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 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, and 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 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.

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’

• 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.

child.stdin

• {Stream object}

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.stdout

• {Stream object}

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.

child.stderr

• {Stream object}

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.stdio

• {Array}

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.

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.pid

• {Integer}

The PID of the child process.

Example:

var spawn = require('child_process').spawn,
    grep  = spawn('grep', ['ssh']);
console.log('Spawned child pid: ' + grep.pid);
grep.stdin.end();

child.connected

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

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

child.kill([signal])

• 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
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: 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.

Please note that the send() method on both the parent and child are
synchronous - sending large chunks of data is not advised (pipes can be used
instead, see
child_process.spawn).

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 74.125.127.100 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 === '74.125.127.100') {
    special.send('socket', socket);
    return;
  }
  // just the usual dudes
  normal.send('socket', socket);
});
server.listen(1337);

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.

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()).

Asynchronous Process Creation

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

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
  • env {Object} Environment key-value pairs
  • stdio {Array|String} Child’s stdio configuration. (See
    below)
  • customFds {Array} Deprecated File descriptors for the child to use
    for stdio. (See below)
  • 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, 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:

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) {
  grep.stdin.write(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.stdin.end();
});
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);
  }
});

options.stdio

As a shorthand, the stdio argument may also 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 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. 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:

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'] });

options.detached

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 ]
 });
 child.unref();

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.

options.customFds

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’)
  • uid {Number} Sets the user identity of the process. (See setuid(2).)
  • gid {Number} Sets the group identity of the process. (See setgid(2).)
• 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;
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 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 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’)
  • uid {Number} Sets the user identity of the process. (See setuid(2).)
  • gid {Number} Sets the group identity of the process. (See setgid(2).)
• 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
  • execPath {String} Executable used to create the child process
  • execArgv {Array} List of string arguments passed to the executable
    (Default: process.execArgv)
  • silent {Boolean} If true, stdin, stdout, and stderr of the child will be
    piped to the parent, otherwise they will be inherited from the parent, see
    the “pipe” and “inherit” options for spawn()‘s stdio for more details
    (default is false)
  • 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

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.

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.spawnSync(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
  • input {String|Buffer} The value which will be passed as stdin to the spawned process
    • supplying this value will override stdio[0]
  • stdio {Array} Child’s stdio configuration.
  • env {Object} Environment key-value pairs
  • uid {Number} Sets the user identity of the process. (See setuid(2).)
  • gid {Number} Sets the group identity of the process. (See setgid(2).)
  • timeout {Number} In milliseconds the maximum amount of time the process is allowed to run. (Default: undefined)
  • killSignal {String} The signal value to be used when the spawned process will be killed. (Default: ‘SIGTERM’)
  • maxBuffer {Number}
  • encoding {String} The encoding used for all stdio inputs and outputs. (Default: ‘buffer’)
• return: {Object}
  • pid {Number} Pid of the child process
  • output {Array} Array of results from stdio output
  • stdout {Buffer|String} The contents of output[1]
  • stderr {Buffer|String} The contents of output[2]
  • status {Number} The exit code of the child process
  • signal {String} The signal used to kill the child process
  • error {Error} The error object if the child process failed or timed out

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.

child_process.execFileSync(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
  • input {String|Buffer} The value which will be passed as stdin to the spawned process
    • supplying this value will override stdio[0]
  • stdio {Array} Child’s stdio configuration. (Default: ‘pipe’)
    • stderr by default will be output to the parent process’ stderr unless
      stdio is specified
  • env {Object} Environment key-value pairs
  • uid {Number} Sets the user identity of the process. (See setuid(2).)
  • gid {Number} Sets the group identity of the process. (See setgid(2).)
  • timeout {Number} In milliseconds the maximum amount of time the process is allowed to run. (Default: undefined)
  • killSignal {String} The signal value to be used when the spawned process will be killed. (Default: ‘SIGTERM’)
  • maxBuffer {Number}
  • encoding {String} The encoding used for all stdio inputs and outputs. (Default: ‘buffer’)
• return: {Buffer|String} The stdout from the command

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])

• command {String} The command to run
• options {Object}
  • cwd {String} Current working directory of the child process
  • input {String|Buffer} The value which will be passed as stdin to the spawned process
    • supplying this value will override stdio[0]
  • stdio {Array} Child’s stdio configuration. (Default: ‘pipe’)
    • stderr by default will be output to the parent process’ stderr unless
      stdio is specified
  • env {Object} Environment key-value pairs
  • uid {Number} Sets the user identity of the process. (See setuid(2).)
  • gid {Number} Sets the group identity of the process. (See setgid(2).)
  • timeout {Number} In milliseconds the maximum amount of time the process is allowed to run. (Default: undefined)
  • killSignal {String} The signal value to be used when the spawned process will be killed. (Default: ‘SIGTERM’)
  • maxBuffer {Number}
  • encoding {String} The encoding used for all stdio inputs and outputs. (Default: ‘buffer’)
• return: {Buffer|String} The stdout from the command

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