ChildProcess

class ChildProcess extends EventEmitter

Instances of the ChildProcess represent spawned child processes.

Instances of ChildProcess are not intended to be created directly. Rather, use the child_process.spawn(), child_process.exec(), child_process.execFile(), or child_process.fork() methods to create instances of ChildProcess.

The 'close' event is emitted after a process has ended and the stdio streams of a child process have been closed. This is distinct from the 'exit' event, since multiple processes might share the same stdio streams. The 'close' event will always emit after 'exit' was already emitted, or 'error' if the child process failed to spawn.

If the process exited, code is the final exit code of the process, otherwise null. If the process terminated due to receipt of a signal, signal is the string name of the signal, otherwise null. One of the two will always be non-null.

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

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

ls.on('close', (code) => {
  console.log(`child process close all stdio with code ${code}`);
});

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

import { spawn } from 'node:child_process';
import { once } from 'node:events';
const ls = spawn('ls', ['-lh', '/usr']);

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

ls.on('close', (code) => {
  console.log(`child process close all stdio with code ${code}`);
});

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

const [code] = await once(ls, 'close');
console.log(`child process close all stdio with code ${code}`);

The 'disconnect' event is emitted after calling the subprocess.disconnect() method in parent process or process.disconnect() in child process. After disconnecting it is no longer possible to send or receive messages, and the subprocess.connected property is false.

The 'error' event is emitted whenever:

• The process could not be spawned.
• The process could not be killed.
• Sending a message to the child process failed.
• The child process was aborted via the signal option.

The 'exit' event may or may not fire after an error has occurred. When listening to both the 'exit' and 'error' events, guard against accidentally invoking handler functions multiple times.

See also subprocess.kill() and subprocess.send().

The 'exit' event is emitted after the child process ends. If the process exited, code is the final exit code of the process, otherwise null. If the process terminated due to receipt of a signal, signal is the string name of the signal, otherwise null. One of the two will always be non-null.

When the 'exit' event is triggered, child process stdio streams might still be open.

Node.js establishes signal handlers for SIGINT and SIGTERM and Node.js processes will not terminate immediately due to receipt of those signals. Rather, Node.js will perform a sequence of cleanup actions and then will re-raise the handled signal.

See waitpid(2).

When code is null due to signal termination, you can use util.convertProcessSignalToExitCode() to convert the signal to a POSIX exit code.

The 'message' event is triggered when a child process uses process.send() to send messages.

The message goes through serialization and parsing. The resulting message might not be the same as what is originally sent.

If the serialization option was set to 'advanced' used when spawning the child process, the message argument can contain data that JSON is not able to represent. See Advanced serialization for more details.

The 'spawn' event is emitted once the child process has spawned successfully. If the child process does not spawn successfully, the 'spawn' event is not emitted and the 'error' event is emitted instead.

If emitted, the 'spawn' event comes before all other events and before any data is received via stdout or stderr.

The 'spawn' event will fire regardless of whether an error occurs within the spawned process. For example, if bash some-command spawns successfully, the 'spawn' event will fire, though bash may fail to spawn some-command. This caveat also applies when using { shell: true }.

The subprocess.channel property is a reference to the child's IPC channel. If no IPC channel exists, this property is undefined.

subprocess.channel.ref(): void

This method makes the IPC channel keep the event loop of the parent process running if .unref() has been called before.

subprocess.channel.unref(): void

This method makes the IPC channel not keep the event loop of the parent process running, and lets it finish even while the channel is open.

The subprocess.connected property indicates whether it is still possible to send and receive messages from a child process. When subprocess.connected is false, it is no longer possible to send or receive messages.

subprocess.disconnect(): void

Closes the IPC channel between parent and child processes, allowing the child process to exit gracefully once there are no other connections keeping it alive. After calling this method the subprocess.connected and process.connected properties in both the parent and child processes (respectively) will be set to false, and it will be no longer possible to pass messages between the processes.

The 'disconnect' event will be emitted when there are no messages in the process of being received. This will most often be triggered immediately after calling subprocess.disconnect().

When the child process is a Node.js instance (e.g. spawned using child_process.fork()), the process.disconnect() method can be invoked within the child process to close the IPC channel as well.

The subprocess.exitCode property indicates the exit code of the child process. If the child process is still running, the field will be null.

