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 * `customFds` {Array} **Deprecated** File descriptors for the child to use
249 for stdio. (See below)
250 * `env` {Object} Environment key-value pairs
252 * return: {ChildProcess object}
254 Launches a new process with the given `command`, with command line arguments in `args`.
255 If omitted, `args` defaults to an empty Array.
257 The third argument is used to specify additional options, which defaults to:
263 `cwd` allows you to specify the working directory from which the process is spawned.
264 Use `env` to specify environment variables that will be visible to the new process.
266 Example of running `ls -lh /usr`, capturing `stdout`, `stderr`, and the exit code:
268 var util = require('util'),
269 spawn = require('child_process').spawn,
270 ls = spawn('ls', ['-lh', '/usr']);
272 ls.stdout.on('data', function (data) {
273 console.log('stdout: ' + data);
276 ls.stderr.on('data', function (data) {
277 console.log('stderr: ' + data);
280 ls.on('exit', function (code) {
281 console.log('child process exited with code ' + code);
285 Example: A very elaborate way to run 'ps ax | grep ssh'
287 var util = require('util'),
288 spawn = require('child_process').spawn,
289 ps = spawn('ps', ['ax']),
290 grep = spawn('grep', ['ssh']);
292 ps.stdout.on('data', function (data) {
293 grep.stdin.write(data);
296 ps.stderr.on('data', function (data) {
297 console.log('ps stderr: ' + data);
300 ps.on('exit', function (code) {
302 console.log('ps process exited with code ' + code);
307 grep.stdout.on('data', function (data) {
311 grep.stderr.on('data', function (data) {
312 console.log('grep stderr: ' + data);
315 grep.on('exit', function (code) {
317 console.log('grep process exited with code ' + code);
322 Example of checking for failed exec:
324 var spawn = require('child_process').spawn,
325 child = spawn('bad_command');
327 child.stderr.setEncoding('utf8');
328 child.stderr.on('data', function (data) {
329 if (/^execvp\(\)/.test(data)) {
330 console.log('Failed to start child process.');
334 Note that if spawn receives an empty options object, it will result in
335 spawning the process with an empty environment rather than using
336 `process.env`. This due to backwards compatibility issues with a deprecated
339 There is a deprecated option called `customFds` which allows one to specify
340 specific file descriptors for the stdio of the child process. This API was
341 not portable to all platforms and therefore removed.
342 With `customFds` it was possible to hook up the new process' `[stdin, stdout,
343 stderr]` to existing streams; `-1` meant that a new stream should be created.
344 Use at your own risk.
346 There are several internal options. In particular `stdinStream`,
347 `stdoutStream`, `stderrStream`. They are for INTERNAL USE ONLY. As with all
348 undocumented APIs in Node, they should not be used.
350 See also: `child_process.exec()` and `child_process.fork()`
352 ## child_process.exec(command, [options], callback)
354 * `command` {String} The command to run, with space-separated arguments
356 * `cwd` {String} Current working directory of the child process
357 * `customFds` {Array} **Deprecated** File descriptors for the child to use
358 for stdio. (See below)
359 * `env` {Object} Environment key-value pairs
361 * `encoding` {String} (Default: 'utf8')
362 * `timeout` {Number} (Default: 0)
363 * `maxBuffer` {Number} (Default: 200*1024)
364 * `killSignal` {String} (Default: 'SIGTERM')
365 * `callback` {Function} called with the output when process terminates
369 * Return: ChildProcess object
371 Runs a command in a shell and buffers the output.
373 var util = require('util'),
374 exec = require('child_process').exec,
377 child = exec('cat *.js bad_file | wc -l',
378 function (error, stdout, stderr) {
379 console.log('stdout: ' + stdout);
380 console.log('stderr: ' + stderr);
381 if (error !== null) {
382 console.log('exec error: ' + error);
386 The callback gets the arguments `(error, stdout, stderr)`. On success, `error`
387 will be `null`. On error, `error` will be an instance of `Error` and `err.code`
388 will be the exit code of the child process, and `err.signal` will be set to the
389 signal that terminated the process.
391 There is a second optional argument to specify several options. The
397 killSignal: 'SIGTERM',
401 If `timeout` is greater than 0, then it will kill the child process
402 if it runs longer than `timeout` milliseconds. The child process is killed with
403 `killSignal` (default: `'SIGTERM'`). `maxBuffer` specifies the largest
404 amount of data allowed on stdout or stderr - if this value is exceeded then
405 the child process is killed.
408 ## child_process.execFile(file, args, options, callback)
410 * `file` {String} The filename of the program to run
411 * `args` {Array} List of string arguments
413 * `cwd` {String} Current working directory of the child process
414 * `customFds` {Array} **Deprecated** File descriptors for the child to use
415 for stdio. (See below)
416 * `env` {Object} Environment key-value pairs
418 * `encoding` {String} (Default: 'utf8')
419 * `timeout` {Number} (Default: 0)
420 * `maxBuffer` {Number} (Default: 200*1024)
421 * `killSignal` {String} (Default: 'SIGTERM')
422 * `callback` {Function} called with the output when process terminates
426 * Return: ChildProcess object
428 This is similar to `child_process.exec()` except it does not execute a
429 subshell but rather the specified file directly. This makes it slightly
430 leaner than `child_process.exec`. It has the same options.
433 ## child_process.fork(modulePath, [args], [options])
435 * `modulePath` {String} The module to run in the child
436 * `args` {Array} List of string arguments
438 * `cwd` {String} Current working directory of the child process
439 * `customFds` {Array} **Deprecated** File descriptors for the child to use
440 for stdio. (See below)
441 * `env` {Object} Environment key-value pairs
443 * `encoding` {String} (Default: 'utf8')
444 * `timeout` {Number} (Default: 0)
445 * Return: ChildProcess object
447 This is a special case of the `spawn()` functionality for spawning Node
448 processes. In addition to having all the methods in a normal ChildProcess
449 instance, the returned object has a communication channel built-in. See
450 `child.send(message, [sendHandle])` for details.
452 By default the spawned Node process will have the stdout, stderr associated
453 with the parent's. To change this behavior set the `silent` property in the
454 `options` object to `true`.
456 These child Nodes are still whole new instances of V8. Assume at least 30ms
457 startup and 10mb memory for each new Node. That is, you cannot create many