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@nodejs

# 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.

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: '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`.

See `waitpid(2)`.

### 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.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('exit', function (code, signal) {
      console.log('child process terminated due to receipt of signal '+signal);
    });

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

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

Send a message (and, optionally, a handle object) to a child process.

See `child_process.fork()` for details.

## 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
  - `customFds` Array Deprecated File descriptors for the child to use for stdio. (See below)
  - `env` Object Environment key-value pairs
  - `setsid` Boolean
- 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 util  = require('util'),
        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('exit', function (code) {
      console.log('child process exited with code ' + code);
    });

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

    var util  = require('util'),
        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('exit', 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('exit', 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.

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.

There are several internal options. In particular `stdinStream`, `stdoutStream`, `stderrStream`. They are for INTERNAL USE ONLY. As with all undocumented APIs in Node, they should not be used.

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
  - `customFds` Array Deprecated File descriptors for the child to use for stdio. (See below)
  - `env` Object Environment key-value pairs
  - `setsid` Boolean
  - `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
  - `code` Integer Exit code
  - `stdout` Buffer
  - `stderr` Buffer
- Return: ChildProcess object

Runs a command in a shell and buffers the output.

    var util = require('util'),
        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
  - `customFds` Array Deprecated File descriptors for the child to use for stdio. (See below)
  - `env` Object Environment key-value pairs
  - `setsid` Boolean
  - `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
  - `code` Integer Exit code
  - `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
  - `customFds` Array Deprecated File descriptors for the child to use for stdio. (See below)
  - `env` Object Environment key-value pairs
  - `setsid` Boolean
  - `encoding` String (Default: 'utf8')
  - `timeout` Number (Default: 0)
- `callback` Function called with the output when process terminates
  - `code` Integer Exit code
  - `stdout` Buffer
  - `stderr` Buffer
- 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. The channel is written to with `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.

By default the spawned Node process will have the stdin, stdout, stderr associated with the parent's.

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 `sendHandle` option to `child.send()` is for sending a handle object to another process. Child will receive the handle as as second argument to the `message` event. Here is an example of sending a handle:

    var server = require('net').createServer();
    var child = require('child_process').fork(__dirname + '/child.js');
    // Open up the server object and send the handle.
    server.listen(1337, function() {
      child.send({ server: true }, server._handle);
    });

Here is an example of receiving the server handle and sharing it between processes:

    process.on('message', function(m, serverHandle) {
      if (serverHandle) {
        var server = require('net').createServer();
        server.listen(serverHandle);
      }
    });