When the child process is terminated by a signal, subprocess.exitCode will be null and subprocess.signalCode will be set. To get the corresponding POSIX exit code, use util.convertProcessSignalToExitCode(subprocess.signalCode).

subprocess.kill(signal?): boolean

The subprocess.kill() method sends a signal to the child process. If no argument is given, the process will be sent the 'SIGTERM' signal. See signal(7) for a list of available signals. This function returns true if kill(2) succeeds, and false otherwise.

const { spawn } = require('node:child_process');
const grep = spawn('grep', ['ssh']);

grep.on('close', (code, signal) => {
  console.log(
    `child process terminated due to receipt of signal ${signal}`);
});

// Send SIGHUP to process.
grep.kill('SIGHUP');

import { spawn } from 'node:child_process';
const grep = spawn('grep', ['ssh']);

grep.on('close', (code, signal) => {
  console.log(
    `child process terminated due to receipt of signal ${signal}`);
});

// Send SIGHUP to process.
grep.kill('SIGHUP');

The ChildProcess object may emit an 'error' event if the signal cannot be delivered. Sending a signal to a child process that has already exited is not an error but may have unforeseen consequences. Specifically, if the process identifier (PID) has been reassigned to another process, the signal will be delivered to that process instead which can have unexpected results.

While the function is called kill, the signal delivered to the child process may not actually terminate the process.

See kill(2) for reference.

On Windows, where POSIX signals do not exist, signals are handled as follows. 'SIGKILL', 'SIGTERM', 'SIGINT' and 'SIGQUIT' terminate the process forcefully and abruptly (similar to 'SIGKILL'); any other signal whose name is known on Windows (such as 'SIGHUP') does the same. 'SIGWINCH' is not terminal and is not coerced: subprocess.kill() throws an ENOSYS error and the child keeps running. A signal name that does not exist on Windows (such as 'SIGSTOP') throws an ERR_UNKNOWN_SIGNAL error. See Signal Events for more details.

On Linux, child processes of child processes will not be terminated when attempting to kill their parent. This is likely to happen when running a new process in a shell or with the use of the shell option of ChildProcess:

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

const subprocess = spawn(
  'sh',
  [
    '-c',
    `node -e "setInterval(() => {
      console.log(process.pid, 'is alive')
    }, 500);"`,
  ], {
    stdio: ['inherit', 'inherit', 'inherit'],
  },
);

setTimeout(() => {
  subprocess.kill(); // Does not terminate the Node.js process in the shell.
}, 2000);

import { spawn } from 'node:child_process';

const subprocess = spawn(
  'sh',
  [
    '-c',
    `node -e "setInterval(() => {
      console.log(process.pid, 'is alive')
    }, 500);"`,
  ], {
    stdio: ['inherit', 'inherit', 'inherit'],
  },
);

setTimeout(() => {
  subprocess.kill(); // Does not terminate the Node.js process in the shell.
}, 2000);

subprocess[Symbol.dispose](): void

Calls subprocess.kill() with 'SIGTERM'.

The subprocess.killed property indicates whether the child process successfully received a signal from subprocess.kill(). The killed property does not indicate that the child process has been terminated.

Returns the process identifier (PID) of the child process. If the child process fails to spawn due to errors, then the value is undefined and error is emitted.

const { spawn } = require('node:child_process');
const grep = spawn('grep', ['ssh']);

console.log(`Spawned child pid: ${grep.pid}`);
grep.stdin.end();

import { spawn } from 'node:child_process';
const grep = spawn('grep', ['ssh']);

console.log(`Spawned child pid: ${grep.pid}`);
grep.stdin.end();

subprocess.ref(): void

Calling subprocess.ref() after making a call to subprocess.unref() will restore the removed reference count for the child process, forcing the parent process to wait for the child process to exit before exiting itself.

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

const subprocess = spawn(process.argv[0], ['child_program.js'], {
  detached: true,
  stdio: 'ignore',
});

subprocess.unref();
subprocess.ref();

import { spawn } from 'node:child_process';
import process from 'node:process';

const subprocess = spawn(process.argv[0], ['child_program.js'], {
  detached: true,
  stdio: 'ignore',
});

subprocess.unref();
subprocess.ref();

subprocess.send(message, sendHandle?, options?, callback?): boolean

When an IPC channel has been established between the parent and child processes ( i.e. when using child_process.fork()), the subprocess.send() method can be used to send messages to the child process. When the child process is a Node.js instance, these messages can be received via the 'message' event.

The message goes through serialization and parsing. The resulting message might not be the same as what is originally sent.

For example, in the parent script:

const { fork } = require('node:child_process');
const forkedProcess = fork(`${__dirname}/sub.js`);

forkedProcess.on('message', (message) => {
  console.log('PARENT got message:', message);
});

// Causes the child to print: CHILD got message: { hello: 'world' }
forkedProcess.send({ hello: 'world' });

import { fork } from 'node:child_process';
const forkedProcess = fork(`${import.meta.dirname}/sub.js`);

forkedProcess.on('message', (message) => {
  console.log('PARENT got message:', message);
});

// Causes the child to print: CHILD got message: { hello: 'world' }
forkedProcess.send({ hello: 'world' });

And then the child script, 'sub.js' might look like this:

process.on('message', (message) => {
  console.log('CHILD got message:', message);
});

// Causes the parent to print: PARENT got message: { foo: 'bar', baz: null }
process.send({ foo: 'bar', baz: NaN });

Child Node.js processes will have a process.send() method of their own that allows the child process to send messages back to the parent process.

