Table of Contents

• Child Process
  • Class: ChildProcess
    • Event: 'close'
    • Event: 'disconnect'
    • Event: 'error'
    • Event: 'exit'
    • Event: 'message'
    • child.connected
    • child.disconnect()
    • child.kill([signal])
    • child.pid
    • child.send(message[, sendHandle][, callback])
      • Example: sending server object
      • Example: sending socket object
    • child.stderr
    • child.stdin
    • child.stdio
    • child.stdout
  • Asynchronous Process Creation
    • child_process.exec(command[, options], callback)
    • child_process.execFile(file[, args][, options][, callback])
    • child_process.fork(modulePath[, args][, options])
    • child_process.spawn(command[, args][, options])
      • options.detached
      • options.stdio
  • Synchronous Process Creation
    • child_process.execFileSync(file[, args][, options])
    • child_process.execSync(command[, options])
    • child_process.spawnSync(command[, args][, options])

Child Process#

Stability: 2 - Stable

Node.js provides a tri-directional popen(3) facility through the child_process module.

It is possible to stream data through a child's stdin, stdout, and stderr in a fully non-blocking way. (Note that some programs use line-buffered I/O internally. That doesn't affect Node.js but it means data you send to the child process may not be immediately consumed.)

To create a child process, use require('child_process').spawn() or require('child_process').fork(). The semantics of each are slightly different as explained below.

For scripting purposes you may find the synchronous counterparts more convenient.

Class: ChildProcess#

ChildProcess is an EventEmitter.

Child processes always have three streams associated with them. child.stdin, child.stdout, and child.stderr. These may be shared with the stdio streams of the parent process, or they may be separate stream objects which can be piped to and from.

The ChildProcess class is not intended to be used directly. Use the spawn(), exec(), execFile(), or fork() methods to create an instance of ChildProcess.

Event: 'close'#

This event is emitted when the stdio streams of a child process have all terminated. This is distinct from 'exit', since multiple processes might share the same stdio streams.

Event: 'disconnect'#

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

Event: 'error'#

Emitted when:

1. The process could not be spawned, or
2. The process could not be killed, or
3. Sending a message to the child process failed.

Note that the 'exit' event may or may not fire after an error has occurred. If you are listening on both events to fire a function, remember to guard against calling your function twice.

See also ChildProcess#kill() and ChildProcess#send().

Event: 'exit'#

This event is emitted after the child process ends. If the process terminated normally, 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.

Note that the child process stdio streams might still be open.

Also, note that Node.js establishes signal handlers for SIGINT and SIGTERM. It will not terminate due to receipt of those signals. It will exit.

See waitpid(2).

Event: 'message'#

Messages sent by .send(message, [sendHandle]) are obtained using the 'message' event.

child.connected#

If .connected is false, it is no longer possible to send messages.

child.disconnect()#

Close the IPC channel between parent and child, allowing the child to exit gracefully once there are no other connections keeping it alive. After calling this method the .connected flag will be set to false in both the parent and child, and it is no longer possible to send messages.

The 'disconnect' event will be emitted when there are no messages in the process of being received, most likely immediately.

Note that you can also call process.disconnect() in the child process when the child process has any open IPC channels with the parent (i.e fork()).

child.kill([signal])#

Send a signal to the child process. If no argument is given, the process will be sent 'SIGTERM'. See signal(7) for a list of available signals.

const spawn = require('child_process').spawn;
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');

May emit an 'error' event when 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. What happens next is anyone's guess.

Note that while the function is called kill, the signal delivered to the child process may not actually kill it. kill really just sends a signal to a process.

See kill(2)

child.pid#

The process identifier (PID) of the child process.

Example:

const spawn = require('child_process').spawn;
const grep = spawn('grep', ['ssh']);

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

child.send(message[, sendHandle][, callback])#

When using child_process.fork() you can write to the child using child.send(message[, sendHandle][, callback]) and messages are received by a 'message' event on the child.

