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. (Note that some programs use
10 line-buffered I/O internally. That doesn't affect node.js but it means
11 data you send to the child process is not immediately consumed.)
13 To create a child process use `require('child_process').spawn()` or
14 `require('child_process').fork()`. The semantics of each are slightly
15 different, and explained below.
17 ## Class: ChildProcess
19 `ChildProcess` is an [EventEmitter][].
21 Child processes always have three streams associated with them. `child.stdin`,
22 `child.stdout`, and `child.stderr`. These may be shared with the stdio
23 streams of the parent process, or they may be separate stream objects
24 which can be piped to and from.
26 The ChildProcess class is not intended to be used directly. Use the
27 `spawn()` or `fork()` methods to create a Child Process instance.
31 * `code` {Number} the exit code, if it exited normally.
32 * `signal` {String} the signal passed to kill the child process, if it
33 was killed by the parent.
35 This event is emitted after the child process ends. If the process terminated
36 normally, `code` is the final exit code of the process, otherwise `null`. If
37 the process terminated due to receipt of a signal, `signal` is the string name
38 of the signal, otherwise `null`.
40 Note that the child process stdio streams might still be open.
46 * `code` {Number} the exit code, if it exited normally.
47 * `signal` {String} the signal passed to kill the child process, if it
48 was killed by the parent.
50 This event is emitted when the stdio streams of a child process have all
51 terminated. This is distinct from 'exit', since multiple processes
52 might share the same stdio streams.
54 ### Event: 'disconnect'
56 This event is emitted after using the `.disconnect()` method in the parent or
57 in the child. After disconnecting it is no longer possible to send messages.
58 An alternative way to check if you can send messages is to see if the
59 `child.connected` property is `true`.
63 * `message` {Object} a parsed JSON object or primitive value
64 * `sendHandle` {Handle object} a Socket or Server object
66 Messages send by `.send(message, [sendHandle])` are obtained using the
73 A `Writable Stream` that represents the child process's `stdin`.
74 Closing this stream via `end()` often causes the child process to terminate.
76 If the child stdio streams are shared with the parent, then this will
83 A `Readable Stream` that represents the child process's `stdout`.
85 If the child stdio streams are shared with the parent, then this will
92 A `Readable Stream` that represents the child process's `stderr`.
94 If the child stdio streams are shared with the parent, then this will
101 The PID of the child process.
105 var spawn = require('child_process').spawn,
106 grep = spawn('grep', ['ssh']);
108 console.log('Spawned child pid: ' + grep.pid);
111 ### child.kill([signal])
115 Send a signal to the child process. If no argument is given, the process will
116 be sent `'SIGTERM'`. See `signal(7)` for a list of available signals.
118 var spawn = require('child_process').spawn,
119 grep = spawn('grep', ['ssh']);
121 grep.on('close', function (code, signal) {
122 console.log('child process terminated due to receipt of signal '+signal);
125 // send SIGHUP to process
128 Note that while the function is called `kill`, the signal delivered to the child
129 process may not actually kill it. `kill` really just sends a signal to a process.
133 ### child.send(message, [sendHandle])
136 * `sendHandle` {Handle object}
138 When using `child_process.fork()` you can write to the child using
139 `child.send(message, [sendHandle])` and messages are received by
140 a `'message'` event on the child.
144 var cp = require('child_process');
146 var n = cp.fork(__dirname + '/sub.js');
148 n.on('message', function(m) {
149 console.log('PARENT got message:', m);
152 n.send({ hello: 'world' });
154 And then the child script, `'sub.js'` might look like this:
156 process.on('message', function(m) {
157 console.log('CHILD got message:', m);
160 process.send({ foo: 'bar' });
162 In the child the `process` object will have a `send()` method, and `process`
163 will emit objects each time it receives a message on its channel.
165 There is a special case when sending a `{cmd: 'NODE_foo'}` message. All messages
166 containing a `NODE_` prefix in its `cmd` property will not be emitted in
167 the `message` event, since they are internal messages used by node core.
168 Messages containing the prefix are emitted in the `internalMessage` event, you
169 should by all means avoid using this feature, it is subject to change without notice.
