node/doc/api/child_process.markdown

# 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.

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

See also [`ChildProcess#kill()`](#child_process_child_kill_signal) and
[`ChildProcess#send()`](#child_process_child_send_message_sendhandle).

### 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`.
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.

### child.stdout

* {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.

### child.stderr

* {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.

### 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`](#child_process_child_process_spawn_command_args_options)).

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.

## 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) {
      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);
      }
    });


Example of checking for failed exec:

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

    child.stderr.setEncoding('utf8');
    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. 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'`.

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]`

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'] });

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.

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')
  * `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;

    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
  * `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)
* 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.

[EventEmitter]: events.html#events_class_events_eventemitter