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 This event is emitted when the stdio streams of a child process have all
47 terminated. This is distinct from 'exit', since multiple processes
48 might share the same stdio streams.
50 ### Event: 'disconnect'
52 This event is emitted after using the `.disconnect()` method in the parent or
53 in the child. After disconnecting it is no longer possible to send messages.
54 An alternative way to check if you can send messages is to see if the
55 `child.connected` property is `true`.
59 * `message` {Object} a parsed JSON object or primitive value
60 * `sendHandle` {Handle object} a Socket or Server object
62 Messages send by `.send(message, [sendHandle])` are obtained using the
69 A `Writable Stream` that represents the child process's `stdin`.
70 Closing this stream via `end()` often causes the child process to terminate.
72 If the child stdio streams are shared with the parent, then this will
79 A `Readable Stream` that represents the child process's `stdout`.
81 If the child stdio streams are shared with the parent, then this will
88 A `Readable Stream` that represents the child process's `stderr`.
90 If the child stdio streams are shared with the parent, then this will
97 The PID of the child process.
101 var spawn = require('child_process').spawn,
102 grep = spawn('grep', ['ssh']);
104 console.log('Spawned child pid: ' + grep.pid);
107 ### child.kill([signal])
111 Send a signal to the child process. If no argument is given, the process will
112 be sent `'SIGTERM'`. See `signal(7)` for a list of available signals.
114 var spawn = require('child_process').spawn,
115 grep = spawn('grep', ['ssh']);
117 grep.on('exit', function (code, signal) {
118 console.log('child process terminated due to receipt of signal '+signal);
121 // send SIGHUP to process
124 Note that while the function is called `kill`, the signal delivered to the child
125 process may not actually kill it. `kill` really just sends a signal to a process.
129 ### child.send(message, [sendHandle])
132 * `sendHandle` {Handle object}
134 When using `child_process.fork()` you can write to the child using
135 `child.send(message, [sendHandle])` and messages are received by
136 a `'message'` event on the child.
140 var cp = require('child_process');
142 var n = cp.fork(__dirname + '/sub.js');
144 n.on('message', function(m) {
145 console.log('PARENT got message:', m);
148 n.send({ hello: 'world' });
150 And then the child script, `'sub.js'` might look like this:
152 process.on('message', function(m) {
153 console.log('CHILD got message:', m);
156 process.send({ foo: 'bar' });
158 In the child the `process` object will have a `send()` method, and `process`
159 will emit objects each time it receives a message on its channel.
161 There is a special case when sending a `{cmd: 'NODE_foo'}` message. All messages
162 containing a `NODE_` prefix in its `cmd` property will not be emitted in
163 the `message` event, since they are internal messages used by node core.
164 Messages containing the prefix are emitted in the `internalMessage` event, you
165 should by all means avoid using this feature, it is subject to change without notice.
167 The `sendHandle` option to `child.send()` is for sending a TCP server or
168 socket object to another process. The child will receive the object as its
169 second argument to the `message` event.
171 **send server object**
173 Here is an example of sending a server:
175 var child = require('child_process').fork('child.js');
177 // Open up the server object and send the handle.
178 var server = require('net').createServer();
179 server.on('connection', function (socket) {
180 socket.end('handled by parent');
182 server.listen(1337, function() {
183 child.send('server', server);
186 And the child would the receive the server object as:
188 process.on('message', function(m, server) {
189 if (m === 'server') {
190 server.on('connection', function (socket) {
191 socket.end('handled by child');
196 Note that the server is now shared between the parent and child, this means
197 that some connections will be handled by the parent and some by the child.
199 **send socket object**
201 Here is an example of sending a socket. It will spawn two children and handle
202 connections with the remote address `74.125.127.100` as VIP by sending the
203 socket to a "special" child process. Other sockets will go to a "normal" process.
