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], [options])
130 * `sendHandle` {Handle object}
133 When using `child_process.fork()` you can write to the child using
134 `child.send(message, [sendHandle])` and messages are received by
135 a `'message'` event on the child.
139 var cp = require('child_process');
141 var n = cp.fork(__dirname + '/sub.js');
143 n.on('message', function(m) {
144 console.log('PARENT got message:', m);
147 n.send({ hello: 'world' });
149 And then the child script, `'sub.js'` might look like this:
151 process.on('message', function(m) {
152 console.log('CHILD got message:', m);
155 process.send({ foo: 'bar' });
157 In the child the `process` object will have a `send()` method, and `process`
158 will emit objects each time it receives a message on its channel.
160 There is a special case when sending a `{cmd: 'NODE_foo'}` message. All messages
161 containing a `NODE_` prefix in its `cmd` property will not be emitted in
162 the `message` event, since they are internal messages used by node core.
163 Messages containing the prefix are emitted in the `internalMessage` event, you
164 should by all means avoid using this feature, it is subject to change without notice.
166 The `sendHandle` option to `child.send()` is for sending a TCP server or
167 socket object to another process. The child will receive the object as its
168 second argument to the `message` event.
170 The `options` object may have the following properties:
172 * `track` - Notify master process when `sendHandle` will be closed in child
173 process. (`false` by default)
175 **send server object**
177 Here is an example of sending a server:
179 var child = require('child_process').fork('child.js');
181 // Open up the server object and send the handle.
182 var server = require('net').createServer();
183 server.on('connection', function (socket) {
184 socket.end('handled by parent');
186 server.listen(1337, function() {
187 child.send('server', server);
190 And the child would the receive the server object as:
192 process.on('message', function(m, server) {
193 if (m === 'server') {
194 server.on('connection', function (socket) {
195 socket.end('handled by child');
200 Note that the server is now shared between the parent and child, this means
201 that some connections will be handled by the parent and some by the child.
203 **send socket object**
205 Here is an example of sending a socket. It will spawn two children and handle
206 connections with the remote address `74.125.127.100` as VIP by sending the
207 socket to a "special" child process. Other sockets will go to a "normal" process.
209 var normal = require('child_process').fork('child.js', ['normal']);
210 var special = require('child_process').fork('child.js', ['special']);
212 // Open up the server and send sockets to child
213 var server = require('net').createServer();
214 server.on('connection', function (socket) {
217 if (socket.remoteAddress === '74.125.127.100') {
218 special.send('socket', socket);
221 // just the usual dudes
222 normal.send('socket', socket);
226 The `child.js` could look like this:
228 process.on('message', function(m, socket) {
229 if (m === 'socket') {
230 socket.end('You were handled as a ' + process.argv[2] + ' person');
234 Note that once a single socket has been sent to a child the parent can no
235 longer keep track of when the socket is destroyed. To indicate this condition
236 the `.connections` property becomes `null`.
237 It is also recommended not to use `.maxConnections` in this condition.
239 ### child.disconnect()
241 To close the IPC connection between parent and child use the
242 `child.disconnect()` method. This allows the child to exit gracefully since
243 there is no IPC channel keeping it alive. When calling this method the
244 `disconnect` event will be emitted in both parent and child, and the
245 `connected` flag will be set to `false`. Please note that you can also call
246 `process.disconnect()` in the child process.
248 ## child_process.spawn(command, [args], [options])
250 * `command` {String} The command to run
251 * `args` {Array} List of string arguments
253 * `cwd` {String} Current working directory of the child process
254 * `stdio` {Array|String} Child's stdio configuration. (See below)
255 * `customFds` {Array} **Deprecated** File descriptors for the child to use
256 for stdio. (See below)
257 * `env` {Object} Environment key-value pairs
258 * `detached` {Boolean} The child will be a process group leader. (See below)
259 * `uid` {Number} Sets the user identity of the process. (See setuid(2).)
260 * `gid` {Number} Sets the group identity of the process. (See setgid(2).)
261 * return: {ChildProcess object}
263 Launches a new process with the given `command`, with command line arguments in `args`.
264 If omitted, `args` defaults to an empty Array.
266 The third argument is used to specify additional options, which defaults to:
272 `cwd` allows you to specify the working directory from which the process is spawned.
