Child process exit codes in Node.js

Introduction

Child process exit codes tell you if a program finished successfully or had an error. They help you understand what happened after running another program inside your Node.js app.

You run a script or command from your Node.js app and want to know if it worked.

You want to handle errors when a child process stops unexpectedly.

You need to perform different actions based on how a child process ended.

You want to log or debug why a child process failed.

You want to clean up resources only if the child process finished correctly.

Syntax

Node.js

childProcess.on('exit', (code, signal) => {
  // code is the exit code number or null
  // signal is the termination signal or null
});

The code is a number where 0 means success and other numbers mean errors.

The signal tells if the process was stopped by a system signal like SIGTERM.

Examples

This runs the ls -lh command and logs the exit code when done.

Node.js

const { spawn } = require('child_process');
const ls = spawn('ls', ['-lh']);

ls.on('exit', (code) => {
  console.log(`Child exited with code ${code}`);
});

This runs a Node.js script and checks the exit code inside the callback.

Node.js

const { exec } = require('child_process');
exec('node someScript.js', (error, stdout, stderr) => {
  if (error) {
    console.log(`Error code: ${error.code}`);
  } else {
    console.log('Script ran successfully');
  }
});

This starts a sleep command, kills it, and logs the exit code and signal.

Node.js

const { spawn } = require('child_process');
const proc = spawn('sleep', ['10']);

proc.kill('SIGTERM');
proc.on('exit', (code, signal) => {
  console.log(`Exit code: ${code}, Signal: ${signal}`);
});

Sample Program

This program runs a child Node.js process that exits with code 1. It listens for the exit event and prints the exit code. If the process was killed by a signal, it prints that instead.

Node.js

import { spawn } from 'child_process';

const child = spawn('node', ['-e', "process.exit(1)"]);

child.on('exit', (code, signal) => {
  if (code !== null) {
    console.log(`Child process exited with code ${code}`);
  } else if (signal !== null) {
    console.log(`Child process was killed by signal ${signal}`);
  } else {
    console.log('Child process exited');
  }
});

Important Notes

Exit code 0 means success; any other number usually means an error.

If the process is terminated by a signal, the exit code will be null and the signal will be set.

Always listen for the exit event to know when the child process finishes.

Summary

Child process exit codes tell if a process finished well or had errors.

Use the exit event to get the code and signal.

Code 0 means success; other codes or signals mean problems.

Practice

1. What does an exit code of 0 from a Node.js child process usually mean?

easy

A. The process finished successfully without errors.

B. The process was terminated by a signal.

C. The process encountered an error and crashed.

D. The process is still running.

Solution

1. Step 1: Understand exit codes in Node.js child processes

   Exit codes indicate how a process ended. Code 0 means success.

2. Step 2: Interpret exit code 0

   Exit code 0 means the process finished without errors or interruptions.

3. Final Answer:

   The process finished successfully without errors. -> Option A

4. Quick Check:

   Exit code 0 = success [OK]

Hint: Exit code 0 always means success in child processes [OK]

Common Mistakes:

• Confusing exit code 0 with error codes
• Thinking signal termination returns 0
• Assuming non-zero codes mean success

2. Which of the following is the correct way to listen for a child process exit event in Node.js?

easy

A. child.listen('exit', (code) => { /* handle exit */ });

B. child.exit(() => { /* handle exit */ });

C. child.onExit((code) => { /* handle exit */ });

D. child.on('exit', (code, signal) => { /* handle exit */ });

Solution

1. Step 1: Recall Node.js child process event syntax

   Node.js child processes emit events listened with on method.

2. Step 2: Confirm correct event and parameters

   The 'exit' event uses child.on('exit', (code, signal) => {}) syntax.

