promisify-child-process

promisify-child-process

seriously like the best async child process library

Based upon child-process-async, but more thorough, because that package doesn't seem very actively maintained.

promisify-child-process provides a drop-in replacement for the original child_process functions, not just duplicate methods that return a Promise. So when you call exec(...) we still return a ChildProcess instance, just with .then(), .catch(), and .finally() added to make it promise-friendly.

Install and Set-up

npm install --save promisify-child-process

If you are using a old version of Node without built-in Promises or Object.create, you will need to use polyfills (e.g. @babel/polyfill).

// OLD:
const { exec, spawn, fork, execFile } = require('child_process')
// NEW:
const { exec, spawn, fork, execFile } = require('promisify-child-process')

Upgrading to v3

You must now pass maxBuffer or encoding to spawn/fork if you want to capture stdout or stderr.

Resolution/Rejection

The child process promise will only resolve if the process exits with a code of 0. If it exits with any other code, is killed by a signal, or emits an 'error' event, the promise will reject.

Capturing output

exec and execFile capture stdout and stderr by default. But spawn and fork don't capture stdout and stderr unless you pass an encoding or maxBuffer option:

const { spawn } = require('promisify-child-process');

async function() {
  // captures output
  const { stdout, stderr } = await spawn('ls', [ '-al' ], {encoding: 'utf8'});
  const { stdout, stderr } = await spawn('ls', [ '-al' ], {maxBuffer: 200 * 1024});

  // BUG, DOESN'T CAPTURE OUTPUT:
  const { stdout, stderr } = await spawn('ls', [ '-al' ]);
}

Additional properties on rejection errors

If the child process promise rejects, the error may have the following additional properties:

• code - the process' exit code (if it exited)
• signal - the signal the process was killed with (if it was killed)
• stdout - the captured stdout (if output capturing was enabled)
• stderr - the captured stderr (if output capturing was enabled)

Wrapper

If for any reason you need to wrap a ChildProcess you didn't create, you can use the exported promisifyChildProcess function:

const { promisifyChildProcess } = require('promisify-child-process');

async function() {
  const { stdout, stderr } = await promisifyChildProcess(
    some3rdPartyFunctionThatReturnsChildProcess(),
    { encoding: 'utf8' }
  )
}

Examples

exec()

async function() {
  const { stdout, stderr } = await exec('ls -al');
  // OR:
  const child = exec('ls -al', {});
  // do whatever you want with `child` here - it's a ChildProcess instance just
  // with promise-friendly `.then()` & `.catch()` functions added to it!
  child.stdin.write(...);
  child.stdout.pipe(...);
  child.stderr.on('data', (data) => ...);
  const { stdout, stderr } = await child;
}

spawn()

async function() {
  const { stdout, stderr, code } = await spawn('ls', [ '-al' ], {encoding: 'utf8'});
  // OR:
  const child = spawn('ls', [ '-al' ], {});
  // do whatever you want with `child` here - it's a ChildProcess instance just
  // with promise-friendly `.then()` & `.catch()` functions added to it!
  child.stdin.write(...);
  child.stdout.pipe(...);
  child.stderr.on('data', (data) => ...);
  const { stdout, stderr, code } = await child;
}