How to wait for a child process to finish in Node.js?

I'm running a Python script through a child process in Node.js, like this:

require('child_process').exec('python celulas.py', function (error, stdout, stderr) {
    child.stdout.pipe(process.stdout);
});

but Node doesn't wait for it to finish. How can I wait for the process to finish?

EDIT: Is it possible to do this by running the child process in a module I call from the main script?

You should use exec-sync

That allow your script to wait that you exec is done

really easy to use:

var execSync = require('exec-sync');

var user = execSync('python celulas.py');

Take a look at: https://www.npmjs.org/package/exec-sync

Node.js Child Processes: Everything you need to know, We can easily spin a child process using Node's child_process module when the child process exits by registering a handler for the exit event: have synchronous blocking versions that will wait until the child process exits. The child_process.execSync() method is generally identical to child_process.exec() with the exception that the method will not return until the child process has fully closed. When a timeout has been encountered and killSignal is sent, the method won't return until the process has completely exited.

Use exit event for the child process.

var child = require('child_process').exec('python celulas.py')
child.stdout.pipe(process.stdout)
child.on('exit', function() {
  process.exit()
})

PS: It's not really a duplicate, since you don't want to use sync code unless you really really need it.

Node.js Child Processes using spawn, exec, fork & async/await , As ChildProcess inherits from EventEmitter you can register handlers for events on it. child.on('exit', code => { console.log� It's the shorthand for waitpid(-1, NULL, 0);, which will suspends the execution of the calling process until any one child process exits. Here, 1st argument of waitpid indicates wait for any child process to end. In your case, have the parent call it from within your else branch.

NodeJS supports doing this synchronously. Use this:

const exec = require("child_process").execSync;

var result = exec("python celulas.py");

// convert and show the output.
    console.log(result.toString("utf8");

Remember to convert the buffer into a string. Otherwise you'll just be left with hex code.

Wait for all child processes to finish to continue - node.js - html, I would like to know if it is possible to wait for all child process created using the spawn function to finish before continuing execution. I have a code looking like� How to wait for function to finish before continuning in Node.js Tag: javascript , node.js , asynchronous I am trying to create a route in Node.js/Express that reads data from two queries and then increments a count based on that data from the queires.

You need to remove the listeners exec installs to add to the buffered stdout and stderr, even if you pass no callback it still buffers the output. Node will still exit the child process in the buffer is exceeded in this case.

var child = require('child_process').exec('python celulas.py');
child.stdout.removeAllListeners("data");
child.stderr.removeAllListeners("data");
child.stdout.pipe(process.stdout);
child.stderr.pipe(process.stderr);

Child process, The child_process module provides the ability to spawn child processes in a manner that is similar, By default, the parent will wait for the detached child to exit. If you find yourself wishing you could have your Node.js process start another program for you, then look no further than the child_process module. The simplest way is the "fire, forget, and buffer" method using child_process.exec. It runs your process, buffers its output (up to a default maximum of 200kb), and lets you access it from a callback when it is finished.

In my opinion, the best way to handle this is by implementing an event emitter. When the first spawn finishes, emit an event that indicates that it is complete.

const { spawn } = require('child_process');
const events = require('events');
const myEmitter = new events.EventEmitter();


firstSpawn = spawn('echo', ['hello']);
firstSpawn.on('exit'), (exitCode) => {
    if (parseInt(code) !== 0) {
        //Handle non-zero exit
    }
    myEmitter.emit('firstSpawn-finished');
}

myEmitter.on('firstSpawn-finished', () => {
    secondSpawn = spawn('echo', ['BYE!'])
})

Child process, That doesn't affect node.js but it means data you send to the child process may not be immediately By default, the parent will wait for the detached child to exit. Find answers to Node Js -- Wait for Function to complete before ("yitt") process you run a new thread but your code don't wait this thread to finish it's task

Working with stdout and stdin of a child process in Node.js, In line B, we are waiting until the process is completely finished. Waiting for a child process to exit via a Promise #. Function onExit() looks as� by Samer Buna Node.js Child Processes: Everything you need to know How to use spawn(), exec(), execFile(), and fork() Screenshot captured from my Pluralsight course — Advanced Node.js> Update: This article is now part of my book “Node.js Beyond The Basics”.

wait for process with pid=pid to exit � Issue #1132 � nodejs/help , sorry, I wasn't clear - the process that I want to wait for is not a child process of my node process - all I have is the pid. I found an answer here: Node.js runs in a single thread. You can, however take advantage of multiple processes. child_process module allows to create child processes in Node.js. Those processes can easily communicate with each other using a built-in messaging system. There are four different ways to create a child process in Node: spawn(), fork(), exec(), and execFile().

Nodejs does not wait for promise resolution, Also node exits as everything is ok with exit code 0. As promises are a V8 internal thing and the node.js mother process doesn't know new Promise(( resolve, reject) => { // fork() and more child process things in here }); }. Exiting from a Node.js program started at the command line can be as simple as waiting for the script to finish executing. Implicitly, the Node.js process will exit when it reaches the end of the script. You can see this by running the following scipt:

Comments
  • possible duplicate of node.js execute system command synchronously
  • Although this answer is correct, I highly recommend not using a synchronous method in node.js. This will completely block the application until the command is completed. See npmjs.org/package/execSync. It says to not use this for production.
  • Trying to figure out how to install this, but I get a wild list of errors when I try to run the npm install on this particular library.
  • @BryanJohnson perhaps a suggestion on what to use would be helpful.
  • @A.Wentzel I recommend a non-synchronous call described in alex's answer. execSync can work for simple scripts, but it seems I always end up regretting using synchronous methods.
  • exec-sync is broken now(2019). Just you the "vanilla" method. I posted it as an answer on this thread.
  • This is a better answer. You should avoid blocking (synchronous) code in Node wherever possible - it halts your entire program whilst waiting for the child process to finish.
  • There's also an 'end' event, maybe different events are emitted for different calls (exec(), spawn()...)
  • The exit event isn't guaranteed to be fired. For instance, if the process failed to spawn, the error event is called. However, if the process did spawn and another error is caused, both handlers will be called.
  • This is the correct answer. Current correct answer blocks child process logs which are important.
  • This is the the answer to some other, opposite, question.