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 * `err` {Error Object} the error.
35 1. The process could not be spawned, or
36 2. The process could not be killed, or
37 3. Sending a message to the child process failed for whatever reason.
39 Note that the `exit`-event may or may not fire after an error has occured. If
40 you are listening on both events to fire a function, remember to guard against
41 calling your function twice.
43 See also [`ChildProcess#kill()`](#child_process_child_kill_signal) and
44 [`ChildProcess#send()`](#child_process_child_send_message_sendhandle).
48 * `code` {Number} the exit code, if it exited normally.
49 * `signal` {String} the signal passed to kill the child process, if it
50 was killed by the parent.
52 This event is emitted after the child process ends. If the process terminated
53 normally, `code` is the final exit code of the process, otherwise `null`. If
54 the process terminated due to receipt of a signal, `signal` is the string name
55 of the signal, otherwise `null`.
57 Note that the child process stdio streams might still be open.
59 Also, note that node establishes signal handlers for `'SIGINT'` and `'SIGTERM`',
60 so it will not terminate due to receipt of those signals, it will exit.
66 * `code` {Number} the exit code, if it exited normally.
67 * `signal` {String} the signal passed to kill the child process, if it
68 was killed by the parent.
70 This event is emitted when the stdio streams of a child process have all
71 terminated. This is distinct from 'exit', since multiple processes
72 might share the same stdio streams.
74 ### Event: 'disconnect'
76 This event is emitted after calling the `.disconnect()` method in the parent
77 or in the child. After disconnecting it is no longer possible to send messages,
78 and the `.connected` property is false.
82 * `message` {Object} a parsed JSON object or primitive value
83 * `sendHandle` {Handle object} a Socket or Server object
85 Messages send by `.send(message, [sendHandle])` are obtained using the
92 A `Writable Stream` that represents the child process's `stdin`.
93 Closing this stream via `end()` often causes the child process to terminate.
95 If the child stdio streams are shared with the parent, then this will
102 A `Readable Stream` that represents the child process's `stdout`.
104 If the child stdio streams are shared with the parent, then this will
111 A `Readable Stream` that represents the child process's `stderr`.
113 If the child stdio streams are shared with the parent, then this will
120 The PID of the child process.
124 var spawn = require('child_process').spawn,
125 grep = spawn('grep', ['ssh']);
127 console.log('Spawned child pid: ' + grep.pid);
132 * {Boolean} Set to false after `.disconnect' is called
134 If `.connected` is false, it is no longer possible to send messages.
136 ### child.kill([signal])
140 Send a signal to the child process. If no argument is given, the process will
141 be sent `'SIGTERM'`. See `signal(7)` for a list of available signals.
143 var spawn = require('child_process').spawn,
144 grep = spawn('grep', ['ssh']);
146 grep.on('close', function (code, signal) {
147 console.log('child process terminated due to receipt of signal '+signal);
150 // send SIGHUP to process
153 May emit an `'error'` event when the signal cannot be delivered. Sending a
154 signal to a child process that has already exited is not an error but may
155 have unforeseen consequences: if the PID (the process ID) has been reassigned
156 to another process, the signal will be delivered to that process instead.
157 What happens next is anyone's guess.
159 Note that while the function is called `kill`, the signal delivered to the
160 child process may not actually kill it. `kill` really just sends a signal
165 ### child.send(message, [sendHandle])
168 * `sendHandle` {Handle object}
170 When using `child_process.fork()` you can write to the child using
171 `child.send(message, [sendHandle])` and messages are received by
172 a `'message'` event on the child.
176 var cp = require('child_process');
178 var n = cp.fork(__dirname + '/sub.js');
180 n.on('message', function(m) {
181 console.log('PARENT got message:', m);
184 n.send({ hello: 'world' });
186 And then the child script, `'sub.js'` might look like this:
188 process.on('message', function(m) {
189 console.log('CHILD got message:', m);
192 process.send({ foo: 'bar' });
194 In the child the `process` object will have a `send()` method, and `process`
195 will emit objects each time it receives a message on its channel.
197 There is a special case when sending a `{cmd: 'NODE_foo'}` message. All messages
198 containing a `NODE_` prefix in its `cmd` property will not be emitted in
199 the `message` event, since they are internal messages used by node core.
200 Messages containing the prefix are emitted in the `internalMessage` event, you
201 should by all means avoid using this feature, it is subject to change without notice.