171 The `sendHandle` option to `child.send()` is for sending a TCP server or
172 socket object to another process. The child will receive the object as its
173 second argument to the `message` event.
175 #### Example: sending 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 For `dgram` servers the workflow is exactly the same. Here you listen on
204 a `message` event instead of `connection` and use `server.bind` instead of
207 #### Example: sending socket object
209 Here is an example of sending a socket. It will spawn two children and handle
210 connections with the remote address `74.125.127.100` as VIP by sending the
211 socket to a "special" child process. Other sockets will go to a "normal" process.
213 var normal = require('child_process').fork('child.js', ['normal']);
214 var special = require('child_process').fork('child.js', ['special']);
216 // Open up the server and send sockets to child
217 var server = require('net').createServer();
218 server.on('connection', function (socket) {
221 if (socket.remoteAddress === '74.125.127.100') {
222 special.send('socket', socket);
225 // just the usual dudes
226 normal.send('socket', socket);
230 The `child.js` could look like this:
232 process.on('message', function(m, socket) {
233 if (m === 'socket') {
234 socket.end('You were handled as a ' + process.argv[2] + ' person');
238 Note that once a single socket has been sent to a child the parent can no
239 longer keep track of when the socket is destroyed. To indicate this condition
240 the `.connections` property becomes `null`.
241 It is also recommended not to use `.maxConnections` in this condition.
243 ### child.disconnect()
245 To close the IPC connection between parent and child use the
246 `child.disconnect()` method. This allows the child to exit gracefully since
247 there is no IPC channel keeping it alive. When calling this method the
248 `disconnect` event will be emitted in both parent and child, and the
249 `connected` flag will be set to `false`. Please note that you can also call
250 `process.disconnect()` in the child process.
252 ## child_process.spawn(command, [args], [options])
254 * `command` {String} The command to run
255 * `args` {Array} List of string arguments
257 * `cwd` {String} Current working directory of the child process
258 * `stdio` {Array|String} Child's stdio configuration. (See below)
259 * `customFds` {Array} **Deprecated** File descriptors for the child to use
260 for stdio. (See below)
261 * `env` {Object} Environment key-value pairs
262 * `detached` {Boolean} The child will be a process group leader. (See below)
263 * `uid` {Number} Sets the user identity of the process. (See setuid(2).)
264 * `gid` {Number} Sets the group identity of the process. (See setgid(2).)
265 * return: {ChildProcess object}
267 Launches a new process with the given `command`, with command line arguments in `args`.
268 If omitted, `args` defaults to an empty Array.
270 The third argument is used to specify additional options, which defaults to:
276 `cwd` allows you to specify the working directory from which the process is spawned.
277 Use `env` to specify environment variables that will be visible to the new process.
279 Example of running `ls -lh /usr`, capturing `stdout`, `stderr`, and the exit code:
281 var spawn = require('child_process').spawn,
282 ls = spawn('ls', ['-lh', '/usr']);
284 ls.stdout.on('data', function (data) {
285 console.log('stdout: ' + data);
288 ls.stderr.on('data', function (data) {
289 console.log('stderr: ' + data);
292 ls.on('close', function (code) {
293 console.log('child process exited with code ' + code);
297 Example: A very elaborate way to run 'ps ax | grep ssh'
299 var spawn = require('child_process').spawn,
300 ps = spawn('ps', ['ax']),
301 grep = spawn('grep', ['ssh']);
303 ps.stdout.on('data', function (data) {
304 grep.stdin.write(data);
307 ps.stderr.on('data', function (data) {
308 console.log('ps stderr: ' + data);
311 ps.on('close', function (code) {
313 console.log('ps process exited with code ' + code);
318 grep.stdout.on('data', function (data) {
319 console.log('' + data);
322 grep.stderr.on('data', function (data) {
323 console.log('grep stderr: ' + data);
326 grep.on('close', function (code) {
328 console.log('grep process exited with code ' + code);
333 Example of checking for failed exec:
335 var spawn = require('child_process').spawn,
336 child = spawn('bad_command');
338 child.stderr.setEncoding('utf8');
339 child.stderr.on('data', function (data) {
340 if (/^execvp\(\)/.test(data)) {
341 console.log('Failed to start child process.');
345 Note that if spawn receives an empty options object, it will result in
346 spawning the process with an empty environment rather than using
347 `process.env`. This due to backwards compatibility issues with a deprecated
350 The 'stdio' option to `child_process.spawn()` is an array where each
351 index corresponds to a fd in the child. The value is one of the following:
353 1. `'pipe'` - Create a pipe between the child process and the parent process.
