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Node.js - Child Process 

Prashant Surya  |   Nov 19, 2019  |  Views: 12

Description:

Child process (child_process) is a module in Node.js which is used to spawn or copy a existing child process which are similar but not identical.


How to import in application:
const { spawn } = require('child_process');


Different ways to create a new child process:
  • child_process.spawn(command[, args][, options])
  • child_process.fork(modulePath[, args][, options])
  • child_process.exec(command[, options][, callback])
  • child_process.execFile(file[, args][, options][, callback])

Note: 
These all child process executes asynchronously without blocking Node.js event loop.

To execute them synchronously we have different synchronous methods in child process class:

  • child_process.spawnSync()
  • child_process.execSync()
  • child_process.execFileSync()

These synchronous methods creates or spawns new child processes which blocks the event loop until the spawned process either exits or is terminated.



Difference between child_process.spawn() & child_process.fork()

Spawn: It takes commands as input argument parameter to run in new child proecss.
Fork: It takes the module path as input argumnet parameter to run in new child processs.
      An IPC communication channel is established that allows sending messages between parent and child.


Difference between child_process.exec() & child_process.execFile()

Exec: It spawn shell first then executed commands.
ExecFile: It direct execute command without spawning any shell first.


Difference between child_process.spawn() & child_process.exec()
spawn: spawn and fork dont have callback as argument.
exec: exec and execFile have callback options as argument.


The callback in exec/execfile gives err, stdout, stderr

Example:
const { exec } = require('child_process');
exec('my.bat', (err, stdout, stderr) => {
  if (err) {
    console.error(err);
    return;
  }
  console.log(stdout);
});
// Script with spaces in the filename:
const bat = spawn('"my script.cmd"', ['a', 'b'], { shell: true });
// or:
exec('"my script.cmd" a b', (err, stdout, stderr) => {
  // ...
});




Child Process have following events
  • Event: 'close'
  • Event: 'disconnect'
  • Event: 'error'
  • Event: 'exit'
  • Event: 'message'



Ezxample:
const { spawn } = require('child_process');
const ls = spawn('ls', ['-lh', '/usr']);
ls.on('error', (err) => {
  console.log(`child process exited with error ${err}`);
});
ls.on('close', (code) => {
  console.log(`child process exited with code ${code}`);
});


By default, pipes for stdin, stdout, and stderr are established between the parent Node.js process and the spawned child. 

const { spawn } = require('child_process');
const ls = spawn('ls', ['-lh', '/usr']);
ls.stdout.on('data', (data) => {
  console.log(`stdout: ${data}`);
});
ls.stderr.on('data', (data) => {
  console.log(`stderr: ${data}`);
});
ls.on('close', (code) => {
  console.log(`child process exited with code ${code}`);
});

Different arguments needed while creating new child process:
  • file
  • modulePath
  • command
  • args
  • options : stdio,detached
  • callback : function with output(err,stdout,stderr)



option.stdio:

As by default, pipes for stdin, stdout, and stderr are enabled, we have option to ignore it by setting the option.stdio as

  • pipe - is equal to ['pipe', 'pipe', 'pipe'] (the default)
  • ignore - is equal to ['ignore', 'ignore', 'ignore']
  • inherit - is equal to ['inherit', 'inherit', 'inherit'] or [0, 1, 2]
  • Or Array of combination of (pipe,ipc,ignore,inherit,stream object,positive integer,null/undefined)

option.detached:

By default a parent process waits till the exits of child process, so we have to detach it, by setting true, so that child will still running after the parent exits.

If true, it allows child process to continue running after the parent process exits.



A subprocess have following utilities:
  • subprocess.channel
  • subprocess.connected
  • subprocess.disconnect()
  • subprocess.kill([signal])
  • subprocess.killed
  • subprocess.pid
  • subprocess.ref()
  • subprocess.send(message[, sendHandle[, options]][, callback])
  • Example: sending a server object
  • Example: sending a socket object
  • subprocess.stderr
  • subprocess.stdin
  • subprocess.stdio
  • subprocess.stdout
  • subprocess.unref()






Comments:
Content Contributor
Prashant Surya

Prashant Surya

Software developer at Uniserve Data Technologies Pvt. Ltd.  |   Noida, Uttar Pradesh, India

I am a software developer, freelancer and trainer working on technologies likes. MEAN stack, MERN Stack, ASP.NET full stack, WebRTC, socket.io ,PWA, SQL, JavaScript, jQuery, HTML, CSS, SEO, responsive UI/UX designing... having 7 years of experience in development in multiple domains like, eCommerce, RTA, capital markets, education, real estate...


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