Node.js spawn child process and get terminal output live - node.js

I have a script that outputs 'hi', sleeps for a second, outputs 'hi', sleeps for 1 second, and so on and so forth. Now I thought I would be able to tackle this problem with this model.
var spawn = require('child_process').spawn,
temp = spawn('PATH TO SCRIPT WITH THE ABOVE BEHAVIOUR');
temp.stdout.pipe(process.stdout);
Now the problem is that the task needs to be finished in order for the output to be displayed. As I am understanding it, this is due to the fact that the newly spawned process takes execution control. Obviously node.js does not support threads so any solutions? My idea was to possibly run two instances, first one for the specific purpose of creating the task and have it pipe the output to process of the second instance, considering this can be achieved.

It's much easier now (6 years later)!
Spawn returns a childObject, which you can then listen for events with. The events are:
Class: ChildProcess
Event: 'error'
Event: 'exit'
Event: 'close'
Event: 'disconnect'
Event: 'message'
There are also a bunch of objects from childObject, they are:
Class: ChildProcess
child.stdin
child.stdout
child.stderr
child.stdio
child.pid
child.connected
child.kill([signal])
child.send(message[, sendHandle][, callback])
child.disconnect()
See more information here about childObject: https://nodejs.org/api/child_process.html
Asynchronous
If you want to run your process in the background while node is still able to continue to execute, use the asynchronous method. You can still choose to perform actions after your process completes, and when the process has any output (for example if you want to send a script's output to the client).
child_process.spawn(...); (Node v0.1.90)
var spawn = require('child_process').spawn;
var child = spawn('node ./commands/server.js');
// You can also use a variable to save the output
// for when the script closes later
var scriptOutput = "";
child.stdout.setEncoding('utf8');
child.stdout.on('data', function(data) {
//Here is where the output goes
console.log('stdout: ' + data);
data=data.toString();
scriptOutput+=data;
});
child.stderr.setEncoding('utf8');
child.stderr.on('data', function(data) {
//Here is where the error output goes
console.log('stderr: ' + data);
data=data.toString();
scriptOutput+=data;
});
child.on('close', function(code) {
//Here you can get the exit code of the script
console.log('closing code: ' + code);
console.log('Full output of script: ',scriptOutput);
});
Here's how you would use a callback + asynchronous method:
var child_process = require('child_process');
console.log("Node Version: ", process.version);
run_script("ls", ["-l", "/home"], function(output, exit_code) {
console.log("Process Finished.");
console.log('closing code: ' + exit_code);
console.log('Full output of script: ',output);
});
console.log ("Continuing to do node things while the process runs at the same time...");
// This function will output the lines from the script
// AS is runs, AND will return the full combined output
// as well as exit code when it's done (using the callback).
function run_script(command, args, callback) {
console.log("Starting Process.");
var child = child_process.spawn(command, args);
var scriptOutput = "";
child.stdout.setEncoding('utf8');
child.stdout.on('data', function(data) {
console.log('stdout: ' + data);
data=data.toString();
scriptOutput+=data;
});
child.stderr.setEncoding('utf8');
child.stderr.on('data', function(data) {
console.log('stderr: ' + data);
data=data.toString();
scriptOutput+=data;
});
child.on('close', function(code) {
callback(scriptOutput,code);
});
}
Using the method above, you can send every line of output from the script to the client (for example using Socket.io to send each line when you receive events on stdout or stderr).
Synchronous
If you want node to stop what it's doing and wait until the script completes, you can use the synchronous version:
child_process.spawnSync(...); (Node v0.11.12+)
Issues with this method:
If the script takes a while to complete, your server will hang for
that amount of time!
The stdout will only be returned once the script
has finished running. Because it's synchronous, it cannot continue
until the current line has finished. Therefore it's unable to capture
the 'stdout' event until the spawn line has finished.
How to use it:
var child_process = require('child_process');
var child = child_process.spawnSync("ls", ["-l", "/home"], { encoding : 'utf8' });
console.log("Process finished.");
if(child.error) {
console.log("ERROR: ",child.error);
}
console.log("stdout: ",child.stdout);
console.log("stderr: ",child.stderr);
console.log("exist code: ",child.status);