For example:

const cp = require('child_process');
const n = cp.fork(`${__dirname}/sub.js`);

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

n.send({ hello: 'world' });

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

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

process.send({ foo: 'bar' });

In the child, the process object will have a send() method, and process will emit objects each time it receives a message on its channel.

There is a special case when sending a {cmd: 'NODE_foo'} message. All messages containing a NODE_ prefix in its cmd property will not be emitted in the 'message' event, since they are internal messages used by Node.js core. Messages containing the prefix are emitted in the 'internalMessage' event. Avoid using this feature; it is subject to change without notice.

The sendHandle option to child.send() is for sending a TCP server or socket object to another process. The child will receive the object as its second argument to the 'message' event.

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

child.send() emits an 'error' event if no callback was given and the message cannot be sent, for example because the child process has already exited.

Returns true under normal circumstances or false when the backlog of unsent messages exceeds a threshold that makes it unwise to send more. Use the callback mechanism to implement flow control.

Example: sending server object#

Here is an example of sending a server:

const child = require('child_process').fork('child.js');

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

And the child would then receive the server object as:

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

Note that the server is now shared between the parent and child, this means that some connections will be handled by the parent and some by the child.

For dgram servers the workflow is exactly the same. Here you listen on a 'message' event instead of 'connection' and use server.bind instead of server.listen. (Currently only supported on UNIX platforms.)

Example: sending socket object#

Here is an example of sending a socket. It will spawn two children and handle connections with the remote address 74.125.127.100 as VIP by sending the socket to a "special" child process. Other sockets will go to a "normal" process.

const normal = require('child_process').fork('child.js', ['normal']);
const special = require('child_process').fork('child.js', ['special']);

// Open up the server and send sockets to child
const server = require('net').createServer();
server.on('connection', (socket) => {

  // if this is a VIP
  if (socket.remoteAddress === '74.125.127.100') {
    special.send('socket', socket);
    return;
  }
  // just the usual...
  normal.send('socket', socket);
});
server.listen(1337);

The child.js could look like this:

process.on('message', (m, socket) => {
  if (m === 'socket') {
    socket.end(`You were handled as a ${process.argv[2]} person`);
  }
});

Note that once a single socket has been sent to a child the parent can no longer keep track of when the socket is destroyed. To indicate this condition the .connections property becomes null. It is also recommended not to use .maxConnections in this condition.

child.stderr#

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

If the child was not spawned with stdio[2] set to 'pipe', then this will not be set.

child.stderr is shorthand for child.stdio[2]. Both properties will refer to the same object, or null.

child.stdin#

A Writable Stream that represents the child process's stdin. If the child is waiting to read all its input, it will not continue until this stream has been closed via end().

If the child was not spawned with stdio[0] set to 'pipe', then this will not be set.

child.stdin is shorthand for child.stdio[0]. Both properties will refer to the same object, or null.

child.stdio#

A sparse array of pipes to the child process, corresponding with positions in the stdio option to spawn() that have been set to 'pipe'. Note that streams 0-2 are also available as ChildProcess.stdin, ChildProcess.stdout, and ChildProcess.stderr, respectively.

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

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

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

assert.equal(child.stdio[0], null);
assert.equal(child.stdio[0], child.stdin);

assert(child.stdout);
assert.equal(child.stdio[1], child.stdout);

assert.equal(child.stdio[2], null);
assert.equal(child.stdio[2], child.stderr);

child.stdout#

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

If the child was not spawned with stdio[1] set to 'pipe', then this will not be set.

child.stdout is shorthand for child.stdio[1]. Both properties will refer to the same object, or null.

Asynchronous Process Creation#

These methods follow the common async programming patterns (accepting a callback or returning an EventEmitter).

