Child Processes

TL;DR

The child_process module allows Node.js to execute external applications, scripts, or shell commands in a separate OS process, communicating with them via standard input/output streams.

Mental Model

How It Works

There are four primary ways to spawn a child process:

1. spawn(): Starts a process and returns streams (stdout, stdin). Best for large amounts of data (like processing a video via FFmpeg).
2. exec(): Runs a command in a shell and buffers the output in memory. Returns the whole output as a string. Dangerous for large data (buffer overflow).
3. execFile(): Like exec, but executes a file directly without spinning up a shell, making it slightly faster and safer.
4. fork(): A special version of spawn designed specifically to spawn other Node.js processes. It establishes an IPC (Inter-Process Communication) channel so the parent and child can send messages to each other.

Example

const { spawn, exec } = require('child_process');

// 1. Using exec (Buffers output - good for small scripts)
exec('ls -la', (error, stdout, stderr) => {
    if (error) return console.error(`exec error: ${error}`);
    console.log(`Directory listing:\n${stdout}`);
});

// 2. Using spawn (Streams output - good for large data)
const grep = spawn('grep', ['ssh']);

grep.stdout.on('data', (data) => {
  console.log(`grep output: ${data}`);
});

Common Interview Questions

What is the difference between spawn and exec?

spawn works with streams, allowing you to process unlimited amounts of data as it arrives. exec buffers the output in memory and passes the complete string to a callback when finished, which will crash if the output exceeds the default buffer size (usually 1MB).

What is IPC (Inter-Process Communication)?

When you use child_process.fork('script.js'), Node sets up an IPC channel. This allows the parent and child Node.js processes to communicate directly using process.send({ data }) and process.on('message', ...).