5 Node provides a tri-directional `popen(3)` facility through the
6 `child_process` module.
8 It is possible to stream data through a child's `stdin`, `stdout`, and
9 `stderr` in a fully non-blocking way.
11 To create a child process use `require('child_process').spawn()` or
12 `require('child_process').fork()`. The semantics of each are slightly
13 different, and explained below.
15 ## Class: ChildProcess
17 `ChildProcess` is an `EventEmitter`.
19 Child processes always have three streams associated with them. `child.stdin`,
20 `child.stdout`, and `child.stderr`. These may be shared with the stdio
21 streams of the parent process, or they may be separate stream objects
22 which can be piped to and from.
24 The ChildProcess class is not intended to be used directly. Use the
25 `spawn()` or `fork()` methods to create a Child Process instance.
29 * `code` {Number} the exit code, if it exited normally.
30 * `signal` {String} the signal passed to kill the child process, if it
31 was killed by the parent.
33 This event is emitted after the child process ends. If the process terminated
34 normally, `code` is the final exit code of the process, otherwise `null`. If
35 the process terminated due to receipt of a signal, `signal` is the string name
36 of the signal, otherwise `null`.
38 Note that the child process stdio streams might still be open.
44 This event is emitted when the stdio streams of a child process have all
45 terminated. This is distinct from 'exit', since multiple processes
46 might share the same stdio streams.
48 ### Event: 'disconnect'
50 This event is emitted after using the `.disconnect()` method in the parent or
51 in the child. After disconnecting it is no longer possible to send messages.
52 An alternative way to check if you can send messages is to see if the
53 `child.connected` property is `true`.
57 * `message` {Object} a parsed JSON object or primitive value
58 * `sendHandle` {Handle object} a Socket or Server object
60 Messages send by `.send(message, [sendHandle])` are obtained using the
67 A `Writable Stream` that represents the child process's `stdin`.
68 Closing this stream via `end()` often causes the child process to terminate.
70 If the child stdio streams are shared with the parent, then this will
77 A `Readable Stream` that represents the child process's `stdout`.
79 If the child stdio streams are shared with the parent, then this will
86 A `Readable Stream` that represents the child process's `stderr`.
88 If the child stdio streams are shared with the parent, then this will
95 The PID of the child process.
99 var spawn = require('child_process').spawn,
100 grep = spawn('grep', ['ssh']);
102 console.log('Spawned child pid: ' + grep.pid);
105 ### child.kill([signal])
109 Send a signal to the child process. If no argument is given, the process will
110 be sent `'SIGTERM'`. See `signal(7)` for a list of available signals.
112 var spawn = require('child_process').spawn,
113 grep = spawn('grep', ['ssh']);
115 grep.on('exit', function (code, signal) {
116 console.log('child process terminated due to receipt of signal '+signal);
119 // send SIGHUP to process
122 Note that while the function is called `kill`, the signal delivered to the child
123 process may not actually kill it. `kill` really just sends a signal to a process.
127 ### child.send(message, [sendHandle])
130 * `sendHandle` {Handle object}
132 When using `child_process.fork()` you can write to the child using
133 `child.send(message, [sendHandle])` and messages are received by
134 a `'message'` event on the child.
138 var cp = require('child_process');
140 var n = cp.fork(__dirname + '/sub.js');
142 n.on('message', function(m) {
143 console.log('PARENT got message:', m);
146 n.send({ hello: 'world' });
148 And then the child script, `'sub.js'` might look like this:
150 process.on('message', function(m) {
151 console.log('CHILD got message:', m);
154 process.send({ foo: 'bar' });
156 In the child the `process` object will have a `send()` method, and `process`
157 will emit objects each time it receives a message on its channel.
159 There is a special case when sending a `{cmd: 'NODE_foo'}` message. All messages
160 containing a `NODE_` prefix in its `cmd` property will not be emitted in
161 the `message` event, since they are internal messages used by node core.
162 Messages containing the prefix are emitted in the `internalMessage` event, you
163 should by all means avoid using this feature, it is subject to change without notice.
165 The `sendHandle` option to `child.send()` is for sending a TCP server or
166 socket object to another process. The child will receive the object as its
167 second argument to the `message` event.
169 **send server object**
171 Here is an example of sending a server:
173 var child = require('child_process').fork('child.js');
175 // Open up the server object and send the handle.
176 var server = require('net').createServer();
177 server.on('connection', function (socket) {
178 socket.end('handled by parent');
180 server.listen(1337, function() {
181 child.send('server', server);
184 And the child would the recive the server object as:
186 process.on('message', function(m, server) {
187 if (m === 'server') {
188 server.on('connection', function (socket) {
189 socket.end('handled by child');
194 Note that the server is now shared between the parent and child, this means
195 that some connections will be handled by the parent and some by the child.