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

Runs a command in a shell and buffers the output.

const exec = require('child_process').exec;
const child = exec('cat *.js bad_file | wc -l',
  (error, stdout, stderr) => {
    console.log(`stdout: ${stdout}`);
    console.log(`stderr: ${stderr}`);
    if (error !== null) {
      console.log(`exec error: ${error}`);
    }
});

The callback gets the arguments (error, stdout, stderr). On success, error will be null. On error, error will be an instance of Error and error.code will be the exit code of the child process, and error.signal will be set to the signal that terminated the process.

There is a second optional argument to specify several options. The default options are

{ encoding: 'utf8',
  timeout: 0,
  maxBuffer: 200*1024,
  killSignal: 'SIGTERM',
  cwd: null,
  env: null }

If timeout is greater than 0, then it will kill the child process if it runs longer than timeout milliseconds. The child process is killed with killSignal (default: 'SIGTERM'). maxBuffer specifies the largest amount of data (in bytes) allowed on stdout or stderr - if this value is exceeded then the child process is killed.

Note: Unlike the exec() POSIX system call, child_process.exec() does not replace the existing process and uses a shell to execute the command.

child_process.execFile(file[, args][, options][, callback])#

This is similar to child_process.exec() except it does not execute a subshell but rather the specified file directly. This makes it slightly leaner than child_process.exec(). It has the same options.

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

This is a special case of the child_process.spawn() functionality for spawning Node.js processes. In addition to having all the methods in a normal ChildProcess instance, the returned object has a communication channel built-in. See ChildProcess#send() for details.

These child Node.js processes are still whole new instances of V8. Assume at least 30ms startup and 10mb memory for each new Node.js. That is, you cannot create many thousands of them.

The execPath property in the options object allows for a process to be created for the child rather than the current node executable. This should be done with care and by default will talk over the fd represented an environmental variable NODE_CHANNEL_FD on the child process. The input and output on this fd is expected to be line delimited JSON objects.

Note: Unlike the fork() POSIX system call, child_process.fork() does not clone the current process.

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

Launches a new process with the given command, with command line arguments in args. If omitted, args defaults to an empty Array.

The third argument is used to specify additional options, with these defaults:

{ cwd: undefined,
  env: process.env
}

Use cwd to specify the working directory from which the process is spawned. If not given, the default is to inherit the current working directory.

Use env to specify environment variables that will be visible to the new process, the default is process.env.

Example of running ls -lh /usr, capturing stdout, stderr, and the exit code:

const spawn = require('child_process').spawn;
const ls = spawn('ls', ['-lh', '/usr']);

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

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

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

Example: A very elaborate way to run 'ps ax | grep ssh'

const spawn = require('child_process').spawn;
const ps = spawn('ps', ['ax']);
const grep = spawn('grep', ['ssh']);

ps.stdout.on('data', (data) => {
  grep.stdin.write(data);
});

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

ps.on('close', (code) => {
  if (code !== 0) {
    console.log(`ps process exited with code ${code}`);
  }
  grep.stdin.end();
});

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

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

grep.on('close', (code) => {
  if (code !== 0) {
    console.log(`grep process exited with code ${code}`);
  }
});

Example of checking for failed exec:

const spawn = require('child_process').spawn;
const child = spawn('bad_command');

child.on('error', (err) => {
  console.log('Failed to start child process.');
});

options.detached#

On Windows, this makes it possible for the child to continue running after the parent exits. The child will have a new console window (this cannot be disabled).

On non-Windows, if the detached option is set, the child process will be made the leader of a new process group and session. Note that child processes may continue running after the parent exits whether they are detached or not. See setsid(2) for more information.

By default, the parent will wait for the detached child to exit. To prevent the parent from waiting for a given child, use the child.unref() method, and the parent's event loop will not include the child in its reference count.

