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 * `code` {Number} the exit code, if it exited normally.
45 * `signal` {String} the signal passed to kill the child process, if it
46 was killed by the parent.
48 This event is emitted when the stdio streams of a child process have all
49 terminated. This is distinct from 'exit', since multiple processes
50 might share the same stdio streams.
52 ### Event: 'disconnect'
54 This event is emitted after using the `.disconnect()` method in the parent or
55 in the child. After disconnecting it is no longer possible to send messages.
56 An alternative way to check if you can send messages is to see if the
57 `child.connected` property is `true`.
61 * `message` {Object} a parsed JSON object or primitive value
62 * `sendHandle` {Handle object} a Socket or Server object
64 Messages send by `.send(message, [sendHandle])` are obtained using the
71 A `Writable Stream` that represents the child process's `stdin`.
72 Closing this stream via `end()` often causes the child process to terminate.
74 If the child stdio streams are shared with the parent, then this will
81 A `Readable Stream` that represents the child process's `stdout`.
83 If the child stdio streams are shared with the parent, then this will
90 A `Readable Stream` that represents the child process's `stderr`.
92 If the child stdio streams are shared with the parent, then this will
99 The PID of the child process.
103 var spawn = require('child_process').spawn,
104 grep = spawn('grep', ['ssh']);
106 console.log('Spawned child pid: ' + grep.pid);
109 ### child.kill([signal])
113 Send a signal to the child process. If no argument is given, the process will
114 be sent `'SIGTERM'`. See `signal(7)` for a list of available signals.
116 var spawn = require('child_process').spawn,
117 grep = spawn('grep', ['ssh']);
119 grep.on('close', function (code, signal) {
120 console.log('child process terminated due to receipt of signal '+signal);
123 // send SIGHUP to process
126 Note that while the function is called `kill`, the signal delivered to the child
127 process may not actually kill it. `kill` really just sends a signal to a process.
131 ### child.send(message, [sendHandle])
134 * `sendHandle` {Handle object}
136 When using `child_process.fork()` you can write to the child using
137 `child.send(message, [sendHandle])` and messages are received by
138 a `'message'` event on the child.
142 var cp = require('child_process');
144 var n = cp.fork(__dirname + '/sub.js');
146 n.on('message', function(m) {
147 console.log('PARENT got message:', m);
150 n.send({ hello: 'world' });
152 And then the child script, `'sub.js'` might look like this:
154 process.on('message', function(m) {
155 console.log('CHILD got message:', m);
158 process.send({ foo: 'bar' });
160 In the child the `process` object will have a `send()` method, and `process`
161 will emit objects each time it receives a message on its channel.
163 There is a special case when sending a `{cmd: 'NODE_foo'}` message. All messages
164 containing a `NODE_` prefix in its `cmd` property will not be emitted in
165 the `message` event, since they are internal messages used by node core.
166 Messages containing the prefix are emitted in the `internalMessage` event, you
167 should by all means avoid using this feature, it is subject to change without notice.
169 The `sendHandle` option to `child.send()` is for sending a TCP server or
170 socket object to another process. The child will receive the object as its
171 second argument to the `message` event.
173 #### Example: sending server object
175 Here is an example of sending a server:
177 var child = require('child_process').fork('child.js');
179 // Open up the server object and send the handle.
180 var server = require('net').createServer();
181 server.on('connection', function (socket) {
182 socket.end('handled by parent');
184 server.listen(1337, function() {
185 child.send('server', server);
188 And the child would the receive the server object as:
190 process.on('message', function(m, server) {
191 if (m === 'server') {
192 server.on('connection', function (socket) {
193 socket.end('handled by child');
198 Note that the server is now shared between the parent and child, this means
199 that some connections will be handled by the parent and some by the child.
201 #### Example: sending socket object
203 Here is an example of sending a socket. It will spawn two children and handle
204 connections with the remote address `74.125.127.100` as VIP by sending the
205 socket to a "special" child process. Other sockets will go to a "normal" process.
