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 any connections will be load balanced between the servers provided there
196 is a `connection` listener.
198 **send socket object**
200 Here is an example of sending a socket. It will spawn two childs and handle
201 connections with the remote address `74.125.127.100` as VIP by sending the
202 socket to a "special" child process. Other sockets will go to a "normal" process.
204 var normal = require('child_process').fork('child.js', ['normal']);
205 var special = require('child_process').fork('child.js', ['special']);
207 // Open up the server and send sockets to child
208 var server = require('net').createServer();
209 server.on('connection', function (socket) {
212 if (socket.remoteAddress === '74.125.127.100') {
213 special.send('socket', socket);
216 // just the usual dudes
217 normal.send('socket', socket);
221 The `child.js` could look like this:
223 process.on('message', function(m, socket) {
224 if (m === 'socket') {
225 socket.end('You where handled as a ' + process.argv[2] + ' person');
229 Note that once a single socket has been sent to a child the parent can no
230 longer keep track of when the socket is destroyed. To indicate this condition
231 the `.connections` property becomes `null`.
232 It is also recomended not to use `.maxConnections` in this condition.
234 ### child.disconnect()
236 To close the IPC connection between parent and child use the
237 `child.disconnect()` method. This allows the child to exit gracefully since
238 there is no IPC channel keeping it alive. When calling this method the
239 `disconnect` event will be emitted in both parent and child, and the
240 `connected` flag will be set to `false`. Please note that you can also call
241 `process.disconnect()` in the child process.
243 ## child_process.spawn(command, [args], [options])
245 * `command` {String} The command to run
246 * `args` {Array} List of string arguments
248 * `cwd` {String} Current working directory of the child process
249 * `stdio` {Array|String} Child's stdio configuration. (See below)
250 * `customFds` {Array} **Deprecated** File descriptors for the child to use
251 for stdio. (See below)
252 * `env` {Object} Environment key-value pairs
253 * `detached` {Boolean} The child will be a process group leader. (See below)
254 * return: {ChildProcess object}
256 Launches a new process with the given `command`, with command line arguments in `args`.
257 If omitted, `args` defaults to an empty Array.
259 The third argument is used to specify additional options, which defaults to:
265 `cwd` allows you to specify the working directory from which the process is spawned.
266 Use `env` to specify environment variables that will be visible to the new process.
268 Example of running `ls -lh /usr`, capturing `stdout`, `stderr`, and the exit code:
270 var util = require('util'),
271 spawn = require('child_process').spawn,
272 ls = spawn('ls', ['-lh', '/usr']);
274 ls.stdout.on('data', function (data) {
275 console.log('stdout: ' + data);
278 ls.stderr.on('data', function (data) {
279 console.log('stderr: ' + data);
282 ls.on('exit', function (code) {
283 console.log('child process exited with code ' + code);
287 Example: A very elaborate way to run 'ps ax | grep ssh'
289 var util = require('util'),
290 spawn = require('child_process').spawn,
291 ps = spawn('ps', ['ax']),
292 grep = spawn('grep', ['ssh']);
294 ps.stdout.on('data', function (data) {
295 grep.stdin.write(data);
298 ps.stderr.on('data', function (data) {
299 console.log('ps stderr: ' + data);
302 ps.on('exit', function (code) {
304 console.log('ps process exited with code ' + code);
309 grep.stdout.on('data', function (data) {
310 console.log('' + data);
313 grep.stderr.on('data', function (data) {
314 console.log('grep stderr: ' + data);
317 grep.on('exit', function (code) {
319 console.log('grep process exited with code ' + code);
324 Example of checking for failed exec:
326 var spawn = require('child_process').spawn,
327 child = spawn('bad_command');
329 child.stderr.setEncoding('utf8');
330 child.stderr.on('data', function (data) {
331 if (/^execvp\(\)/.test(data)) {
332 console.log('Failed to start child process.');
336 Note that if spawn receives an empty options object, it will result in
337 spawning the process with an empty environment rather than using
338 `process.env`. This due to backwards compatibility issues with a deprecated
341 The 'stdio' option to `child_process.spawn()` is an array where each
342 index corresponds to a fd in the child. The value is one of the following:
344 1. `'pipe'` - Create a pipe between the child process and the parent process.