203 The `sendHandle` option to `child.send()` is for sending a TCP server or
204 socket object to another process. The child will receive the object as its
205 second argument to the `message` event.
207 Emits an `'error'` event if the message cannot be sent, for example because
208 the child process has already exited.
210 #### Example: sending server object
212 Here is an example of sending a server:
214 var child = require('child_process').fork('child.js');
216 // Open up the server object and send the handle.
217 var server = require('net').createServer();
218 server.on('connection', function (socket) {
219 socket.end('handled by parent');
221 server.listen(1337, function() {
222 child.send('server', server);
225 And the child would the receive the server object as:
227 process.on('message', function(m, server) {
228 if (m === 'server') {
229 server.on('connection', function (socket) {
230 socket.end('handled by child');
235 Note that the server is now shared between the parent and child, this means
236 that some connections will be handled by the parent and some by the child.
238 For `dgram` servers the workflow is exactly the same. Here you listen on
239 a `message` event instead of `connection` and use `server.bind` instead of
240 `server.listen`. (Currently only supported on UNIX platforms.)
242 #### Example: sending socket object
244 Here is an example of sending a socket. It will spawn two children and handle
245 connections with the remote address `74.125.127.100` as VIP by sending the
246 socket to a "special" child process. Other sockets will go to a "normal" process.
248 var normal = require('child_process').fork('child.js', ['normal']);
249 var special = require('child_process').fork('child.js', ['special']);
251 // Open up the server and send sockets to child
252 var server = require('net').createServer();
253 server.on('connection', function (socket) {
256 if (socket.remoteAddress === '74.125.127.100') {
257 special.send('socket', socket);
260 // just the usual dudes
261 normal.send('socket', socket);
265 The `child.js` could look like this:
267 process.on('message', function(m, socket) {
268 if (m === 'socket') {
269 socket.end('You were handled as a ' + process.argv[2] + ' person');
273 Note that once a single socket has been sent to a child the parent can no
274 longer keep track of when the socket is destroyed. To indicate this condition
275 the `.connections` property becomes `null`.
276 It is also recommended not to use `.maxConnections` in this condition.
278 ### child.disconnect()
280 Close the IPC channel between parent and child, allowing the child to exit
281 gracefully once there are no other connections keeping it alive. After calling
282 this method the `.connected` flag will be set to `false` in both the parent and
283 child, and it is no longer possible to send messages.
285 The 'disconnect' event will be emitted when there are no messages in the process
286 of being received, most likely immediately.
288 Note that you can also call `process.disconnect()` in the child process.
290 ## child_process.spawn(command, [args], [options])
292 * `command` {String} The command to run
293 * `args` {Array} List of string arguments
295 * `cwd` {String} Current working directory of the child process
296 * `stdio` {Array|String} Child's stdio configuration. (See below)
297 * `customFds` {Array} **Deprecated** File descriptors for the child to use
298 for stdio. (See below)
299 * `env` {Object} Environment key-value pairs
300 * `detached` {Boolean} The child will be a process group leader. (See below)
301 * `uid` {Number} Sets the user identity of the process. (See setuid(2).)
302 * `gid` {Number} Sets the group identity of the process. (See setgid(2).)
303 * return: {ChildProcess object}
305 Launches a new process with the given `command`, with command line arguments in `args`.
306 If omitted, `args` defaults to an empty Array.
308 The third argument is used to specify additional options, which defaults to:
314 `cwd` allows you to specify the working directory from which the process is spawned.