273 Use `env` to specify environment variables that will be visible to the new process.
275 Example of running `ls -lh /usr`, capturing `stdout`, `stderr`, and the exit code:
277 var spawn = require('child_process').spawn,
278 ls = spawn('ls', ['-lh', '/usr']);
280 ls.stdout.on('data', function (data) {
281 console.log('stdout: ' + data);
284 ls.stderr.on('data', function (data) {
285 console.log('stderr: ' + data);
288 ls.on('exit', function (code) {
289 console.log('child process exited with code ' + code);
293 Example: A very elaborate way to run 'ps ax | grep ssh'
295 var spawn = require('child_process').spawn,
296 ps = spawn('ps', ['ax']),
297 grep = spawn('grep', ['ssh']);
299 ps.stdout.on('data', function (data) {
300 grep.stdin.write(data);
303 ps.stderr.on('data', function (data) {
304 console.log('ps stderr: ' + data);
307 ps.on('exit', function (code) {
309 console.log('ps process exited with code ' + code);
314 grep.stdout.on('data', function (data) {
315 console.log('' + data);
318 grep.stderr.on('data', function (data) {
319 console.log('grep stderr: ' + data);
322 grep.on('exit', function (code) {
324 console.log('grep process exited with code ' + code);
329 Example of checking for failed exec:
331 var spawn = require('child_process').spawn,
332 child = spawn('bad_command');
334 child.stderr.setEncoding('utf8');
335 child.stderr.on('data', function (data) {
336 if (/^execvp\(\)/.test(data)) {
337 console.log('Failed to start child process.');
341 Note that if spawn receives an empty options object, it will result in
342 spawning the process with an empty environment rather than using
343 `process.env`. This due to backwards compatibility issues with a deprecated
346 The 'stdio' option to `child_process.spawn()` is an array where each
347 index corresponds to a fd in the child. The value is one of the following:
349 1. `'pipe'` - Create a pipe between the child process and the parent process.
350 The parent end of the pipe is exposed to the parent as a property on the
351 `child_process` object as `ChildProcess.stdio[fd]`. Pipes created for
352 fds 0 - 2 are also available as ChildProcess.stdin, ChildProcess.stdout
353 and ChildProcess.stderr, respectively.
354 2. `'ipc'` - Create an IPC channel for passing messages/file descriptors
355 between parent and child. A ChildProcess may have at most *one* IPC stdio
356 file descriptor. Setting this option enables the ChildProcess.send() method.
357 If the child writes JSON messages to this file descriptor, then this will
358 trigger ChildProcess.on('message'). If the child is a Node.js program, then
359 the presence of an IPC channel will enable process.send() and
360 process.on('message').
361 3. `'ignore'` - Do not set this file descriptor in the child. Note that Node
362 will always open fd 0 - 2 for the processes it spawns. When any of these is
363 ignored node will open `/dev/null` and attach it to the child's fd.
364 4. `Stream` object - Share a readable or writable stream that refers to a tty,
365 file, socket, or a pipe with the child process. The stream's underlying
366 file descriptor is duplicated in the child process to the fd that
367 corresponds to the index in the `stdio` array.
368 5. Positive integer - The integer value is interpreted as a file descriptor
369 that is is currently open in the parent process. It is shared with the child
370 process, similar to how `Stream` objects can be shared.
371 6. `null`, `undefined` - Use default value. For stdio fds 0, 1 and 2 (in other
372 words, stdin, stdout, and stderr) a pipe is created. For fd 3 and up, the
373 default is `'ignore'`.
375 As a shorthand, the `stdio` argument may also be one of the following
376 strings, rather than an array:
378 * `ignore` - `['ignore', 'ignore', 'ignore']`
379 * `pipe` - `['pipe', 'pipe', 'pipe']`
380 * `inherit` - `[process.stdin, process.stdout, process.stderr]` or `[0,1,2]`
384 var spawn = require('child_process').spawn;
386 // Child will use parent's stdios
387 spawn('prg', [], { stdio: 'inherit' });
389 // Spawn child sharing only stderr
390 spawn('prg', [], { stdio: ['pipe', 'pipe', process.stderr] });
392 // Open an extra fd=4, to interact with programs present a
393 // startd-style interface.
394 spawn('prg', [], { stdio: ['pipe', null, null, null, 'pipe'] });
396 If the `detached` option is set, the child process will be made the leader of a
397 new process group. This makes it possible for the child to continue running
398 after the parent exits.
400 By default, the parent will wait for the detached child to exit. To prevent
401 the parent from waiting for a given `child`, use the `child.unref()` method,
402 and the parent's event loop will not include the child in its reference count.
404 Example of detaching a long-running process and redirecting its output to a
407 var fs = require('fs'),
408 spawn = require('child_process').spawn,
409 out = fs.openSync('./out.log', 'a'),
410 err = fs.openSync('./out.log', 'a');
412 var child = spawn('prg', [], {
414 stdio: [ 'ignore', out, err ]
419 When using the `detached` option to start a long-running process, the process
420 will not stay running in the background unless it is provided with a `stdio`
421 configuration that is not connected to the parent. If the parent's `stdio` is
422 inherited, the child will remain attached to the controlling terminal.
