the exit code of the child process
• signal: the signal (ex: SIGTERM) used to stop the child process if it did not exit on its own

If there's an error running the child process or it exits with a non-zero status code, spawnAsync rejects the returned promise. The Error object also has the properties listed above.

Accessing the child process

Sometimes you may want to access the child process object--for example, if you wanted to attach event handlers to stdio or stderr and process data as it is available instead of waiting for the process to be resolved.

You can do this by accessing .child on the Promise that is returned by spawnAsync.

Here is an example:

(async () => {
  let ffmpeg$ = spawnAsync('ffmpeg', ['-i', 'path/to/source.flac', '-codec:a', 'libmp3lame', '-b:a', '320k', '-ar', '44100', 'path/to/output.mp3']);
  let childProcess = ffmpeg$.child;
  childProcess.stdout.on('data', (data) => {
    console.log(`ffmpeg stdout: ${data}`);
  });
  childProcess.stderr.on('data', (data) => {
    console.error(`ffmpeg stderr: ${data}`);
  });
  let result = await ffmpeg$;
  console.log(`ffmpeg pid ${result.pid} exited with code ${result.code}`);
})();

Notes

maxBuffer

maxBuffer is a later addition to the API. Set it when child output could exhaust memory and crash the parent process, or when the command or arguments are influenced by untrusted input — an attacker can otherwise force unbounded output to crash the parent.

The default of buffer.constants.MAX_STRING_LENGTH (~512 MiB) is a crash-safe floor, not a memory bound: at that size the materialized string itself can still exhaust process memory.

When maxBuffer is set explicitly, exceeding it rejects the promise immediately with ERR_CHILD_PROCESS_STDIO_MAXBUFFER. When left at the default, exceeding it doesn't reject; the sliding-window tail is still readable, but reading stdout/stderr throws ERR_CHILD_PROCESS_STDIO_MAXBUFFER with the truncated tail attached.