205 var normal = require('child_process').fork('child.js', ['normal']);
206 var special = require('child_process').fork('child.js', ['special']);
208 // Open up the server and send sockets to child
209 var server = require('net').createServer();
210 server.on('connection', function (socket) {
213 if (socket.remoteAddress === '74.125.127.100') {
214 special.send('socket', socket);
217 // just the usual dudes
218 normal.send('socket', socket);
222 The `child.js` could look like this:
224 process.on('message', function(m, socket) {
225 if (m === 'socket') {
226 socket.end('You were handled as a ' + process.argv[2] + ' person');
230 Note that once a single socket has been sent to a child the parent can no
231 longer keep track of when the socket is destroyed. To indicate this condition
232 the `.connections` property becomes `null`.
233 It is also recommended not to use `.maxConnections` in this condition.
235 ### child.disconnect()
237 To close the IPC connection between parent and child use the
238 `child.disconnect()` method. This allows the child to exit gracefully since
239 there is no IPC channel keeping it alive. When calling this method the
240 `disconnect` event will be emitted in both parent and child, and the
241 `connected` flag will be set to `false`. Please note that you can also call
242 `process.disconnect()` in the child process.
244 ## child_process.spawn(command, [args], [options])
246 * `command` {String} The command to run
247 * `args` {Array} List of string arguments
249 * `cwd` {String} Current working directory of the child process
250 * `stdio` {Array|String} Child's stdio configuration. (See below)
251 * `customFds` {Array} **Deprecated** File descriptors for the child to use
252 for stdio. (See below)
253 * `env` {Object} Environment key-value pairs
254 * `detached` {Boolean} The child will be a process group leader. (See below)
255 * `uid` {Number} Sets the user identity of the process. (See setuid(2).)
256 * `gid` {Number} Sets the group identity of the process. (See setgid(2).)
257 * return: {ChildProcess object}
259 Launches a new process with the given `command`, with command line arguments in `args`.
260 If omitted, `args` defaults to an empty Array.
262 The third argument is used to specify additional options, which defaults to:
268 `cwd` allows you to specify the working directory from which the process is spawned.
269 Use `env` to specify environment variables that will be visible to the new process.
271 Example of running `ls -lh /usr`, capturing `stdout`, `stderr`, and the exit code:
273 var spawn = require('child_process').spawn,
274 ls = spawn('ls', ['-lh', '/usr']);
276 ls.stdout.on('data', function (data) {
277 console.log('stdout: ' + data);
280 ls.stderr.on('data', function (data) {
281 console.log('stderr: ' + data);
284 ls.on('exit', function (code) {
285 console.log('child process exited with code ' + code);
289 Example: A very elaborate way to run 'ps ax | grep ssh'
291 var spawn = require('child_process').spawn,
292 ps = spawn('ps', ['ax']),
293 grep = spawn('grep', ['ssh']);
295 ps.stdout.on('data', function (data) {
296 grep.stdin.write(data);
299 ps.stderr.on('data', function (data) {
300 console.log('ps stderr: ' + data);
303 ps.on('exit', function (code) {
305 console.log('ps process exited with code ' + code);
310 grep.stdout.on('data', function (data) {
311 console.log('' + data);
314 grep.stderr.on('data', function (data) {
315 console.log('grep stderr: ' + data);
318 grep.on('exit', function (code) {
320 console.log('grep process exited with code ' + code);
325 Example of checking for failed exec:
327 var spawn = require('child_process').spawn,
328 child = spawn('bad_command');
330 child.stderr.setEncoding('utf8');
331 child.stderr.on('data', function (data) {
332 if (/^execvp\(\)/.test(data)) {
333 console.log('Failed to start child process.');
337 Note that if spawn receives an empty options object, it will result in
338 spawning the process with an empty environment rather than using
339 `process.env`. This due to backwards compatibility issues with a deprecated
342 The 'stdio' option to `child_process.spawn()` is an array where each
343 index corresponds to a fd in the child. The value is one of the following:
345 1. `'pipe'` - Create a pipe between the child process and the parent process.