197 **send socket object**
199 Here is an example of sending a socket. It will spawn two childs and handle
200 connections with the remote address `74.125.127.100` as VIP by sending the
201 socket to a "special" child process. Other sockets will go to a "normal" process.
203 var normal = require('child_process').fork('child.js', ['normal']);
204 var special = require('child_process').fork('child.js', ['special']);
206 // Open up the server and send sockets to child
207 var server = require('net').createServer();
208 server.on('connection', function (socket) {
211 if (socket.remoteAddress === '74.125.127.100') {
212 special.send('socket', socket);
215 // just the usual dudes
216 normal.send('socket', socket);
220 The `child.js` could look like this:
222 process.on('message', function(m, socket) {
223 if (m === 'socket') {
224 socket.end('You where handled as a ' + process.argv[2] + ' person');
228 Note that once a single socket has been sent to a child the parent can no
229 longer keep track of when the socket is destroyed. To indicate this condition
230 the `.connections` property becomes `null`.
231 It is also recomended not to use `.maxConnections` in this condition.
233 ### child.disconnect()
235 To close the IPC connection between parent and child use the
236 `child.disconnect()` method. This allows the child to exit gracefully since
237 there is no IPC channel keeping it alive. When calling this method the
238 `disconnect` event will be emitted in both parent and child, and the
239 `connected` flag will be set to `false`. Please note that you can also call
240 `process.disconnect()` in the child process.
242 ## child_process.spawn(command, [args], [options])
244 * `command` {String} The command to run
245 * `args` {Array} List of string arguments
247 * `cwd` {String} Current working directory of the child process
248 * `stdio` {Array|String} Child's stdio configuration. (See below)
249 * `customFds` {Array} **Deprecated** File descriptors for the child to use
250 for stdio. (See below)
251 * `env` {Object} Environment key-value pairs
253 * return: {ChildProcess object}
255 Launches a new process with the given `command`, with command line arguments in `args`.
256 If omitted, `args` defaults to an empty Array.
258 The third argument is used to specify additional options, which defaults to:
264 `cwd` allows you to specify the working directory from which the process is spawned.
265 Use `env` to specify environment variables that will be visible to the new process.
267 Example of running `ls -lh /usr`, capturing `stdout`, `stderr`, and the exit code:
269 var util = require('util'),
270 spawn = require('child_process').spawn,
271 ls = spawn('ls', ['-lh', '/usr']);
273 ls.stdout.on('data', function (data) {
274 console.log('stdout: ' + data);
277 ls.stderr.on('data', function (data) {
278 console.log('stderr: ' + data);
281 ls.on('exit', function (code) {
282 console.log('child process exited with code ' + code);
286 Example: A very elaborate way to run 'ps ax | grep ssh'
288 var util = require('util'),
289 spawn = require('child_process').spawn,
290 ps = spawn('ps', ['ax']),
291 grep = spawn('grep', ['ssh']);
293 ps.stdout.on('data', function (data) {
294 grep.stdin.write(data);
297 ps.stderr.on('data', function (data) {
298 console.log('ps stderr: ' + data);
301 ps.on('exit', function (code) {
303 console.log('ps process exited with code ' + code);
308 grep.stdout.on('data', function (data) {
312 grep.stderr.on('data', function (data) {
313 console.log('grep stderr: ' + data);
316 grep.on('exit', function (code) {
318 console.log('grep process exited with code ' + code);
323 Example of checking for failed exec:
325 var spawn = require('child_process').spawn,
326 child = spawn('bad_command');
328 child.stderr.setEncoding('utf8');
329 child.stderr.on('data', function (data) {
330 if (/^execvp\(\)/.test(data)) {
331 console.log('Failed to start child process.');
335 Note that if spawn receives an empty options object, it will result in
336 spawning the process with an empty environment rather than using
337 `process.env`. This due to backwards compatibility issues with a deprecated
340 The 'stdio' option to `child_process.spawn()` is an array where each
341 index corresponds to a fd in the child. The value is one of the following:
343 1. `null`, `undefined` - Use default value. For 0,1,2 stdios this is the same
344 as `'pipe'`. For any higher value, `'ignore'`
345 2. `'ignore'` - Open the fd in the child, but do not expose it to the parent
346 3. `'pipe'` - Open the fd and expose as a `Stream` object to parent.
347 4. `'ipc'` - Create IPC channel for passing messages/file descriptors between
350 Note: A ChildProcess may have at most *one* IPC stdio file descriptor.
351 Setting this option enables the ChildProcess.send() method. If the
352 child writes JSON messages to this file descriptor, then this will trigger
353 ChildProcess.on('message'). If the child is a Node.js program, then
354 the presence of an IPC channel will enable process.send() and
355 process.on('message')
356 5. positive integer - Share corresponding fd with child
357 6. Any TTY, TCP, File stream (or any object with `fd` property) - Share
358 corresponding stream with child.
