## Child Processes

Node provides a tri-directional `popen(3)` facility through the `ChildProcess`
class.

It is possible to stream data through the child's `stdin`, `stdout`, and
`stderr` in a fully non-blocking way.

To create a child process use `require('child_process').spawn()`.

Child processes always have three streams associated with them. `child.stdin`,
`child.stdout`, and `child.stderr`.

`ChildProcess` is an `EventEmitter`.

### Event:  'exit'

`function (code, signal) {}`

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

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

### child.stdout

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

### child.stderr

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

### child.pid

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_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, which defaults to:

    { cwd: undefined
    , env: process.env,
    , customFds: [-1, -1, -1]
    }

`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.
With `customFds` it is possible to hook up the new process' [stdin, stout, stderr] to
existing streams; `-1` means that a new stream should be created.

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) {
      util.print('stdout: ' + data);
    });

    ls.stderr.on('data', function (data) {
      util.print('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) {
      util.print('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) {
      util.print(data);
    });

    grep.stderr.on('data', function (data) {
      util.print('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.on('data', function (data) {
      if (/^execvp\(\)/.test(data.asciiSlice(0,data.length))) {
        console.log('Failed to start child process.');
      }
    });


See also: `child_process.exec()`

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

High-level way to execute a command as a child process, buffer the
output, and return it all in a callback.

    var util   = require('util'),
        exec  = require('child_process').exec,
        child;

    child = exec('cat *.js bad_file | wc -l', 
      function (error, stdout, stderr) {
        util.print('stdout: ' + stdout);
        util.print('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.kill(signal='SIGTERM')

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)`