What happens if a child process terminates?

The parent process will receive an exit event. If the child process terminates with an error, an error event will also be generated and an error object will be passed. The standard input/output streams of the child process (stdout, stderr) will be closed, and the parent process should handle these events to avoid resource leaks (for example, by cleaning up event handlers).

Example of handling termination:

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

const child = spawn('ls', ['-lh', '/usr']);

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

child.stderr.on('data', (data) => {
  console.error(`stderr: ${data}`);
});

child.on('error', (err) => {
  // Handle error during process start or execution
  console.error('Failed to start subprocess.', err);
});

child.on('exit', (code, signal) => {
  // Handle process exit
  if (code !== 0) {
    console.warn(`Child process exited with code ${code} and signal ${signal}`);
  } else {
    console.log(`Child process exited successfully.`);
  }
});

If the parent process does not listen for the exit event, it will continue its execution. If the child process terminates with an error and the error event is not handled, an unhandled exception will occur, and most likely, the parent process will terminate.

Zombie processes may also occur if the parent process does not handle the exit event and the operating system cannot properly clean up the child process resources. This happens when the child process has finished, but its PID and exit status remain in the process table until the parent "collects" its status.