315 Use `env` to specify environment variables that will be visible to the new process.
317 Example of running `ls -lh /usr`, capturing `stdout`, `stderr`, and the exit code:
319 var spawn = require('child_process').spawn,
320 ls = spawn('ls', ['-lh', '/usr']);
322 ls.stdout.on('data', function (data) {
323 console.log('stdout: ' + data);
326 ls.stderr.on('data', function (data) {
327 console.log('stderr: ' + data);
330 ls.on('close', function (code) {
331 console.log('child process exited with code ' + code);
335 Example: A very elaborate way to run 'ps ax | grep ssh'
337 var spawn = require('child_process').spawn,
338 ps = spawn('ps', ['ax']),
339 grep = spawn('grep', ['ssh']);
341 ps.stdout.on('data', function (data) {
342 grep.stdin.write(data);
345 ps.stderr.on('data', function (data) {
346 console.log('ps stderr: ' + data);
349 ps.on('close', function (code) {
351 console.log('ps process exited with code ' + code);
356 grep.stdout.on('data', function (data) {
357 console.log('' + data);
360 grep.stderr.on('data', function (data) {
361 console.log('grep stderr: ' + data);
364 grep.on('close', function (code) {
366 console.log('grep process exited with code ' + code);
371 Example of checking for failed exec:
373 var spawn = require('child_process').spawn,
374 child = spawn('bad_command');
376 child.stderr.setEncoding('utf8');
377 child.stderr.on('data', function (data) {
378 if (/^execvp\(\)/.test(data)) {
379 console.log('Failed to start child process.');
383 Note that if spawn receives an empty options object, it will result in
384 spawning the process with an empty environment rather than using
385 `process.env`. This due to backwards compatibility issues with a deprecated
388 The 'stdio' option to `child_process.spawn()` is an array where each
389 index corresponds to a fd in the child. The value is one of the following:
391 1. `'pipe'` - Create a pipe between the child process and the parent process.
392 The parent end of the pipe is exposed to the parent as a property on the
393 `child_process` object as `ChildProcess.stdio[fd]`. Pipes created for
394 fds 0 - 2 are also available as ChildProcess.stdin, ChildProcess.stdout
395 and ChildProcess.stderr, respectively.
396 2. `'ipc'` - Create an IPC channel for passing messages/file descriptors
397 between parent and child. A ChildProcess may have at most *one* IPC stdio
398 file descriptor. Setting this option enables the ChildProcess.send() method.
399 If the child writes JSON messages to this file descriptor, then this will
400 trigger ChildProcess.on('message'). If the child is a Node.js program, then
401 the presence of an IPC channel will enable process.send() and
402 process.on('message').
403 3. `'ignore'` - Do not set this file descriptor in the child. Note that Node
404 will always open fd 0 - 2 for the processes it spawns. When any of these is
405 ignored node will open `/dev/null` and attach it to the child's fd.
406 4. `Stream` object - Share a readable or writable stream that refers to a tty,
407 file, socket, or a pipe with the child process. The stream's underlying
408 file descriptor is duplicated in the child process to the fd that
409 corresponds to the index in the `stdio` array.
410 5. Positive integer - The integer value is interpreted as a file descriptor
411 that is is currently open in the parent process. It is shared with the child
412 process, similar to how `Stream` objects can be shared.
413 6. `null`, `undefined` - Use default value. For stdio fds 0, 1 and 2 (in other
414 words, stdin, stdout, and stderr) a pipe is created. For fd 3 and up, the
415 default is `'ignore'`.
417 As a shorthand, the `stdio` argument may also be one of the following
418 strings, rather than an array:
420 * `ignore` - `['ignore', 'ignore', 'ignore']`
421 * `pipe` - `['pipe', 'pipe', 'pipe']`
422 * `inherit` - `[process.stdin, process.stdout, process.stderr]` or `[0,1,2]`
426 var spawn = require('child_process').spawn;
428 // Child will use parent's stdios
429 spawn('prg', [], { stdio: 'inherit' });
431 // Spawn child sharing only stderr
432 spawn('prg', [], { stdio: ['pipe', 'pipe', process.stderr] });
434 // Open an extra fd=4, to interact with programs present a
435 // startd-style interface.
436 spawn('prg', [], { stdio: ['pipe', null, null, null, 'pipe'] });
438 If the `detached` option is set, the child process will be made the leader of a
439 new process group. This makes it possible for the child to continue running
440 after the parent exits.
442 By default, the parent will wait for the detached child to exit. To prevent
443 the parent from waiting for a given `child`, use the `child.unref()` method,
444 and the parent's event loop will not include the child in its reference count.
446 Example of detaching a long-running process and redirecting its output to a
449 var fs = require('fs'),
450 spawn = require('child_process').spawn,
451 out = fs.openSync('./out.log', 'a'),
452 err = fs.openSync('./out.log', 'a');
454 var child = spawn('prg', [], {
456 stdio: [ 'ignore', out, err ]
461 When using the `detached` option to start a long-running process, the process
462 will not stay running in the background unless it is provided with a `stdio`
463 configuration that is not connected to the parent. If the parent's `stdio` is
464 inherited, the child will remain attached to the controlling terminal.