I'm still getting my feet wet with Node.js, but I have a few ideas. first, I believe you need to use execFile instead of spawn; execFile is for when you have the path to a script, whereas spawn is for executing a well-known command that Node.js can resolve against your system path.
1. Provide a callback to process the buffered output:
var child = require('child_process').execFile('path/to/script', [
'arg1', 'arg2', 'arg3',
], function(err, stdout, stderr) {
// Node.js will invoke this callback when process terminates.
console.log(stdout);
});
2. Add a listener to the child process' stdout stream (9thport.net)
var child = require('child_process').execFile('path/to/script', [
'arg1', 'arg2', 'arg3' ]);
// use event hooks to provide a callback to execute when data are available:
child.stdout.on('data', function(data) {
console.log(data.toString());
});
Further, there appear to be options whereby you can detach the spawned process from Node's controlling terminal, which would allow it to run asynchronously. I haven't tested this yet, but there are examples in the API docs that go something like this:
child = require('child_process').execFile('path/to/script', [
'arg1', 'arg2', 'arg3',
], {
// detachment and ignored stdin are the key here:
detached: true,
stdio: [ 'ignore', 1, 2 ]
});
// and unref() somehow disentangles the child's event loop from the parent's:
child.unref();
child.stdout.on('data', function(data) {
console.log(data.toString());
});

Here is the cleanest approach I've found:
require("child_process").spawn('bash', ['./script.sh'], {
cwd: process.cwd(),
detached: true,
stdio: "inherit"
});

I had a little trouble getting logging output from the "npm install" command when I spawned npm in a child process. The realtime logging of dependencies did not show in the parent console.
The simplest way to do what the original poster wants seems to be this (spawn npm on windows and log everything to parent console):
var args = ['install'];
var options = {
stdio: 'inherit' //feed all child process logging into parent process
};
var childProcess = spawn('npm.cmd', args, options);
childProcess.on('close', function(code) {
process.stdout.write('"npm install" finished with code ' + code + '\n');
});

PHP-like passthru
import { spawn } from 'child_process';
export default async function passthru(exe, args, options) {
return new Promise((resolve, reject) => {
const env = Object.create(process.env);
const child = spawn(exe, args, {
...options,
env: {
...env,
...options.env,
},
});
child.stdout.setEncoding('utf8');
child.stderr.setEncoding('utf8');
child.stdout.on('data', data => console.log(data));
child.stderr.on('data', data => console.log(data));
child.on('error', error => reject(error));
child.on('close', exitCode => {
console.log('Exit code:', exitCode);
resolve(exitCode);
});
});
}
Usage
const exitCode = await passthru('ls', ['-al'], { cwd: '/var/www/html' })

child:
setInterval(function() {
process.stdout.write("hi");
}, 1000); // or however else you want to run a timer
parent:
require('child_process').fork('./childfile.js');
// fork'd children use the parent's stdio

I found myself requiring this functionality often enough that I packaged it into a library called std-pour. It should let you execute a command and view the output in real time. To install simply:
npm install std-pour
Then it's simple enough to execute a command and see the output in realtime:
const { pour } = require('std-pour');
pour('ping', ['8.8.8.8', '-c', '4']).then(code => console.log(`Error Code: ${code}`));
It's promised based so you can chain multiple commands. It's even function signature-compatible with child_process.spawn so it should be a drop in replacement anywhere you're using it.

Adding a sample for exec as I too had needed live feedback and wasn't getting any until after the script finished. exec does return an EventEmitter, contrary to the many claims that only spawn works in such a way.
This supplements the comment I made to the accepted answer more thoroughly.
The interface for exec is similar to spawn:
// INCLUDES
import * as childProcess from 'child_process'; // ES6 Syntax
// DEFINES
let exec = childProcess.exec; // Use 'var' for more proper
// semantics, or 'const' it all
// if that's your thing; though 'let' is
// true-to-scope;
// Return an EventEmitter to work with, though
// you can also chain stdout too:
// (i.e. exec( ... ).stdout.on( ... ); )
let childProcess = exec
(
'./binary command -- --argument argumentValue',
( error, stdout, stderr ) =>
{ // When the process completes:
if( error )
{
console.log( `${error.name}: ${error.message}` );
console.log( `[STACK] ${error.stack}` );
}
console.log( stdout );
console.log( stderr );
callback(); // Gulp stuff
}
);
Now its as simple as registering an event handler for stdout:
childProcess.stdout.on( 'data', data => console.log( data ) );
And for stderr:
childProcess.stderr.on( 'data', data => console.log( `[ERROR]: ${data}` ) );
You can also pipe stdout to the main process' stdout:
childProcess.stdout.pipe( process.stdout );
Not too bad at all - HTH

