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Child Process

Stability: 3 - Stable

Node 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 is not immediately consumed.)

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

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() or fork() methods to create a Child Process instance.

Event: 'error'

  • err {Error Object} the 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 for whatever reason.

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

Event: 'exit'

  • code {Number} the exit code, if it exited normally.
  • signal {String} the signal passed to kill the child process, if it was killed by the parent.

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 establishes signal handlers for 'SIGINT' and 'SIGTERM', so it will not terminate due to receipt of those signals, it will exit.

See waitpid(2).

Event: 'close'

  • code {Number} the exit code, if it exited normally.
  • signal {String} the signal passed to kill the child process, if it was killed by the parent.

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: 'message'

  • message {Object} a parsed JSON object or primitive value
  • sendHandle {Handle object} a Socket or Server object

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


  • {Stream object}

A Writable Stream that represents the child process's stdin. Closing this stream via end() often causes the child process to terminate.

If the child stdio streams are shared with the parent, then this will not be set.


  • {Stream object}

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

If the child stdio streams are shared with the parent, then this will not be set.


  • {Stream object}

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

If the child stdio streams are shared with the parent, then this will not be set.

  • {Integer}

The PID of the child process.


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

console.log('Spawned child pid: ' +;


  • {Boolean} Set to false after `.disconnect' is called

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


  • signal {String}

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.

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

grep.on('close', function (code, signal) {
  console.log('child process terminated due to receipt of signal '+signal);

// send SIGHUP to process

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: if the PID (the process ID) 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.send(message, [sendHandle])

  • message {Object}
  • sendHandle {Handle object}

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

For example:

var cp = require('child_process');

var n = cp.fork(__dirname + '/sub.js');

n.on('message', function(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', function(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 core. Messages containing the prefix are emitted in the internalMessage event, you should by all means 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.

Emits an 'error' event if the message cannot be sent, for example because the child process has already exited.

Example: sending server object

Here is an example of sending a server:

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

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

And the child would the receive the server object as:

process.on('message', function(m, server) {
  if (m === 'server') {
    server.on('connection', function (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 as VIP by sending the socket to a "special" child process. Other sockets will go to a "normal" process.

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

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

  // if this is a VIP
  if (socket.remoteAddress === '') {
    special.send('socket', socket);
  // just the usual dudes
  normal.send('socket', socket);

The child.js could look like this:

process.on('message', function(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.


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.

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

  • command {String} The command to run
  • args {Array} List of string arguments
  • options {Object}
    • cwd {String} Current working directory of the child process
    • stdio {Array|String} Child's stdio configuration. (See below)
    • customFds {Array} Deprecated File descriptors for the child to use for stdio. (See below)
    • env {Object} Environment key-value pairs
    • detached {Boolean} The child will be a process group leader. (See below)
    • uid {Number} Sets the user identity of the process. (See setuid(2).)
    • gid {Number} Sets the group identity of the process. (See setgid(2).)
  • return: {ChildProcess object}

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, which defaults to:

{ cwd: undefined,
  env: process.env

cwd allows you to specify the working directory from which the process is spawned. Use env to specify environment variables that will be visible to the new process.

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

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

ls.stdout.on('data', function (data) {
  console.log('stdout: ' + data);

ls.stderr.on('data', function (data) {
  console.log('stderr: ' + data);

ls.on('close', function (code) {
  console.log('child process exited with code ' + code);

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

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

ps.stdout.on('data', function (data) {

ps.stderr.on('data', function (data) {
  console.log('ps stderr: ' + data);

ps.on('close', function (code) {
  if (code !== 0) {
    console.log('ps process exited with code ' + code);

grep.stdout.on('data', function (data) {
  console.log('' + data);

grep.stderr.on('data', function (data) {
  console.log('grep stderr: ' + data);

grep.on('close', function (code) {
  if (code !== 0) {
    console.log('grep process exited with code ' + code);

Example of checking for failed exec:

var spawn = require('child_process').spawn,
    child = spawn('bad_command');

child.stderr.on('data', function (data) {
  if (/^execvp\(\)/.test(data)) {
    console.log('Failed to start child process.');

Note that if spawn receives an empty options object, it will result in spawning the process with an empty environment rather than using process.env. This due to backwards compatibility issues with a deprecated API.

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 a 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 will always open fd 0 - 2 for the processes it spawns. When any of these is ignored node 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.
  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'.

As a shorthand, the stdio argument may also be one of the following strings, rather than an array:

  • ignore - ['ignore', 'ignore', 'ignore']
  • pipe - ['pipe', 'pipe', 'pipe']
  • inherit - [process.stdin, process.stdout, process.stderr] or [0,1,2]


var 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'] });

If the detached option is set, the child process will be made the leader of a new process group. This makes it possible for the child to continue running after the parent exits.

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:

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

 var child = spawn('prg', [], {
   detached: true,
   stdio: [ 'ignore', out, err ]


When using the detached option to start a long-running process, the process will not stay running in the background 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.

There is a deprecated option called customFds which allows one to specify specific file descriptors for the stdio of the child process. This API was not portable to all platforms and therefore removed. With customFds it was possible to hook up the new process' [stdin, stdout, stderr] to existing streams; -1 meant that a new stream should be created. Use at your own risk.

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

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

  • command {String} The command to run, with space-separated arguments
  • options {Object}
    • cwd {String} Current working directory of the child process
    • env {Object} Environment key-value pairs
    • encoding {String} (Default: 'utf8')
    • shell {String} Shell to execute the command with (Default: '/bin/sh' on UNIX, 'cmd.exe' on Windows, The shell should understand the -c switch on UNIX or /s /c on Windows. On Windows, command line parsing should be compatible with cmd.exe.)
    • timeout {Number} (Default: 0)
    • maxBuffer {Number} (Default: 200*1024)
    • killSignal {String} (Default: 'SIGTERM')
  • callback {Function} called with the output when process terminates
    • error {Error}
    • stdout {Buffer}
    • stderr {Buffer}
  • Return: ChildProcess object

Runs a command in a shell and buffers the output.

var exec = require('child_process').exec,

child = exec('cat *.js bad_file | wc -l',
  function (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 err.code will be the exit code of the child process, and err.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 allowed on stdout or stderr - if this value is exceeded then the child process is killed.

child_process.execFile(file, args, options, callback)

  • file {String} The filename of the program to run
  • args {Array} List of string arguments
  • options {Object}
    • cwd {String} Current working directory of the child process
    • env {Object} Environment key-value pairs
    • encoding {String} (Default: 'utf8')
    • timeout {Number} (Default: 0)
    • maxBuffer {Number} (Default: 200*1024)
    • killSignal {String} (Default: 'SIGTERM')
  • callback {Function} called with the output when process terminates
    • error {Error}
    • stdout {Buffer}
    • stderr {Buffer}
  • Return: ChildProcess object

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])

  • modulePath {String} The module to run in the child
  • args {Array} List of string arguments
  • options {Object}
    • cwd {String} Current working directory of the child process
    • env {Object} Environment key-value pairs
    • encoding {String} (Default: 'utf8')
    • execPath {String} Executable used to create the child process
    • silent {Boolean} If true, prevent stdout and stderr in the spawned node process from being associated with the parent's (default is false)
  • Return: ChildProcess object

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

These child Nodes are still whole new instances of V8. Assume at least 30ms startup and 10mb memory for each new Node. 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.

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