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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.

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
  • 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
    • 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 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
    • 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
    • customFds {Array} Deprecated File descriptors for the child to use for stdio. (See below)
    • env {Object} Environment key-value pairs
    • encoding {String} (Default: 'utf8')
    • timeout {Number} (Default: 0)
  • 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);
  }
});
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