345 The parent end of the pipe is exposed to the parent as a property on the
346 `child_process` object as `ChildProcess.stdio[fd]`. Pipes created for
347 fds 0 - 2 are also available as ChildProcess.stdin, ChildProcess.stdout
348 and ChildProcess.stderr, respectively.
349 2. `'ipc'` - Create an IPC channel for passing messages/file descriptors
350 between parent and child. A ChildProcess may have at most *one* IPC stdio
351 file descriptor. Setting this option enables the ChildProcess.send() method.
352 If the child writes JSON messages to this file descriptor, then this will
353 trigger 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 3. `'ignore'` - Do not set this file descriptor in the child. Note that Node
357 will always open fd 0 - 2 for the processes it spawns. When any of these is
358 ignored node will open `/dev/null` and attach it to the child's fd.
359 4. `Stream` object - Share a readable or writable stream that refers to a tty,
360 file, socket, or a pipe with the child process. The stream's underlying
361 file descriptor is duplicated in the child process to the fd that
362 corresponds to the index in the `stdio` array.
363 5. Positive integer - The integer value is interpreted as a file descriptor
364 that is is currently open in the parent process. It is shared with the child
365 process, similar to how `Stream` objects can be shared.
366 6. `null`, `undefined` - Use default value. For stdio fds 0, 1 and 2 (in other
367 words, stdin, stdout, and stderr) a pipe is created. For fd 3 and up, the
368 default is `'ignore'`.
370 As a shorthand, the `stdio` argument may also be one of the following
371 strings, rather than an array:
373 * `ignore` - `['ignore', 'ignore', 'ignore']`
374 * `pipe` - `['pipe', 'pipe', 'pipe']`
375 * `inherit` - `[process.stdin, process.stdout, process.stderr]` or `[0,1,2]`
379 var spawn = require('child_process').spawn;
381 // Child will use parent's stdios
382 spawn('prg', [], { stdio: 'inherit' });
384 // Spawn child sharing only stderr
385 spawn('prg', [], { stdio: ['pipe', 'pipe', process.stderr] });
387 // Open an extra fd=4, to interact with programs present a
388 // startd-style interface.
389 spawn('prg', [], { stdio: ['pipe', null, null, null, 'pipe'] });
391 If the `detached` option is set, the child process will be made the leader of a
392 new process group. This makes it possible for the child to continue running
393 after the parent exits.
395 By default, the parent will wait for the detached child to exit. To prevent
396 the parent from waiting for a given `child`, use the `child.unref()` method,
397 and the parent's event loop will not include the child in its reference count.
399 Example of detaching a long-running process and redirecting its output to a
402 var fs = require('fs'),
403 spawn = require('child_process').spawn,
404 out = fs.openSync('./out.log', 'a'),
405 err = fs.openSync('./out.log', 'a');
407 var child = spawn('prg', [], {
409 stdio: [ 'ignore', out, err ]
414 When using the `detached` option to start a long-running process, the process
415 will not stay running in the background unless it is provided with a `stdio`
416 configuration that is not connected to the parent. If the parent's `stdio` is
417 inherited, the child will remain attached to the controlling terminal.
419 There is a deprecated option called `customFds` which allows one to specify
420 specific file descriptors for the stdio of the child process. This API was
421 not portable to all platforms and therefore removed.
422 With `customFds` it was possible to hook up the new process' `[stdin, stdout,
423 stderr]` to existing streams; `-1` meant that a new stream should be created.
