promisify-child-process

What is promisify-child-process?

The promisify-child-process package is a utility that allows you to use Node.js child processes with promises, making it easier to work with asynchronous operations. It wraps the standard child process methods to return promises, enabling the use of async/await syntax for cleaner and more readable code.

What are promisify-child-process's main functionalities?

Promisified exec

This feature allows you to execute shell commands using the exec function and handle the result with promises. The code sample demonstrates how to run the 'ls' command and handle its output using async/await.

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

async function runCommand() {
  try {
    const { stdout, stderr } = await exec('ls');
    console.log('Output:', stdout);
  } catch (error) {
    console.error('Error:', error);
  }
}

runCommand();

Promisified spawn

This feature allows you to spawn a new process using the spawn function and handle its lifecycle with promises. The code sample demonstrates how to spawn a process to list directory contents and handle its output and errors.

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

async function runSpawn() {
  const child = spawn('ls', ['-lh', '/usr']);

  child.stdout.on('data', (data) => {
    console.log(`stdout: ${data}`);
  });

  child.stderr.on('data', (data) => {
    console.error(`stderr: ${data}`);
  });

  try {
    await child;
    console.log('Process completed');
  } catch (error) {
    console.error('Error:', error);
  }
}

runSpawn();

Other packages similar to promisify-child-process

child-process-promise

The child-process-promise package provides similar functionality by wrapping Node.js child process methods with promises. It offers a similar API for exec and spawn, allowing for promise-based handling of child processes. Compared to promisify-child-process, it also focuses on providing a promise-based interface but may have different implementation details and additional features.

execa

Execa is a higher-level wrapper around child processes that provides a promise-based API. It offers more features than promisify-child-process, such as improved error handling, cross-platform compatibility, and additional options for running commands. Execa is often preferred for more complex use cases due to its extensive feature set.

README

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;
}