There is a special case when sending a {cmd: 'NODE_foo'} message. Messages containing a NODE_ prefix in the cmd property are reserved for use within Node.js core and will not be emitted in the child's 'message' event. Rather, such messages are emitted using the 'internalMessage' event and are consumed internally by Node.js. Applications should avoid using such messages or listening for 'internalMessage' events as it is subject to change without notice.

The optional sendHandle argument that may be passed to subprocess.send() is for passing a TCP server or socket object to the child process. The child process will receive the object as the second argument passed to the callback function registered on the 'message' event. Any data that is received and buffered in the socket will not be sent to the child. Sending IPC sockets is not supported on Windows.

The optional callback is a function that is invoked after the message is sent but before the child process may have received it. The function is called with a single argument: null on success, or an Error object on failure.

If no callback function is provided and the message cannot be sent, an 'error' event will be emitted by the ChildProcess object. This can happen, for instance, when the child process has already exited.

subprocess.send() will return false if the channel has closed or when the backlog of unsent messages exceeds a threshold that makes it unwise to send more. Otherwise, the method returns true. The callback function can be used to implement flow control.

The sendHandle argument can be used, for instance, to pass the handle of a TCP server object to the child process as illustrated in the example below:

const { fork } = require('node:child_process');
const { createServer } = require('node:net');

const subprocess = fork('subprocess.js');

// Open up the server object and send the handle.
const server = createServer();
server.on('connection', (socket) => {
  socket.end('handled by parent');
});
server.listen(1337, () => {
  subprocess.send('server', server);
});

import { fork } from 'node:child_process';
import { createServer } from 'node:net';

const subprocess = fork('subprocess.js');

// Open up the server object and send the handle.
const server = createServer();
server.on('connection', (socket) => {
  socket.end('handled by parent');
});
server.listen(1337, () => {
  subprocess.send('server', server);
});

The child process would then receive the server object as:

process.on('message', (m, server) => {
  if (m === 'server') {
    server.on('connection', (socket) => {
      socket.end('handled by child');
    });
  }
});

Once the server is now shared between the parent and child, some connections can be handled by the parent and some by the child.

While the example above uses a server created using the node:net module, node:dgram module servers use exactly the same workflow with the exceptions of listening on a 'message' event instead of 'connection' and using server.bind() instead of server.listen(). This is, however, only supported on Unix platforms.

Similarly, the sendHandler argument can be used to pass the handle of a socket to the child process. The example below spawns two children that each handle connections with "normal" or "special" priority:

const { fork } = require('node:child_process');
const { createServer } = require('node:net');

const normal = fork('subprocess.js', ['normal']);
const special = fork('subprocess.js', ['special']);

// Open up the server and send sockets to child. Use pauseOnConnect to prevent
// the sockets from being read before they are sent to the child process.
const server = createServer({ pauseOnConnect: true });
server.on('connection', (socket) => {

  // If this is special priority...
  if (socket.remoteAddress === '74.125.127.100') {
    special.send('socket', socket);
    return;
  }
  // This is normal priority.
  normal.send('socket', socket);
});
server.listen(1337);

import { fork } from 'node:child_process';
import { createServer } from 'node:net';

const normal = fork('subprocess.js', ['normal']);
const special = fork('subprocess.js', ['special']);

// Open up the server and send sockets to child. Use pauseOnConnect to prevent
// the sockets from being read before they are sent to the child process.
const server = createServer({ pauseOnConnect: true });
server.on('connection', (socket) => {

  // If this is special priority...
  if (socket.remoteAddress === '74.125.127.100') {
    special.send('socket', socket);
    return;
  }
  // This is normal priority.
  normal.send('socket', socket);
});
server.listen(1337);

The subprocess.js would receive the socket handle as the second argument passed to the event callback function:

process.on('message', (m, socket) => {
  if (m === 'socket') {
    if (socket) {
      // Check that the client socket exists.
      // It is possible for the socket to be closed between the time it is
      // sent and the time it is received in the child process.
      socket.end(`Request handled with ${process.argv[2]} priority`);
    }
  }
});

Do not use .maxConnections on a socket that has been passed to a subprocess. The parent cannot track when the socket is destroyed.