360 As a shorthand, the `stdio` argument may also be one of the following
361 strings, rather than an array:
363 * `ignore` - `['ignore', 'ignore', 'ignore']`
364 * `pipe` - `['pipe', 'pipe', 'pipe']`
365 * `inherit` - `[process.stdin, process.stdout, process.stderr]` or `[0,1,2]`
369 var spawn = require('child_process').spawn;
371 // Child will use parent's stdios
372 spawn('prg', [], { stdio: 'inherit' });
374 // Spawn child sharing only stderr
375 spawn('prg', [], { stdio: ['pipe', 'pipe', process.stderr] });
377 // Open an extra fd=4, to interact with programs present a
378 // startd-style interface.
379 spawn('prg', [], { stdio: ['pipe', null, null, null, 'pipe'] });
381 There is a deprecated option called `customFds` which allows one to specify
382 specific file descriptors for the stdio of the child process. This API was
383 not portable to all platforms and therefore removed.
384 With `customFds` it was possible to hook up the new process' `[stdin, stdout,
385 stderr]` to existing streams; `-1` meant that a new stream should be created.
386 Use at your own risk.
388 There are several internal options. In particular `stdinStream`,
389 `stdoutStream`, `stderrStream`. They are for INTERNAL USE ONLY. As with all
390 undocumented APIs in Node, they should not be used.
392 See also: `child_process.exec()` and `child_process.fork()`
394 ## child_process.exec(command, [options], callback)
396 * `command` {String} The command to run, with space-separated arguments
398 * `cwd` {String} Current working directory of the child process
399 * `stdio` {Array|String} Child's stdio configuration. (See above)
400 * `customFds` {Array} **Deprecated** File descriptors for the child to use
401 for stdio. (See above)
402 * `env` {Object} Environment key-value pairs
404 * `encoding` {String} (Default: 'utf8')
405 * `timeout` {Number} (Default: 0)
406 * `maxBuffer` {Number} (Default: 200*1024)
407 * `killSignal` {String} (Default: 'SIGTERM')
408 * `callback` {Function} called with the output when process terminates
412 * Return: ChildProcess object
414 Runs a command in a shell and buffers the output.
416 var util = require('util'),
417 exec = require('child_process').exec,
420 child = exec('cat *.js bad_file | wc -l',
421 function (error, stdout, stderr) {
422 console.log('stdout: ' + stdout);
423 console.log('stderr: ' + stderr);
424 if (error !== null) {
425 console.log('exec error: ' + error);
429 The callback gets the arguments `(error, stdout, stderr)`. On success, `error`
430 will be `null`. On error, `error` will be an instance of `Error` and `err.code`
431 will be the exit code of the child process, and `err.signal` will be set to the
432 signal that terminated the process.
434 There is a second optional argument to specify several options. The
440 killSignal: 'SIGTERM',
444 If `timeout` is greater than 0, then it will kill the child process
445 if it runs longer than `timeout` milliseconds. The child process is killed with
446 `killSignal` (default: `'SIGTERM'`). `maxBuffer` specifies the largest
447 amount of data allowed on stdout or stderr - if this value is exceeded then
448 the child process is killed.
451 ## child_process.execFile(file, args, options, callback)
453 * `file` {String} The filename of the program to run
454 * `args` {Array} List of string arguments
456 * `cwd` {String} Current working directory of the child process
457 * `stdio` {Array|String} Child's stdio configuration. (See above)
458 * `customFds` {Array} **Deprecated** File descriptors for the child to use
459 for stdio. (See above)
460 * `env` {Object} Environment key-value pairs
462 * `encoding` {String} (Default: 'utf8')
463 * `timeout` {Number} (Default: 0)
464 * `maxBuffer` {Number} (Default: 200*1024)
465 * `killSignal` {String} (Default: 'SIGTERM')
466 * `callback` {Function} called with the output when process terminates
470 * Return: ChildProcess object
472 This is similar to `child_process.exec()` except it does not execute a
473 subshell but rather the specified file directly. This makes it slightly
474 leaner than `child_process.exec`. It has the same options.
477 ## child_process.fork(modulePath, [args], [options])
479 * `modulePath` {String} The module to run in the child
480 * `args` {Array} List of string arguments
482 * `cwd` {String} Current working directory of the child process
483 * `env` {Object} Environment key-value pairs
485 * `encoding` {String} (Default: 'utf8')
486 * `timeout` {Number} (Default: 0)
487 * Return: ChildProcess object
489 This is a special case of the `spawn()` functionality for spawning Node
490 processes. In addition to having all the methods in a normal ChildProcess
491 instance, the returned object has a communication channel built-in. See
492 `child.send(message, [sendHandle])` for details.
494 By default the spawned Node process will have the stdout, stderr associated
495 with the parent's. To change this behavior set the `silent` property in the
496 `options` object to `true`.
498 These child Nodes are still whole new instances of V8. Assume at least 30ms
499 startup and 10mb memory for each new Node. That is, you cannot create many