Node.js Child Process

Node.js Child Process

The Node.js child_process module allows developers to spawn child processes, similar to the popen(3) system call in UNIX. This is useful when you need to execute system commands, run external scripts, or manage multiple Node.js processes simultaneously.

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There are three major ways to create a child process in Node.js:

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1. child_process.exec() – runs a command in a shell and buffers the output.
2. child_process.spawn() – launches a new process with a given command, suitable for handling large data streams.
3. child_process.fork() – a special case of spawn() designed for spawning Node.js processes with an additional communication channel.

1. Node.js child_process.exec() Method

The exec() method executes a shell command and buffers the entire output before returning it in the callback.

Syntax:

child_process.exec(command[, options], callback)

Parameters:

• command: The command string to execute.
• options (optional):
  • cwd: Current working directory of the child process.
  • env: Environment key-value pairs.
  • encoding: Output encoding (default: utf8).
  • shell: Shell to execute with (/bin/sh on UNIX, cmd.exe on Windows).
  • timeout: Time in ms before the process is killed (default: 0).
  • maxBuffer: Maximum output buffer (default: 200*1024).
  • killSignal: Kill signal to terminate the process (default: SIGTERM).
  • uid/gid: Set user and group identity of the process.
• callback: Invoked with (error, stdout, stderr) after process termination.

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Example 1 – Running a Batch File

child_process_example1.js

const exec = require('child_process').exec;
exec('my.bat', (err, stdout, stderr) => {
  if (err) {
    console.error(err);
    return;
  }
  console.log(stdout);
});

my.bat

dir
mkdir child

Running node child_process_example1.js will list directory contents (dir) and create a new folder (child). On Linux/macOS, replace dir with ls.

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Example 2 – Multiple Child Processes

support.js

console.log("Child Process " + process.argv[2] + " executed.");

master.js

const child_process = require('child_process');
for (let i = 0; i < 3; i++) {
  let workerProcess = child_process.exec('node support.js ' + i, (error, stdout, stderr) => {
    if (error) {
      console.log(error.stack);
      console.log('Error code: ' + error.code);
      console.log('Signal received: ' + error.signal);
    }
    console.log('stdout: ' + stdout);
    console.log('stderr: ' + stderr);
  });
  workerProcess.on('exit', (code) => {
    console.log('Child process exited with exit code ' + code);
  });
}

Running node master.js will execute support.js three times, each with different arguments.

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2. Node.js child_process.spawn() Method

The spawn() method launches a process and returns streams (stdout & stderr). Unlike exec(), it does not buffer the output, making it suitable for processes generating large data.

Syntax:

child_process.spawn(command[, args][, options])

Parameters:

• command: The command to run.
• args: Array of string arguments.
• options: Similar to exec(), including cwd, env, stdio, uid, gid, etc.

Example – Using spawn()

support.js

console.log("Child Process " + process.argv[2] + " executed.");

master.js

const child_process = require('child_process');
for (let i = 0; i < 3; i++) {
  let workerProcess = child_process.spawn('node', ['support.js', i]);
  workerProcess.stdout.on('data', (data) => {
    console.log('stdout: ' + data);
  });
  workerProcess.stderr.on('data', (data) => {
    console.log('stderr: ' + data);
  });
  workerProcess.on('close', (code) => {
    console.log('child process exited with code ' + code);
  });
}

Run node master.js to see each child process print its execution message.

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3. Node.js child_process.fork() Method

The fork() method is a specialized version of spawn() for creating Node.js processes. It includes a built-in communication channel (process.send() and process.on('message')).

Syntax:

child_process.fork(modulePath[, args][, options])

Parameters:

• modulePath: Path to the module executed in the child.
• args: Array of string arguments.
• options: Includes cwd, env, execPath, execArgv, silent, uid, gid, etc.

Example – Using fork()

support.js

console.log("Child process " + process.argv[2] + " executed.");

master.js

const child_process = require('child_process');
for (let i = 0; i < 3; i++) {
  let workerProcess = child_process.fork('support.js', [i]);
  workerProcess.on('close', (code) => {
    console.log('child process exited with code ' + code);
  });
}

Running node master.js spawns three child processes using fork().

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✔️ With these three methods—exec(), spawn(), and fork()—Node.js gives developers powerful tools to manage external processes and scale applications efficiently.

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