207 var normal = require('child_process').fork('child.js', ['normal']);
208 var special = require('child_process').fork('child.js', ['special']);
210 // Open up the server and send sockets to child
211 var server = require('net').createServer();
212 server.on('connection', function (socket) {
215 if (socket.remoteAddress === '74.125.127.100') {
216 special.send('socket', socket);
219 // just the usual dudes
220 normal.send('socket', socket);
224 The `child.js` could look like this:
226 process.on('message', function(m, socket) {
227 if (m === 'socket') {
228 socket.end('You were handled as a ' + process.argv[2] + ' person');
232 Note that once a single socket has been sent to a child the parent can no
233 longer keep track of when the socket is destroyed. To indicate this condition
234 the `.connections` property becomes `null`.
235 It is also recommended not to use `.maxConnections` in this condition.
237 ### child.disconnect()
239 To close the IPC connection between parent and child use the
240 `child.disconnect()` method. This allows the child to exit gracefully since
241 there is no IPC channel keeping it alive. When calling this method the
242 `disconnect` event will be emitted in both parent and child, and the
243 `connected` flag will be set to `false`. Please note that you can also call
244 `process.disconnect()` in the child process.
246 ## child_process.spawn(command, [args], [options])
248 * `command` {String} The command to run
249 * `args` {Array} List of string arguments
251 * `cwd` {String} Current working directory of the child process
252 * `stdio` {Array|String} Child's stdio configuration. (See below)
253 * `customFds` {Array} **Deprecated** File descriptors for the child to use
254 for stdio. (See below)
255 * `env` {Object} Environment key-value pairs
256 * `detached` {Boolean} The child will be a process group leader. (See below)
257 * `uid` {Number} Sets the user identity of the process. (See setuid(2).)
258 * `gid` {Number} Sets the group identity of the process. (See setgid(2).)
259 * return: {ChildProcess object}
261 Launches a new process with the given `command`, with command line arguments in `args`.
262 If omitted, `args` defaults to an empty Array.
264 The third argument is used to specify additional options, which defaults to:
270 `cwd` allows you to specify the working directory from which the process is spawned.
271 Use `env` to specify environment variables that will be visible to the new process.
273 Example of running `ls -lh /usr`, capturing `stdout`, `stderr`, and the exit code:
275 var spawn = require('child_process').spawn,
276 ls = spawn('ls', ['-lh', '/usr']);
278 ls.stdout.on('data', function (data) {
279 console.log('stdout: ' + data);
282 ls.stderr.on('data', function (data) {
283 console.log('stderr: ' + data);
286 ls.on('close', function (code) {
287 console.log('child process exited with code ' + code);
291 Example: A very elaborate way to run 'ps ax | grep ssh'
293 var spawn = require('child_process').spawn,
294 ps = spawn('ps', ['ax']),
295 grep = spawn('grep', ['ssh']);
297 ps.stdout.on('data', function (data) {
298 grep.stdin.write(data);
301 ps.stderr.on('data', function (data) {
302 console.log('ps stderr: ' + data);
305 ps.on('close', function (code) {
307 console.log('ps process exited with code ' + code);
312 grep.stdout.on('data', function (data) {
313 console.log('' + data);
316 grep.stderr.on('data', function (data) {
317 console.log('grep stderr: ' + data);
320 grep.on('close', function (code) {
322 console.log('grep process exited with code ' + code);
327 Example of checking for failed exec:
329 var spawn = require('child_process').spawn,
330 child = spawn('bad_command');
332 child.stderr.setEncoding('utf8');
333 child.stderr.on('data', function (data) {
334 if (/^execvp\(\)/.test(data)) {
335 console.log('Failed to start child process.');
339 Note that if spawn receives an empty options object, it will result in
340 spawning the process with an empty environment rather than using
341 `process.env`. This due to backwards compatibility issues with a deprecated
344 The 'stdio' option to `child_process.spawn()` is an array where each
345 index corresponds to a fd in the child. The value is one of the following:
347 1. `'pipe'` - Create a pipe between the child process and the parent process.
