如何使用 Bluebird 保证 Node 的 child_process.exec 和 child_process.execFile 函数？

Question

I'm using the Bluebird promise library under Node.js, it's great! But I have a question:

If you take a look at the documentation of Node's child_process.exec and child_process.execFile you can see that both of these functions are returning a ChildProcess object.

So what's the recommended way to promisify such functions?

Note that the following works (I get a Promise object):

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

But how can one get access to the original return value of the original Node.js functions? (In these cases I would need to be able to access the originally returned ChildProcess objects.)

Any suggestion would be appreciated!

EDIT:

Here is an example code which is using the return value of the child_process.exec function:

var exec = require('child_process').exec;
var child = exec('node ./commands/server.js');
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);
});

But if I would use the promisified version of the exec function ( execAsync from above ) then the return value will be a promise, not a ChildProcess object. This is the real problem I am talking about.

Answer 1

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 .

Answer 2

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);
});

Answer 3

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);
}

Answer 4

Since Node v12 the built-in util.promisify allows access to the ChildProcess object in the returned Promise for built-in functions where it would have been returned by the un-promisified call. From the docs :

The returned ChildProcess instance is attached to the Promise as a child property.

This correctly and simply satisfies the need to access ChildProcess in the original question and makes other answers out of date providing that Node v12+ can be used.

Adapting the example (and concise style) provided by the questioner, access to the ChildProcess can be achieved like:

const util = require('util');
const exec = util.promisify(require('child_process').exec);
const promise = exec('node ./commands/server.js');
const child = promise.child; 

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);
});

// i.e. can then await for promisified exec call to complete
const { stdout, stderr } = await promise;

Answer 5

There's probably not a way to do nicely that covers all use cases. But for limited cases, you can do something like this:

/**
 * Promisified child_process.exec
 *
 * @param cmd
 * @param opts See child_process.exec node docs
 * @param {stream.Writable} opts.stdout If defined, child process stdout will be piped to it.
 * @param {stream.Writable} opts.stderr If defined, child process stderr will be piped to it.
 *
 * @returns {Promise<{ stdout: string, stderr: stderr }>}
 */
function execp(cmd, opts) {
    opts || (opts = {});
    return new Promise((resolve, reject) => {
        const child = exec(cmd, opts,
            (err, stdout, stderr) => err ? reject(err) : resolve({
                stdout: stdout,
                stderr: stderr
            }));

        if (opts.stdout) {
            child.stdout.pipe(opts.stdout);
        }
        if (opts.stderr) {
            child.stderr.pipe(opts.stderr);
        }
    });
}

This accepts opts.stdout and opts.stderr arguments, so that stdio can be captured from the child process.

For example:

execp('ls ./', {
    stdout: new stream.Writable({
        write: (chunk, enc, next) => {
            console.log(chunk.toString(enc));
            next();
        }
    }),
    stderr: new stream.Writable({
        write: (chunk, enc, next) => {
            console.error(chunk.toString(enc));
            next();
        }
    })
}).then(() => console.log('done!'));

Or simply:

execp('ls ./', {
    stdout: process.stdout,
    stderr: process.stderr
}).then(() => console.log('done!'));

Answer 6

Just want to mention that there's a nice tool that will solve your problem completely:

https://www.npmjs.com/package/core-worker

This package makes it a lot easier to handle processes.

import { process } from "CoreWorker";
import fs from "fs";

const result = await process("node Server.js", "Server is ready.").ready(1000);
const result = await process("cp path/to/file /newLocation/newFile").death();

or combine these functions:

import { process } from "core-worker";

const simpleChat = process("node chat.js", "Chat ready");

setTimeout(() => simpleChat.kill(), 360000); // wait an hour and close the chat

simpleChat.ready(500)
    .then(console.log.bind(console, "You are now able to send messages."))
    .then(::simpleChat.death)
    .then(console.log.bind(console, "Chat closed"))
    .catch(() => /* handle err */);

Answer 7

Here's mine. It doesn't deal with stdin or stdout, so if you need those then use one of the other answers on this page. :)

// promisify `child_process`
// This is a very nice trick :-)
this.promiseFromChildProcess = function (child) {
    return new Promise((resolve, reject) => {
        child.addListener('error', (code, signal) => {
            console.log('ChildProcess error', code, signal);
            reject(code);
        });
        child.addListener('exit', (code, signal) => {
            if (code === 0) {
                resolve(code);
            } else {
                console.log('ChildProcess error', code, signal);
                reject(code);
            }
        });
    });
};

Answer 8

Here are my two cents. Uses spawn which streams the output and writes to stdout and stderr . The error and standard output is captured in buffers and are returned or rejected.

This is written I Typescript, feel free to remove typings if using JavaScript:

import { spawn, SpawnOptionsWithoutStdio } from 'child_process'

const spawnAsync = async (
  command: string,
  options?: SpawnOptionsWithoutStdio
) =>
  new Promise<Buffer>((resolve, reject) => {
    const [spawnCommand, ...args] = command.split(/\s+/);
    const spawnProcess = spawn(spawnCommand, args, options);
    const chunks: Buffer[] = [];
    const errorChunks: Buffer[] = [];
    spawnProcess.stdout.on("data", (data) => {
      process.stdout.write(data.toString());
      chunks.push(data);
    });
    spawnProcess.stderr.on("data", (data) => {
      process.stderr.write(data.toString());
      errorChunks.push(data);
    });
    spawnProcess.on("error", (error) => {
      reject(error);
    });
    spawnProcess.on("close", (code) => {
      if (code === 1) {
        reject(Buffer.concat(errorChunks).toString());
        return;
      }
      resolve(Buffer.concat(chunks));
    });
  });

Answer 9

Just another example you might run into issues when running multiple commands when destructuring with the same const's you can rename them like this.

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

async function runCommands() {
    try {
        const { stdout, stderr } = await exec('ls');
        console.log('stdout:', stdout);
        console.log('stderr:', stderr);

        const { stdout: stdoutTwo, stderr: stderrTwo } = await exec('ls');
        console.log('stdoutTwo:', stdoutTwo);
        console.log('stderrTwo:', stderrTwo);

        const { stdout: stdoutThree, stderr: stderrThree } = await exec('ls');
        console.log('stdoutThree:', stdoutThree);
        console.log('stderrThree:', stderrThree);

    } catch (e) {
        console.error(e); // should contain code (exit code) and signal (that caused the termination).
    }
}
runCommands()