354 The parent end of the pipe is exposed to the parent as a property on the
355 `child_process` object as `ChildProcess.stdio[fd]`. Pipes created for
356 fds 0 - 2 are also available as ChildProcess.stdin, ChildProcess.stdout
357 and ChildProcess.stderr, respectively.
358 2. `'ipc'` - Create an IPC channel for passing messages/file descriptors
359 between parent and child. A ChildProcess may have at most *one* IPC stdio
360 file descriptor. Setting this option enables the ChildProcess.send() method.
361 If the child writes JSON messages to this file descriptor, then this will
362 trigger ChildProcess.on('message'). If the child is a Node.js program, then
363 the presence of an IPC channel will enable process.send() and
364 process.on('message').
365 3. `'ignore'` - Do not set this file descriptor in the child. Note that Node
366 will always open fd 0 - 2 for the processes it spawns. When any of these is
367 ignored node will open `/dev/null` and attach it to the child's fd.
368 4. `Stream` object - Share a readable or writable stream that refers to a tty,
369 file, socket, or a pipe with the child process. The stream's underlying
370 file descriptor is duplicated in the child process to the fd that
371 corresponds to the index in the `stdio` array.
372 5. Positive integer - The integer value is interpreted as a file descriptor
373 that is is currently open in the parent process. It is shared with the child
374 process, similar to how `Stream` objects can be shared.
375 6. `null`, `undefined` - Use default value. For stdio fds 0, 1 and 2 (in other
376 words, stdin, stdout, and stderr) a pipe is created. For fd 3 and up, the
377 default is `'ignore'`.
379 As a shorthand, the `stdio` argument may also be one of the following
380 strings, rather than an array:
382 * `ignore` - `['ignore', 'ignore', 'ignore']`
383 * `pipe` - `['pipe', 'pipe', 'pipe']`
384 * `inherit` - `[process.stdin, process.stdout, process.stderr]` or `[0,1,2]`
388 var spawn = require('child_process').spawn;
390 // Child will use parent's stdios
391 spawn('prg', [], { stdio: 'inherit' });
393 // Spawn child sharing only stderr
394 spawn('prg', [], { stdio: ['pipe', 'pipe', process.stderr] });
396 // Open an extra fd=4, to interact with programs present a
397 // startd-style interface.
398 spawn('prg', [], { stdio: ['pipe', null, null, null, 'pipe'] });
400 If the `detached` option is set, the child process will be made the leader of a
401 new process group. This makes it possible for the child to continue running
402 after the parent exits.
404 By default, the parent will wait for the detached child to exit. To prevent
405 the parent from waiting for a given `child`, use the `child.unref()` method,
406 and the parent's event loop will not include the child in its reference count.
408 Example of detaching a long-running process and redirecting its output to a
411 var fs = require('fs'),
412 spawn = require('child_process').spawn,
413 out = fs.openSync('./out.log', 'a'),
414 err = fs.openSync('./out.log', 'a');
416 var child = spawn('prg', [], {
418 stdio: [ 'ignore', out, err ]
423 When using the `detached` option to start a long-running process, the process
424 will not stay running in the background unless it is provided with a `stdio`
425 configuration that is not connected to the parent. If the parent's `stdio` is
426 inherited, the child will remain attached to the controlling terminal.
428 There is a deprecated option called `customFds` which allows one to specify
429 specific file descriptors for the stdio of the child process. This API was
430 not portable to all platforms and therefore removed.
431 With `customFds` it was possible to hook up the new process' `[stdin, stdout,
432 stderr]` to existing streams; `-1` meant that a new stream should be created.
433 Use at your own risk.
435 There are several internal options. In particular `stdinStream`,
436 `stdoutStream`, `stderrStream`. They are for INTERNAL USE ONLY. As with all
437 undocumented APIs in Node, they should not be used.