I was interested into running a script that gets the input and outputs from my terminal, and that will close my process once the child script finishes.
import { spawn } from 'node:child_process'
import process from 'node:process'
const script = spawn('path/to/script', { stdio: 'inherit' })
script.on('close', process.exit)

I ran into a situation where none of the above worked when I was spawning a Python 3 script. I would get data from stdout, but only once the child terminated.
As it turns out, Python buffers stdout by default. It's possible to disable stdout buffering by including -u as a command line parameter to python3.

Related

Nodejs listing directories with child_process.spawn ends before receiving all data

const spawn = require('child_process').spawn;
function main() {
ls = spawn('dir', {shell: true});
ls.stdout.on('data', data => console.log('DATA RECEIVED'));
ls.on('exit', code => console.log('EXITED'));
}
main();
This runs on Windows, and the result is
DATA RECEIVED
EXITED
DATA RECEIVED
Why is EXITED printed out before the last line? Is there any way safe to check whether all the data is received or not?

Node run command that prompts the user

I'm trying to run a command (aws configure) from a .js file called with node. I've been using child_process which allows me to execute a command or batch file. However when that file would normally prompt the user for something like a username and password, I am unable to have that interaction take place.
I tried using process.stdin with node in order to perhaps use node as the middle man of a user's input, but I am unable to link the two together.
If anyone has any ideas that would be great!
var spawn = require('child_process').spawn;
var mystdin = process.stdin;
var conf = spawn('aws configure', {input: mystdin, shell: true});
//When command returns
conf.stdout.on('data', function(data) {
console.log('stdout: ' + data);
//prompt for input
mystdin.resume();
mystdin.setEncoding('utf8');
var util = require('util');
mystdin.on('data', function (text) {
//INSERT ANSWER HERE
console.log('received data:', util.inspect(text));
if (text === 'quit\n') {
done();
}
});
function done() {
console.log('Now that process.stdin is paused, there is nothing more to do.');
process.exit();
}
});
conf.stderr.on('data', function(data) {
console.log('stdout: ' + data);
});
conf.on('close', function(code) {
console.log('child precess exited with code ' + code);
});
This is just the path I was currently going down. If anyone has a better suggestion let me know!
Thanks
Two ideas: first, I think the arguments are supposed to be in the second parameters. Second, maybe the aws command doesn't realize it is an interactive terminal or something. You may try with child_process like that, or if that doesn't work, with pty.js:
var pty = require('pty.js');
var term = pty.spawn('aws', ['configure'], {
name: 'xterm-color',
cols: 80,
rows: 30,
cwd: process.env.HOME,
env: process.env
});
term.on('data', function(data) {
console.log(data);
});