466 There is a deprecated option called `customFds` which allows one to specify
467 specific file descriptors for the stdio of the child process. This API was
468 not portable to all platforms and therefore removed.
469 With `customFds` it was possible to hook up the new process' `[stdin, stdout,
470 stderr]` to existing streams; `-1` meant that a new stream should be created.
471 Use at your own risk.
473 See also: `child_process.exec()` and `child_process.fork()`
475 ## child_process.exec(command, [options], callback)
477 * `command` {String} The command to run, with space-separated arguments
479 * `cwd` {String} Current working directory of the child process
480 * `env` {Object} Environment key-value pairs
481 * `encoding` {String} (Default: 'utf8')
482 * `shell` {String} Shell to execute the command with
483 (Default: '/bin/sh' on UNIX, 'cmd.exe' on Windows, The shell should
484 understand the `-c` switch on UNIX or `/s /c` on Windows. On Windows,
485 command line parsing should be compatible with `cmd.exe`.)
486 * `timeout` {Number} (Default: 0)
487 * `maxBuffer` {Number} (Default: `200*1024`)
488 * `killSignal` {String} (Default: 'SIGTERM')
489 * `callback` {Function} called with the output when process terminates
493 * Return: ChildProcess object
495 Runs a command in a shell and buffers the output.
497 var exec = require('child_process').exec,
500 child = exec('cat *.js bad_file | wc -l',
501 function (error, stdout, stderr) {
502 console.log('stdout: ' + stdout);
503 console.log('stderr: ' + stderr);
504 if (error !== null) {
505 console.log('exec error: ' + error);
509 The callback gets the arguments `(error, stdout, stderr)`. On success, `error`
510 will be `null`. On error, `error` will be an instance of `Error` and `err.code`
511 will be the exit code of the child process, and `err.signal` will be set to the
512 signal that terminated the process.
514 There is a second optional argument to specify several options. The
520 killSignal: 'SIGTERM',
524 If `timeout` is greater than 0, then it will kill the child process
525 if it runs longer than `timeout` milliseconds. The child process is killed with
526 `killSignal` (default: `'SIGTERM'`). `maxBuffer` specifies the largest
527 amount of data allowed on stdout or stderr - if this value is exceeded then
528 the child process is killed.
531 ## child_process.execFile(file, args, options, callback)
533 * `file` {String} The filename of the program to run
534 * `args` {Array} List of string arguments
536 * `cwd` {String} Current working directory of the child process
537 * `env` {Object} Environment key-value pairs
538 * `encoding` {String} (Default: 'utf8')
539 * `timeout` {Number} (Default: 0)
540 * `maxBuffer` {Number} (Default: 200\*1024)
541 * `killSignal` {String} (Default: 'SIGTERM')
542 * `callback` {Function} called with the output when process terminates
546 * Return: ChildProcess object
548 This is similar to `child_process.exec()` except it does not execute a
549 subshell but rather the specified file directly. This makes it slightly
550 leaner than `child_process.exec`. It has the same options.
553 ## child_process.fork(modulePath, [args], [options])
555 * `modulePath` {String} The module to run in the child
556 * `args` {Array} List of string arguments
558 * `cwd` {String} Current working directory of the child process
559 * `env` {Object} Environment key-value pairs
560 * `encoding` {String} (Default: 'utf8')
561 * `execPath` {String} Executable used to create the child process
562 * `execArgv` {Array} List of string arguments passed to the executable
563 (Default: `process.execArgv`)
564 * `silent` {Boolean} If true, prevent stdout and stderr in the spawned node
565 process from being associated with the parent's (default is false)
566 * Return: ChildProcess object
568 This is a special case of the `spawn()` functionality for spawning Node
569 processes. In addition to having all the methods in a normal ChildProcess
570 instance, the returned object has a communication channel built-in. See
571 `child.send(message, [sendHandle])` for details.
573 These child Nodes are still whole new instances of V8. Assume at least 30ms
574 startup and 10mb memory for each new Node. That is, you cannot create many
577 The `execPath` property in the `options` object allows for a process to be
578 created for the child rather than the current `node` executable. This should be
579 done with care and by default will talk over the fd represented an
580 environmental variable `NODE_CHANNEL_FD` on the child process. The input and
581 output on this fd is expected to be line delimited JSON objects.
583 [EventEmitter]: events.html#events_class_events_eventemitter