346 The parent end of the pipe is exposed to the parent as a property on the
347 `child_process` object as `ChildProcess.stdio[fd]`. Pipes created for
348 fds 0 - 2 are also available as ChildProcess.stdin, ChildProcess.stdout
349 and ChildProcess.stderr, respectively.
350 2. `'ipc'` - Create an IPC channel for passing messages/file descriptors
351 between parent and child. A ChildProcess may have at most *one* IPC stdio
352 file descriptor. Setting this option enables the ChildProcess.send() method.
353 If the child writes JSON messages to this file descriptor, then this will
354 trigger ChildProcess.on('message'). If the child is a Node.js program, then
355 the presence of an IPC channel will enable process.send() and
356 process.on('message').
357 3. `'ignore'` - Do not set this file descriptor in the child. Note that Node
358 will always open fd 0 - 2 for the processes it spawns. When any of these is
359 ignored node will open `/dev/null` and attach it to the child's fd.
360 4. `Stream` object - Share a readable or writable stream that refers to a tty,
361 file, socket, or a pipe with the child process. The stream's underlying
362 file descriptor is duplicated in the child process to the fd that
363 corresponds to the index in the `stdio` array.
364 5. Positive integer - The integer value is interpreted as a file descriptor
365 that is is currently open in the parent process. It is shared with the child
366 process, similar to how `Stream` objects can be shared.
367 6. `null`, `undefined` - Use default value. For stdio fds 0, 1 and 2 (in other
368 words, stdin, stdout, and stderr) a pipe is created. For fd 3 and up, the
369 default is `'ignore'`.
371 As a shorthand, the `stdio` argument may also be one of the following
372 strings, rather than an array:
374 * `ignore` - `['ignore', 'ignore', 'ignore']`
375 * `pipe` - `['pipe', 'pipe', 'pipe']`
376 * `inherit` - `[process.stdin, process.stdout, process.stderr]` or `[0,1,2]`
380 var spawn = require('child_process').spawn;
382 // Child will use parent's stdios
383 spawn('prg', [], { stdio: 'inherit' });
385 // Spawn child sharing only stderr
386 spawn('prg', [], { stdio: ['pipe', 'pipe', process.stderr] });
388 // Open an extra fd=4, to interact with programs present a
389 // startd-style interface.
390 spawn('prg', [], { stdio: ['pipe', null, null, null, 'pipe'] });
392 If the `detached` option is set, the child process will be made the leader of a
393 new process group. This makes it possible for the child to continue running
394 after the parent exits.
396 By default, the parent will wait for the detached child to exit. To prevent
397 the parent from waiting for a given `child`, use the `child.unref()` method,
398 and the parent's event loop will not include the child in its reference count.
400 Example of detaching a long-running process and redirecting its output to a
403 var fs = require('fs'),
404 spawn = require('child_process').spawn,
405 out = fs.openSync('./out.log', 'a'),
406 err = fs.openSync('./out.log', 'a');
408 var child = spawn('prg', [], {
410 stdio: [ 'ignore', out, err ]
415 When using the `detached` option to start a long-running process, the process
416 will not stay running in the background unless it is provided with a `stdio`
417 configuration that is not connected to the parent. If the parent's `stdio` is
418 inherited, the child will remain attached to the controlling terminal.
420 There is a deprecated option called `customFds` which allows one to specify
421 specific file descriptors for the stdio of the child process. This API was
422 not portable to all platforms and therefore removed.
423 With `customFds` it was possible to hook up the new process' `[stdin, stdout,
424 stderr]` to existing streams; `-1` meant that a new stream should be created.
425 Use at your own risk.
427 There are several internal options. In particular `stdinStream`,
428 `stdoutStream`, `stderrStream`. They are for INTERNAL USE ONLY. As with all
429 undocumented APIs in Node, they should not be used.