Example of detaching a long-running process and redirecting its output to a file:

 const fs = require('fs');
 const spawn = require('child_process').spawn;
 const out = fs.openSync('./out.log', 'a');
 const err = fs.openSync('./out.log', 'a');

 const child = spawn('prg', [], {
   detached: true,
   stdio: [ 'ignore', out, err ]
 });

 child.unref();

When using the detached option to start a long-running process, the process will not stay running in the background after the parent exits unless it is provided with a stdio configuration that is not connected to the parent. If the parent's stdio is inherited, the child will remain attached to the controlling terminal.

options.stdio#

As a shorthand, the stdio argument may be one of the following strings:

• 'pipe' - ['pipe', 'pipe', 'pipe'], this is the default value
• 'ignore' - ['ignore', 'ignore', 'ignore']
• 'inherit' - [process.stdin, process.stdout, process.stderr] or [0,1,2]

Otherwise, the 'stdio' option to child_process.spawn() is an array where each index corresponds to a fd in the child. The value is one of the following:

1. 'pipe' - Create a pipe between the child process and the parent process. The parent end of the pipe is exposed to the parent as a property on the child_process object as ChildProcess.stdio[fd]. Pipes created for fds 0 - 2 are also available as ChildProcess.stdin, ChildProcess.stdout and ChildProcess.stderr, respectively.
2. 'ipc' - Create an IPC channel for passing messages/file descriptors between parent and child. A ChildProcess may have at most one IPC stdio file descriptor. Setting this option enables the ChildProcess.send() method. If the child writes JSON messages to this file descriptor, then this will trigger ChildProcess.on('message'). If the child is an Node.js program, then the presence of an IPC channel will enable process.send() and process.on('message').
3. 'ignore' - Do not set this file descriptor in the child. Note that Node.js will always open fd 0 - 2 for the processes it spawns. When any of these is ignored Node.js will open /dev/null and attach it to the child's fd.
4. Stream object - Share a readable or writable stream that refers to a tty, file, socket, or a pipe with the child process. The stream's underlying file descriptor is duplicated in the child process to the fd that corresponds to the index in the stdio array. Note that the stream must have an underlying descriptor (file streams do not until the 'open' event has occurred).
5. Positive integer - The integer value is interpreted as a file descriptor that is is currently open in the parent process. It is shared with the child process, similar to how Stream objects can be shared.
6. null, undefined - Use default value. For stdio fds 0, 1 and 2 (in other words, stdin, stdout, and stderr) a pipe is created. For fd 3 and up, the default is 'ignore'.

Example:

const spawn = require('child_process').spawn;

// Child will use parent's stdios
spawn('prg', [], { stdio: 'inherit' });

// Spawn child sharing only stderr
spawn('prg', [], { stdio: ['pipe', 'pipe', process.stderr] });

// Open an extra fd=4, to interact with programs present a
// startd-style interface.
spawn('prg', [], { stdio: ['pipe', null, null, null, 'pipe'] });

See also: child_process.exec() and child_process.fork()

Synchronous Process Creation#

These methods are synchronous, meaning they WILL block the event loop, pausing execution of your code until the spawned process exits.

Blocking calls like these are mostly useful for simplifying general purpose scripting tasks and for simplifying the loading/processing of application configuration at startup.

child_process.execFileSync(file[, args][, options])#

execFileSync will not return until the child process has fully closed. When a timeout has been encountered and killSignal is sent, the method won't return until the process has completely exited. That is to say, if the process handles the SIGTERM signal and doesn't exit, your process will wait until the child process has exited.

If the process times out, or has a non-zero exit code, this method will throw. The Error object will contain the entire result from child_process.spawnSync()

child_process.execSync(command[, options])#

execSync will not return until the child process has fully closed. When a timeout has been encountered and killSignal is sent, the method won't return until the process has completely exited. That is to say, if the process handles the SIGTERM signal and doesn't exit, your process will wait until the child process has exited.

If the process times out, or has a non-zero exit code, this method will throw. The Error object will contain the entire result from child_process.spawnSync()

child_process.spawnSync(command[, args][, options])#

spawnSync will not return until the child process has fully closed. When a timeout has been encountered and killSignal is sent, the method won't return until the process has completely exited. That is to say, if the process handles the SIGTERM signal and doesn't exit, your process will wait until the child process has exited.