439 See also: `child_process.exec()` and `child_process.fork()`
441 ## child_process.exec(command, [options], callback)
443 * `command` {String} The command to run, with space-separated arguments
445 * `cwd` {String} Current working directory of the child process
446 * `stdio` {Array|String} Child's stdio configuration. (See above)
447 Only stdin is configurable, anything else will lead to unpredictable
449 * `customFds` {Array} **Deprecated** File descriptors for the child to use
450 for stdio. (See above)
451 * `env` {Object} Environment key-value pairs
452 * `encoding` {String} (Default: 'utf8')
453 * `timeout` {Number} (Default: 0)
454 * `maxBuffer` {Number} (Default: 200*1024)
455 * `killSignal` {String} (Default: 'SIGTERM')
456 * `callback` {Function} called with the output when process terminates
460 * Return: ChildProcess object
462 Runs a command in a shell and buffers the output.
464 var exec = require('child_process').exec,
467 child = exec('cat *.js bad_file | wc -l',
468 function (error, stdout, stderr) {
469 console.log('stdout: ' + stdout);
470 console.log('stderr: ' + stderr);
471 if (error !== null) {
472 console.log('exec error: ' + error);
476 The callback gets the arguments `(error, stdout, stderr)`. On success, `error`
477 will be `null`. On error, `error` will be an instance of `Error` and `err.code`
478 will be the exit code of the child process, and `err.signal` will be set to the
479 signal that terminated the process.
481 There is a second optional argument to specify several options. The
487 killSignal: 'SIGTERM',
491 If `timeout` is greater than 0, then it will kill the child process
492 if it runs longer than `timeout` milliseconds. The child process is killed with
493 `killSignal` (default: `'SIGTERM'`). `maxBuffer` specifies the largest
494 amount of data allowed on stdout or stderr - if this value is exceeded then
495 the child process is killed.
498 ## child_process.execFile(file, args, options, callback)
500 * `file` {String} The filename of the program to run
501 * `args` {Array} List of string arguments
503 * `cwd` {String} Current working directory of the child process
504 * `stdio` {Array|String} Child's stdio configuration. (See above)
505 * `customFds` {Array} **Deprecated** File descriptors for the child to use
506 for stdio. (See above)
507 * `env` {Object} Environment key-value pairs
508 * `encoding` {String} (Default: 'utf8')
509 * `timeout` {Number} (Default: 0)
510 * `maxBuffer` {Number} (Default: 200\*1024)
511 * `killSignal` {String} (Default: 'SIGTERM')
512 * `callback` {Function} called with the output when process terminates
516 * Return: ChildProcess object
518 This is similar to `child_process.exec()` except it does not execute a
519 subshell but rather the specified file directly. This makes it slightly
520 leaner than `child_process.exec`. It has the same options.
523 ## child\_process.fork(modulePath, [args], [options])
525 * `modulePath` {String} The module to run in the child
526 * `args` {Array} List of string arguments
528 * `cwd` {String} Current working directory of the child process
529 * `env` {Object} Environment key-value pairs
530 * `encoding` {String} (Default: 'utf8')
531 * `execPath` {String} Executable used to create the child process
532 * Return: ChildProcess object
534 This is a special case of the `spawn()` functionality for spawning Node
535 processes. In addition to having all the methods in a normal ChildProcess
536 instance, the returned object has a communication channel built-in. See
537 `child.send(message, [sendHandle])` for details.
539 By default the spawned Node process will have the stdout, stderr associated
540 with the parent's. To change this behavior set the `silent` property in the
541 `options` object to `true`.
543 The child process does not automatically exit once it's done, you need to call
544 `process.exit()` explicitly. This limitation may be lifted in the future.
546 These child Nodes are still whole new instances of V8. Assume at least 30ms
547 startup and 10mb memory for each new Node. That is, you cannot create many
550 The `execPath` property in the `options` object allows for a process to be
551 created for the child rather than the current `node` executable. This should be
552 done with care and by default will talk over the fd represented an
553 environmental variable `NODE_CHANNEL_FD` on the child process. The input and
554 output on this fd is expected to be line delimited JSON objects.
556 [EventEmitter]: events.html#events_class_events_eventemitter