execute a node command from a grunt task

I have a grunt task from which I would like to run a node command. The command is not giving any error when I run it, but I was expecting some console output from the task, which I don't seem to be getting at all.
What am I missing in order to run this node task?
grunt.registerTask('asyncfoo', 'My "asyncfoo" task.', function() {
// Force task into async mode and grab a handle to the "done" function.
var done = this.async();
// Run some sync stuff.
grunt.log.writeln('Processing task...');
grunt.util.spawn({ cmd: 'node', args: ['S3ListBuckets.js']});
// And some async stuff.
setTimeout(function() {
grunt.log.writeln('All done!');
done();
}, 1000);
});
!-- if someone else is wanting to do something similar here is the code
module.exports = function(grunt) {
grunt.registerTask('asyncfoo', 'My "asyncfoo" task.', function() {
// Force task into async mode and grab a handle to the "done" function.
var done = this.async();
// Run some sync stuff.
grunt.log.writeln('Processing task...');
grunt.util.spawn({ cmd: 'node', args: ['S3ListBuckets.js'], opts: {stdio: 'inherit'}});
});
};
!-- list buckets
var fs = require('fs');
var aws = require('aws-sdk');
aws.config.loadFromPath('./grunt-aws.json');
var s3 = new aws.S3();
s3.listBuckets(function (err, data) {
if (err) {
console.log("Error:", err);
}
else {
for (var index in data.Buckets) {
var bucket = data.Buckets[index];
console.log("Bucket: ", bucket.Name, ' : ', bucket.CreationDate);
}
}
});
The answer https://stackoverflow.com/a/15045126/519995 suggests using the parameter opts: {stdio: 'inherit'} to have the spawned output streamed into the parent output stream.
That same answer also lists other alternatives: listening to data event, or piping the streams as you wish.
Also, using timeouts to wait for async tasks is NOT a good idea. If all you are waiting for is the spawned process you can use a callback to know when its done. If you have more complex sync I suggest starting a new StackOverflow question.

Node Kue and Child Process - get error from spawned process

I try to spawn a child process performing cpu intensive calculations through a job queue with Kue. My code at the moment looks like this:
consumer.js
var kue = require('kue');
var util = require('util');
var spawn = require('child_process').spawn;
var jobs = kue.createQueue();
jobs.process('calc', 2, function(job, done){
var work = spawn('Rscript', ['opti2.R', job.data.file]);
work.stderr.on('data', function (data) {
job.log('stderr: ' + data);
});
work.stdout.on('data', function (data) {
job.log('stdout: ' + data);
});
work.on('exit', function (code, signal) {
console.log('child process exited with code ' + code + ' with singal ' + signal);
if(code != 0){
done(****How to get the stderr of the child process as an error here***);
} else {
done(Error());
}
});
});
The code somewhat do what i would like it to do, but is there a better way to report the job as failed (to Kue) and get the stderr from the spawned process?
You can use job.log method to send data directly to Kue.
I would also recommend you to switch from .spawn to .exec, because it returns stdout and stderr as strings in its final callback along with a good error, which suits your needs well:
var exec = require('child_process').exec;
jobs.process('calc', 2, function(job, done){
exec('Rscript opti2.R ' + job.data.file, function (error, stdout, stderr) {
if (stdout.length > 0) job.log('stdout: ' + stdout);
if (stderr.length > 0) job.log('stderr: ' + stderr);
done(error);
});
});
Though solution should work with .spawn as well: simply replace each console.log call in your code with job.log.
Though, you may want to bufferize your stderr in order to send it to Kue in one chunk:
jobs.process('calc', 2, function(job, done){
var work = spawn('Rscript', ['opti2.R', job.data.file]);
var stderr = '';
work.stderr.on('data', function (data) {
stderr += data;
});
work.stdout.on('data', function (data) {
job.log(data); // sending arriving `stdout` chunks as normal log events
});
work.on('close', function (code, signal) {
console.log('child process exited with code ' + code + ' with singal ' + signal);
if(code != 0){
done(stderr); // sending all collected stderr as an explanation
} else {
done();
}
});
});
I would also recommend using close event instead of exit, because it waits for child's stdio streams.
For more information see Event: 'exit' docs:
This event is emitted after the child process ends.
Note that the child process stdio streams might still be open.
and Event: 'close' docs:
This event is emitted when the stdio streams of a child process have
all terminated.

Exec : display stdout "live"