348 The parent end of the pipe is exposed to the parent as a property on the
349 `child_process` object as `ChildProcess.stdio[fd]`. Pipes created for
350 fds 0 - 2 are also available as ChildProcess.stdin, ChildProcess.stdout
351 and ChildProcess.stderr, respectively.
352 2. `'ipc'` - Create an IPC channel for passing messages/file descriptors
353 between parent and child. A ChildProcess may have at most *one* IPC stdio
354 file descriptor. Setting this option enables the ChildProcess.send() method.
355 If the child writes JSON messages to this file descriptor, then this will
356 trigger ChildProcess.on('message'). If the child is a Node.js program, then
357 the presence of an IPC channel will enable process.send() and
358 process.on('message').
359 3. `'ignore'` - Do not set this file descriptor in the child. Note that Node
360 will always open fd 0 - 2 for the processes it spawns. When any of these is
361 ignored node will open `/dev/null` and attach it to the child's fd.
362 4. `Stream` object - Share a readable or writable stream that refers to a tty,
363 file, socket, or a pipe with the child process. The stream's underlying
364 file descriptor is duplicated in the child process to the fd that
365 corresponds to the index in the `stdio` array.
366 5. Positive integer - The integer value is interpreted as a file descriptor
367 that is is currently open in the parent process. It is shared with the child
368 process, similar to how `Stream` objects can be shared.
369 6. `null`, `undefined` - Use default value. For stdio fds 0, 1 and 2 (in other
370 words, stdin, stdout, and stderr) a pipe is created. For fd 3 and up, the
371 default is `'ignore'`.
373 As a shorthand, the `stdio` argument may also be one of the following
374 strings, rather than an array:
376 * `ignore` - `['ignore', 'ignore', 'ignore']`
377 * `pipe` - `['pipe', 'pipe', 'pipe']`
378 * `inherit` - `[process.stdin, process.stdout, process.stderr]` or `[0,1,2]`
382 var spawn = require('child_process').spawn;
384 // Child will use parent's stdios
385 spawn('prg', [], { stdio: 'inherit' });
387 // Spawn child sharing only stderr
388 spawn('prg', [], { stdio: ['pipe', 'pipe', process.stderr] });
390 // Open an extra fd=4, to interact with programs present a
391 // startd-style interface.
392 spawn('prg', [], { stdio: ['pipe', null, null, null, 'pipe'] });
394 If the `detached` option is set, the child process will be made the leader of a
395 new process group. This makes it possible for the child to continue running
396 after the parent exits.
398 By default, the parent will wait for the detached child to exit. To prevent
399 the parent from waiting for a given `child`, use the `child.unref()` method,
400 and the parent's event loop will not include the child in its reference count.
402 Example of detaching a long-running process and redirecting its output to a
405 var fs = require('fs'),
406 spawn = require('child_process').spawn,
407 out = fs.openSync('./out.log', 'a'),
408 err = fs.openSync('./out.log', 'a');
410 var child = spawn('prg', [], {
412 stdio: [ 'ignore', out, err ]
417 When using the `detached` option to start a long-running process, the process
418 will not stay running in the background unless it is provided with a `stdio`
419 configuration that is not connected to the parent. If the parent's `stdio` is
420 inherited, the child will remain attached to the controlling terminal.
422 There is a deprecated option called `customFds` which allows one to specify
423 specific file descriptors for the stdio of the child process. This API was
424 not portable to all platforms and therefore removed.
425 With `customFds` it was possible to hook up the new process' `[stdin, stdout,
426 stderr]` to existing streams; `-1` meant that a new stream should be created.
427 Use at your own risk.
429 There are several internal options. In particular `stdinStream`,
430 `stdoutStream`, `stderrStream`. They are for INTERNAL USE ONLY. As with all
431 undocumented APIs in Node, they should not be used.
433 See also: `child_process.exec()` and `child_process.fork()`
435 ## child_process.exec(command, [options], callback)
437 * `command` {String} The command to run, with space-separated arguments
439 * `cwd` {String} Current working directory of the child process
440 * `stdio` {Array|String} Child's stdio configuration. (See above)
441 Only stdin is configurable, anything else will lead to unpredictable
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