424 Use at your own risk.
426 There are several internal options. In particular `stdinStream`,
427 `stdoutStream`, `stderrStream`. They are for INTERNAL USE ONLY. As with all
428 undocumented APIs in Node, they should not be used.
430 See also: `child_process.exec()` and `child_process.fork()`
432 ## child_process.exec(command, [options], callback)
434 * `command` {String} The command to run, with space-separated arguments
436 * `cwd` {String} Current working directory of the child process
437 * `stdio` {Array|String} Child's stdio configuration. (See above)
438 * `customFds` {Array} **Deprecated** File descriptors for the child to use
439 for stdio. (See above)
440 * `env` {Object} Environment key-value pairs
441 * `encoding` {String} (Default: 'utf8')
442 * `timeout` {Number} (Default: 0)
443 * `maxBuffer` {Number} (Default: 200*1024)
444 * `killSignal` {String} (Default: 'SIGTERM')
445 * `callback` {Function} called with the output when process terminates
449 * Return: ChildProcess object
451 Runs a command in a shell and buffers the output.
453 var exec = require('child_process').exec,
456 child = exec('cat *.js bad_file | wc -l',
457 function (error, stdout, stderr) {
458 console.log('stdout: ' + stdout);
459 console.log('stderr: ' + stderr);
460 if (error !== null) {
461 console.log('exec error: ' + error);
465 The callback gets the arguments `(error, stdout, stderr)`. On success, `error`
466 will be `null`. On error, `error` will be an instance of `Error` and `err.code`
467 will be the exit code of the child process, and `err.signal` will be set to the
468 signal that terminated the process.
470 There is a second optional argument to specify several options. The
476 killSignal: 'SIGTERM',
480 If `timeout` is greater than 0, then it will kill the child process
481 if it runs longer than `timeout` milliseconds. The child process is killed with
482 `killSignal` (default: `'SIGTERM'`). `maxBuffer` specifies the largest
483 amount of data allowed on stdout or stderr - if this value is exceeded then
484 the child process is killed.
487 ## child_process.execFile(file, args, options, callback)
489 * `file` {String} The filename of the program to run
490 * `args` {Array} List of string arguments
492 * `cwd` {String} Current working directory of the child process
493 * `stdio` {Array|String} Child's stdio configuration. (See above)
494 * `customFds` {Array} **Deprecated** File descriptors for the child to use
495 for stdio. (See above)
496 * `env` {Object} Environment key-value pairs
497 * `encoding` {String} (Default: 'utf8')
498 * `timeout` {Number} (Default: 0)
499 * `maxBuffer` {Number} (Default: 200\*1024)
500 * `killSignal` {String} (Default: 'SIGTERM')
501 * `callback` {Function} called with the output when process terminates
505 * Return: ChildProcess object
507 This is similar to `child_process.exec()` except it does not execute a
508 subshell but rather the specified file directly. This makes it slightly
509 leaner than `child_process.exec`. It has the same options.
512 ## child\_process.fork(modulePath, [args], [options])
514 * `modulePath` {String} The module to run in the child
515 * `args` {Array} List of string arguments
517 * `cwd` {String} Current working directory of the child process
518 * `env` {Object} Environment key-value pairs
519 * `encoding` {String} (Default: 'utf8')
520 * Return: ChildProcess object
522 This is a special case of the `spawn()` functionality for spawning Node
523 processes. In addition to having all the methods in a normal ChildProcess
524 instance, the returned object has a communication channel built-in. See
525 `child.send(message, [sendHandle])` for details.
527 By default the spawned Node process will have the stdout, stderr associated
528 with the parent's. To change this behavior set the `silent` property in the
529 `options` object to `true`.
531 The child process does not automatically exit once it's done, you need to call
532 `process.exit()` explicitly. This limitation may be lifted in the future.
534 These child Nodes are still whole new instances of V8. Assume at least 30ms
535 startup and 10mb memory for each new Node. That is, you cannot create many
538 [EventEmitter]: events.html#events_class_events_eventemitter