431 See also: `child_process.exec()` and `child_process.fork()`
433 ## child_process.exec(command, [options], callback)
435 * `command` {String} The command to run, with space-separated arguments
437 * `cwd` {String} Current working directory of the child process
438 * `stdio` {Array|String} Child's stdio configuration. (See above)
439 Only stdin is configurable, anything else will lead to unpredictable
441 * `customFds` {Array} **Deprecated** File descriptors for the child to use
442 for stdio. (See above)
443 * `env` {Object} Environment key-value pairs
444 * `encoding` {String} (Default: 'utf8')
445 * `timeout` {Number} (Default: 0)
446 * `maxBuffer` {Number} (Default: 200*1024)
447 * `killSignal` {String} (Default: 'SIGTERM')
448 * `callback` {Function} called with the output when process terminates
452 * Return: ChildProcess object
454 Runs a command in a shell and buffers the output.
456 var exec = require('child_process').exec,
459 child = exec('cat *.js bad_file | wc -l',
460 function (error, stdout, stderr) {
461 console.log('stdout: ' + stdout);
462 console.log('stderr: ' + stderr);
463 if (error !== null) {
464 console.log('exec error: ' + error);
468 The callback gets the arguments `(error, stdout, stderr)`. On success, `error`
469 will be `null`. On error, `error` will be an instance of `Error` and `err.code`
470 will be the exit code of the child process, and `err.signal` will be set to the
471 signal that terminated the process.
473 There is a second optional argument to specify several options. The
479 killSignal: 'SIGTERM',
483 If `timeout` is greater than 0, then it will kill the child process
484 if it runs longer than `timeout` milliseconds. The child process is killed with
485 `killSignal` (default: `'SIGTERM'`). `maxBuffer` specifies the largest
486 amount of data allowed on stdout or stderr - if this value is exceeded then
487 the child process is killed.
490 ## child_process.execFile(file, args, options, callback)
492 * `file` {String} The filename of the program to run
493 * `args` {Array} List of string arguments
495 * `cwd` {String} Current working directory of the child process
496 * `stdio` {Array|String} Child's stdio configuration. (See above)
497 * `customFds` {Array} **Deprecated** File descriptors for the child to use
498 for stdio. (See above)
499 * `env` {Object} Environment key-value pairs
500 * `encoding` {String} (Default: 'utf8')
501 * `timeout` {Number} (Default: 0)
502 * `maxBuffer` {Number} (Default: 200\*1024)
503 * `killSignal` {String} (Default: 'SIGTERM')
504 * `callback` {Function} called with the output when process terminates
508 * Return: ChildProcess object
510 This is similar to `child_process.exec()` except it does not execute a
511 subshell but rather the specified file directly. This makes it slightly
512 leaner than `child_process.exec`. It has the same options.
515 ## child\_process.fork(modulePath, [args], [options])
517 * `modulePath` {String} The module to run in the child
518 * `args` {Array} List of string arguments
520 * `cwd` {String} Current working directory of the child process
521 * `env` {Object} Environment key-value pairs
522 * `encoding` {String} (Default: 'utf8')
523 * Return: ChildProcess object
525 This is a special case of the `spawn()` functionality for spawning Node
526 processes. In addition to having all the methods in a normal ChildProcess
527 instance, the returned object has a communication channel built-in. See
528 `child.send(message, [sendHandle])` for details.
530 By default the spawned Node process will have the stdout, stderr associated
531 with the parent's. To change this behavior set the `silent` property in the
532 `options` object to `true`.
534 The child process does not automatically exit once it's done, you need to call
535 `process.exit()` explicitly. This limitation may be lifted in the future.
537 These child Nodes are still whole new instances of V8. Assume at least 30ms
538 startup and 10mb memory for each new Node. That is, you cannot create many
541 [EventEmitter]: events.html#events_class_events_eventemitter