How to promisify Node's child_process.exec and child_process.execFile functions with Bluebird?

I would recommend using standard JS promises built into the language over an additional library dependency like Bluebird.

If you're using Node 10+, the Node.js docs recommend using util.promisify which returns a Promise<{ stdout, stderr }> object. See an example below:

const util = require('util');
const exec = util.promisify(require('child_process').exec);

async function lsExample() {
  try {
    const { stdout, stderr } = await exec('ls');
    console.log('stdout:', stdout);
    console.log('stderr:', stderr);
  } catch (e) {
    console.error(e); // should contain code (exit code) and signal (that caused the termination).
  }
}
lsExample()

Handle errors first from stderr.

It sounds like you'd like to return two things from the call:

• the ChildProcess
• a promise that resolves when the ChildProcess completes

So "the recommended way to promisify such functions"? Don't.

You're outside the convention. Promise returning functions are expected to return a promise, and that's it. You could return an object with two members (the ChildProcess & the promise), but that'll just confuse people.

I'd suggest calling the unpromisified function, and creating a promise based off the returned childProcess. (Maybe wrap that into a helper function)

This way, it's quite explicit for the next person who reads the code.

Something like:

var Promise = require('bluebird');
var exec = require('child_process').execFile;

function promiseFromChildProcess(child) {
    return new Promise(function (resolve, reject) {
        child.addListener("error", reject);
        child.addListener("exit", resolve);
    });
}

var child = exec('ls');

promiseFromChildProcess(child).then(function (result) {
    console.log('promise complete: ' + result);
}, function (err) {
    console.log('promise rejected: ' + err);
});

child.stdout.on('data', function (data) {
    console.log('stdout: ' + data);
});
child.stderr.on('data', function (data) {
    console.log('stderr: ' + data);
});
child.on('close', function (code) {
    console.log('closing code: ' + code);
});

Here's another way:

function execPromise(command) {
    return new Promise(function(resolve, reject) {
        exec(command, (error, stdout, stderr) => {
            if (error) {
                reject(error);
                return;
            }

            resolve(stdout.trim());
        });
    });
}

Use the function:

execPromise(command).then(function(result) {
    console.log(result);
}).catch(function(e) {
    console.error(e.message);
});

Or with async/await:

try {
    var result = await execPromise(command);
} catch (e) {
    console.error(e.message);
}