3. Final Answer:

   child.on('exit', (code, signal) => { /* handle exit */ }); -> Option D

4. Quick Check:

   Use child.on('exit', callback) [OK]

Hint: Use child.on('exit', callback) to catch exit events [OK]

Common Mistakes:

• Using non-existent methods like exit() or onExit()
• Using listen() instead of on()
• Missing parameters in the callback

3. Consider this code snippet:

const { spawn } = require('child_process');
const child = spawn('node', ['-e', "process.exit(5)"]);
child.on('exit', (code, signal) => {
  console.log(`Exit code: ${code}, Signal: ${signal}`);
});

What will be printed when this code runs?

medium

A. Exit code: 0, Signal: null

B. Exit code: 5, Signal: null

C. Exit code: null, Signal: SIGTERM

D. Exit code: 1, Signal: null

Solution

1. Step 1: Understand the child process exit code

   The child process runs process.exit(5), so it exits with code 5.

2. Step 2: Check the exit event parameters

   The exit event callback receives code 5 and signal null because it exited normally with code 5.

3. Final Answer:

   Exit code: 5, Signal: null -> Option B

4. Quick Check:

   process.exit(5) sets exit code 5 [OK]

Hint: process.exit(n) sets exit code n, signal is null if normal exit [OK]

Common Mistakes:

• Assuming exit code is always 0
• Confusing signal with exit code
• Expecting signal to be set on normal exit

4. You wrote this code to spawn a child process and listen for exit:

const { spawn } = require('child_process');
const child = spawn('node', ['-e', "setTimeout(() => process.exit(0), 1000)"]);
child.unref();
child.on('exit', (code) => {
  console.log(`Exited with code ${code}`);
});

But the console never logs anything. What is the likely problem?

medium

A. The exit event callback is missing the signal parameter.

B. The child process is not exiting because the event loop is blocked.

C. The child process exits after 1 second, but the main program ends before that.

D. The spawn command syntax is incorrect.

Solution

1. Step 1: Analyze the child process behavior

   The child process exits after 1 second due to setTimeout.

2. Step 2: Consider the main program lifecycle

   If the main program ends before 1 second, it may exit before child finishes, so no log appears.

3. Final Answer:

   The child process exits after 1 second, but the main program ends before that. -> Option C

4. Quick Check:

   Main program must stay alive to see child exit [OK]

Hint: Keep main program alive to catch delayed child exit events [OK]

Common Mistakes:

• Thinking missing signal parameter breaks event
• Assuming spawn syntax is wrong without error
• Ignoring asynchronous timing of child exit

5. You want to run a child process and handle both normal exit and signal termination. Which code snippet correctly logs the exit code or signal received?

hard

A. child.on('exit', (code, signal) => {
if (code !== null) console.log(`Exited with code ${code}`);
else console.log(`Terminated by signal ${signal}`);
});

B. child.on('exit', (code) => {
if (code === 0) console.log('Success');
else console.log('Error');
});

C. child.on('close', (signal) => {
console.log(`Closed with signal ${signal}`);
});

D. child.on('exit', () => {
console.log('Process ended');
});

Solution

1. Step 1: Understand exit event parameters

   The 'exit' event provides two parameters: code (number or null) and signal (string or null).

2. Step 2: Handle both exit code and signal

   If code is not null, process ended normally; if null, it was terminated by a signal.

3. Step 3: Check code snippet correctness

   child.on('exit', (code, signal) => { if (code !== null) console.log(`Exited with code ${code}`); else console.log(`Terminated by signal ${signal}`); }); correctly checks code and signal and logs accordingly.

4. Final Answer:

   child.on('exit', (code, signal) => { if (code !== null) console.log(`Exited with code ${code}`); else console.log(`Terminated by signal ${signal}`); }); -> Option A

5. Quick Check:

   Check code !== null to distinguish exit vs signal [OK]

Hint: Check if exit code is null to detect signal termination [OK]

Common Mistakes:

• Ignoring the signal parameter in exit event
• Using 'close' event instead of 'exit' for exit codes
• Not checking for null exit code before logging