I have this simple script :
var exec = require('child_process').exec;
exec('coffee -cw my_file.coffee', function(error, stdout, stderr) {
console.log(stdout);
});
where I simply execute a command to compile a coffee-script file. But stdout never get displayed in the console, because the command never ends (because of the -w option of coffee).
If I execute the command directly from the console I get message like this :
18:05:59 - compiled my_file.coffee
My question is : is it possible to display these messages with the node.js exec ? If yes how ? !
Thanks
Don't use exec. Use spawn which is an EventEmmiter object. Then you can listen to stdout/stderr events (spawn.stdout.on('data',callback..)) as they happen.
From NodeJS documentation:
var spawn = require('child_process').spawn,
ls = spawn('ls', ['-lh', '/usr']);
ls.stdout.on('data', function (data) {
console.log('stdout: ' + data.toString());
});
ls.stderr.on('data', function (data) {
console.log('stderr: ' + data.toString());
});
ls.on('exit', function (code) {
console.log('child process exited with code ' + code.toString());
});
exec buffers the output and usually returns it when the command has finished executing.
exec will also return a ChildProcess object that is an EventEmitter.
var exec = require('child_process').exec;
var coffeeProcess = exec('coffee -cw my_file.coffee');
coffeeProcess.stdout.on('data', function(data) {
console.log(data);
});
OR pipe the child process's stdout to the main stdout.
coffeeProcess.stdout.pipe(process.stdout);
OR inherit stdio using spawn
spawn('coffee -cw my_file.coffee', { stdio: 'inherit' });
There are already several answers however none of them mention the best (and easiest) way to do this, which is using spawn and the { stdio: 'inherit' } option. It seems to produce the most accurate output, for example when displaying the progress information from a git clone.
Simply do this:
var spawn = require('child_process').spawn;
spawn('coffee', ['-cw', 'my_file.coffee'], { stdio: 'inherit' });
Credit to #MorganTouvereyQuilling for pointing this out in this comment.
Inspired by Nathanael Smith's answer and Eric Freese's comment, it could be as simple as:
var exec = require('child_process').exec;
exec('coffee -cw my_file.coffee').stdout.pipe(process.stdout);
I'd just like to add that one small issue with outputting the buffer strings from a spawned process with console.log() is that it adds newlines, which can spread your spawned process output over additional lines. If you output stdout or stderr with process.stdout.write() instead of console.log(), then you'll get the console output from the spawned process 'as is'.
I saw that solution here:
Node.js: printing to console without a trailing newline?
Hope that helps someone using the solution above (which is a great one for live output, even if it is from the documentation).
I have found it helpful to add a custom exec script to my utilities that do this.
utilities.js
const { exec } = require('child_process')
module.exports.exec = (command) => {
const process = exec(command)
process.stdout.on('data', (data) => {
console.log('stdout: ' + data.toString())
})
process.stderr.on('data', (data) => {
console.log('stderr: ' + data.toString())
})
process.on('exit', (code) => {
console.log('child process exited with code ' + code.toString())
})
}
app.js
const { exec } = require('./utilities.js')
exec('coffee -cw my_file.coffee')
After reviewing all the other answers, I ended up with this:
function oldSchoolMakeBuild(cb) {
var makeProcess = exec('make -C ./oldSchoolMakeBuild',
function (error, stdout, stderr) {
stderr && console.error(stderr);
cb(error);
});
makeProcess.stdout.on('data', function(data) {
process.stdout.write('oldSchoolMakeBuild: '+ data);
});
}
Sometimes data will be multiple lines, so the oldSchoolMakeBuild header will appear once for multiple lines. But this didn't bother me enough to change it.
child_process.spawn returns an object with stdout and stderr streams.
You can tap on the stdout stream to read data that the child process sends back to Node. stdout being a stream has the "data", "end", and other events that streams have. spawn is best used to when you want the child process to return a large amount of data to Node - image processing, reading binary data etc.
so you can solve your problem using child_process.spawn as used below.
var spawn = require('child_process').spawn,
ls = spawn('coffee -cw my_file.coffee');
ls.stdout.on('data', function (data) {
console.log('stdout: ' + data.toString());
});
ls.stderr.on('data', function (data) {
console.log('stderr: ' + data.toString());
});
ls.on('exit', function (code) {
console.log('code ' + code.toString());
});
Here is an async helper function written in typescript that seems to do the trick for me. I guess this will not work for long-lived processes but still might be handy for someone?
import * as child_process from "child_process";
private async spawn(command: string, args: string[]): Promise<{code: number | null, result: string}> {
return new Promise((resolve, reject) => {
const spawn = child_process.spawn(command, args)
let result: string
spawn.stdout.on('data', (data: any) => {
if (result) {
reject(Error('Helper function does not work for long lived proccess'))
}
result = data.toString()
})
spawn.stderr.on('data', (error: any) => {
reject(Error(error.toString()))
})
spawn.on('exit', code => {
resolve({code, result})
})
})
}

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