424 There is a deprecated option called `customFds` which allows one to specify
425 specific file descriptors for the stdio of the child process. This API was
426 not portable to all platforms and therefore removed.
427 With `customFds` it was possible to hook up the new process' `[stdin, stdout,
428 stderr]` to existing streams; `-1` meant that a new stream should be created.
429 Use at your own risk.
431 There are several internal options. In particular `stdinStream`,
432 `stdoutStream`, `stderrStream`. They are for INTERNAL USE ONLY. As with all
433 undocumented APIs in Node, they should not be used.
435 See also: `child_process.exec()` and `child_process.fork()`
437 ## child_process.exec(command, [options], callback)
439 * `command` {String} The command to run, with space-separated arguments
441 * `cwd` {String} Current working directory of the child process
442 * `stdio` {Array|String} Child's stdio configuration. (See above)
443 * `customFds` {Array} **Deprecated** File descriptors for the child to use
444 for stdio. (See above)
445 * `env` {Object} Environment key-value pairs
446 * `encoding` {String} (Default: 'utf8')
447 * `timeout` {Number} (Default: 0)
448 * `maxBuffer` {Number} (Default: 200*1024)
449 * `killSignal` {String} (Default: 'SIGTERM')
450 * `callback` {Function} called with the output when process terminates
454 * Return: ChildProcess object
456 Runs a command in a shell and buffers the output.
458 var exec = require('child_process').exec,
461 child = exec('cat *.js bad_file | wc -l',
462 function (error, stdout, stderr) {
463 console.log('stdout: ' + stdout);
464 console.log('stderr: ' + stderr);
465 if (error !== null) {
466 console.log('exec error: ' + error);
470 The callback gets the arguments `(error, stdout, stderr)`. On success, `error`
471 will be `null`. On error, `error` will be an instance of `Error` and `err.code`
472 will be the exit code of the child process, and `err.signal` will be set to the
473 signal that terminated the process.
475 There is a second optional argument to specify several options. The
481 killSignal: 'SIGTERM',
485 If `timeout` is greater than 0, then it will kill the child process
486 if it runs longer than `timeout` milliseconds. The child process is killed with
487 `killSignal` (default: `'SIGTERM'`). `maxBuffer` specifies the largest
488 amount of data allowed on stdout or stderr - if this value is exceeded then
489 the child process is killed.
492 ## child_process.execFile(file, args, options, callback)
494 * `file` {String} The filename of the program to run
495 * `args` {Array} List of string arguments
497 * `cwd` {String} Current working directory of the child process
498 * `stdio` {Array|String} Child's stdio configuration. (See above)
499 * `customFds` {Array} **Deprecated** File descriptors for the child to use
500 for stdio. (See above)
501 * `env` {Object} Environment key-value pairs
502 * `encoding` {String} (Default: 'utf8')
503 * `timeout` {Number} (Default: 0)
504 * `maxBuffer` {Number} (Default: 200\*1024)
505 * `killSignal` {String} (Default: 'SIGTERM')
506 * `callback` {Function} called with the output when process terminates
510 * Return: ChildProcess object
512 This is similar to `child_process.exec()` except it does not execute a
513 subshell but rather the specified file directly. This makes it slightly
514 leaner than `child_process.exec`. It has the same options.
517 ## child\_process.fork(modulePath, [args], [options])
519 * `modulePath` {String} The module to run in the child
520 * `args` {Array} List of string arguments
522 * `cwd` {String} Current working directory of the child process
523 * `env` {Object} Environment key-value pairs
524 * `encoding` {String} (Default: 'utf8')
525 * `execPath` {String} Executable used to create the child process
526 * Return: ChildProcess object
528 This is a special case of the `spawn()` functionality for spawning Node
529 processes. In addition to having all the methods in a normal ChildProcess
530 instance, the returned object has a communication channel built-in. See
531 `child.send(message, [sendHandle])` for details.
533 By default the spawned Node process will have the stdout, stderr associated
534 with the parent's. To change this behavior set the `silent` property in the
535 `options` object to `true`.
537 The child process does not automatically exit once it's done, you need to call
538 `process.exit()` explicitly. This limitation may be lifted in the future.
540 These child Nodes are still whole new instances of V8. Assume at least 30ms
541 startup and 10mb memory for each new Node. That is, you cannot create many
544 The `execPath` property in the `options` object allows for a process to be
545 created for the child rather than the current `node` executable. This should be
546 done with care and by default will talk over the fd represented an
547 environmental variable `NODE_CHANNEL_FD` on the child process. The input and
548 output on this fd is expected to be line delimited JSON objects.
550 [EventEmitter]: events.html#events_class_events_eventemitter