Any 'message' handlers in the subprocess should verify that socket exists, as the connection may have been closed during the time it takes to send the connection to the child.

The subprocess.signalCode property indicates the signal received by the child process if any, else null.

When the child process is terminated by a signal, subprocess.exitCode will be null. To get the corresponding POSIX exit code, use util.convertProcessSignalToExitCode(subprocess.signalCode).

The subprocess.spawnargs property represents the full list of command-line arguments the child process was launched with.

The subprocess.spawnfile property indicates the executable file name of the child process that is launched.

For child_process.fork(), its value will be equal to process.execPath. For child_process.spawn(), its value will be the name of the executable file. For child_process.exec(), its value will be the name of the shell in which the child process is launched.

A Readable Stream that represents the child process's stderr.

If the child process was spawned with stdio[2] set to anything other than 'pipe', then this will be null.

subprocess.stderr is an alias for subprocess.stdio[2]. Both properties will refer to the same value.

The subprocess.stderr property can be null or undefined if the child process could not be successfully spawned.

A Writable Stream that represents the child process's stdin.

If a child process waits to read all of its input, the child process will not continue until this stream has been closed via end().

If the child process was spawned with stdio[0] set to anything other than 'pipe', then this will be null.

subprocess.stdin is an alias for subprocess.stdio[0]. Both properties will refer to the same value.

The subprocess.stdin property can be null or undefined if the child process could not be successfully spawned.

A sparse array of pipes to the child process, corresponding with positions in the stdio option passed to child_process.spawn() that have been set to the value 'pipe'. subprocess.stdio[0], subprocess.stdio[1], and subprocess.stdio[2] are also available as subprocess.stdin, subprocess.stdout, and subprocess.stderr, respectively.

In the following example, only the child's fd 1 (stdout) is configured as a pipe, so only the parent's subprocess.stdio[1] is a stream, all other values in the array are null.

const assert = require('node:assert');
const fs = require('node:fs');
const child_process = require('node:child_process');

const subprocess = child_process.spawn('ls', {
  stdio: [
    0, // Use parent's stdin for child.
    'pipe', // Pipe child's stdout to parent.
    fs.openSync('err.out', 'w'), // Direct child's stderr to a file.
  ],
});

assert.strictEqual(subprocess.stdio[0], null);
assert.strictEqual(subprocess.stdio[0], subprocess.stdin);

assert(subprocess.stdout);
assert.strictEqual(subprocess.stdio[1], subprocess.stdout);

assert.strictEqual(subprocess.stdio[2], null);
assert.strictEqual(subprocess.stdio[2], subprocess.stderr);

import assert from 'node:assert';
import fs from 'node:fs';
import child_process from 'node:child_process';

const subprocess = child_process.spawn('ls', {
  stdio: [
    0, // Use parent's stdin for child.
    'pipe', // Pipe child's stdout to parent.
    fs.openSync('err.out', 'w'), // Direct child's stderr to a file.
  ],
});

assert.strictEqual(subprocess.stdio[0], null);
assert.strictEqual(subprocess.stdio[0], subprocess.stdin);

assert(subprocess.stdout);
assert.strictEqual(subprocess.stdio[1], subprocess.stdout);

assert.strictEqual(subprocess.stdio[2], null);
assert.strictEqual(subprocess.stdio[2], subprocess.stderr);

The subprocess.stdio property can be undefined if the child process could not be successfully spawned.

A Readable Stream that represents the child process's stdout.

If the child process was spawned with stdio[1] set to anything other than 'pipe', then this will be null.

subprocess.stdout is an alias for subprocess.stdio[1]. Both properties will refer to the same value.

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

const subprocess = spawn('ls');

subprocess.stdout.on('data', (data) => {
  console.log(`Received chunk ${data}`);
});

import { spawn } from 'node:child_process';

const subprocess = spawn('ls');

subprocess.stdout.on('data', (data) => {
  console.log(`Received chunk ${data}`);
});

The subprocess.stdout property can be null or undefined if the child process could not be successfully spawned.

subprocess.unref(): void

By default, the parent process will wait for the detached child process to exit. To prevent the parent process from waiting for a given subprocess to exit, use the subprocess.unref() method. Doing so will cause the parent's event loop to not include the child process in its reference count, allowing the parent to exit independently of the child, unless there is an established IPC channel between the child and the parent processes.

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

const subprocess = spawn(process.argv[0], ['child_program.js'], {
  detached: true,
  stdio: 'ignore',
});

subprocess.unref();

import { spawn } from 'node:child_process';
import process from 'node:process';

const subprocess = spawn(process.argv[0], ['child_program.js'], {
  detached: true,
  stdio: 'ignore',
});

subprocess.unref();