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 See also [`ChildProcess#kill()`](#child_process_child_kill_signal) and
40 [`ChildProcess#send()`](#child_process_child_send_message_sendhandle).
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 after the child process ends. If the process terminated
49 normally, `code` is the final exit code of the process, otherwise `null`. If
50 the process terminated due to receipt of a signal, `signal` is the string name
51 of the signal, otherwise `null`.
53 Note that the child process stdio streams might still be open.
55 Also, note that node establishes signal handlers for `'SIGINT'` and `'SIGTERM`',
56 so it will not terminate due to receipt of those signals, it will exit.
62 * `code` {Number} the exit code, if it exited normally.
63 * `signal` {String} the signal passed to kill the child process, if it
64 was killed by the parent.
66 This event is emitted when the stdio streams of a child process have all
67 terminated. This is distinct from 'exit', since multiple processes
68 might share the same stdio streams.
70 ### Event: 'disconnect'
72 This event is emitted after calling the `.disconnect()` method in the parent
73 or in the child. After disconnecting it is no longer possible to send messages,
74 and the `.connected` property is false.
78 * `message` {Object} a parsed JSON object or primitive value
79 * `sendHandle` {Handle object} a Socket or Server object
81 Messages send by `.send(message, [sendHandle])` are obtained using the
88 A `Writable Stream` that represents the child process's `stdin`.
89 Closing this stream via `end()` often causes the child process to terminate.
91 If the child stdio streams are shared with the parent, then this will
98 A `Readable Stream` that represents the child process's `stdout`.
100 If the child stdio streams are shared with the parent, then this will
107 A `Readable Stream` that represents the child process's `stderr`.
109 If the child stdio streams are shared with the parent, then this will
116 The PID of the child process.
120 var spawn = require('child_process').spawn,
121 grep = spawn('grep', ['ssh']);
123 console.log('Spawned child pid: ' + grep.pid);
128 * {Boolean} Set to false after `.disconnect' is called
130 If `.connected` is false, it is no longer possible to send messages.
132 ### child.kill([signal])
136 Send a signal to the child process. If no argument is given, the process will
137 be sent `'SIGTERM'`. See `signal(7)` for a list of available signals.
139 var spawn = require('child_process').spawn,
140 grep = spawn('grep', ['ssh']);
142 grep.on('close', function (code, signal) {
143 console.log('child process terminated due to receipt of signal '+signal);
146 // send SIGHUP to process
149 May emit an `'error'` event when the signal cannot be delivered. Sending a
150 signal to a child process that has already exited is not an error but may
151 have unforeseen consequences: if the PID (the process ID) has been reassigned
152 to another process, the signal will be delivered to that process instead.
153 What happens next is anyone's guess.
155 Note that while the function is called `kill`, the signal delivered to the
156 child process may not actually kill it. `kill` really just sends a signal
161 ### child.send(message, [sendHandle])
164 * `sendHandle` {Handle object}
166 When using `child_process.fork()` you can write to the child using
167 `child.send(message, [sendHandle])` and messages are received by
168 a `'message'` event on the child.
172 var cp = require('child_process');
174 var n = cp.fork(__dirname + '/sub.js');
176 n.on('message', function(m) {
177 console.log('PARENT got message:', m);
180 n.send({ hello: 'world' });
182 And then the child script, `'sub.js'` might look like this:
184 process.on('message', function(m) {
185 console.log('CHILD got message:', m);
188 process.send({ foo: 'bar' });
190 In the child the `process` object will have a `send()` method, and `process`
191 will emit objects each time it receives a message on its channel.
193 There is a special case when sending a `{cmd: 'NODE_foo'}` message. All messages
194 containing a `NODE_` prefix in its `cmd` property will not be emitted in
195 the `message` event, since they are internal messages used by node core.
196 Messages containing the prefix are emitted in the `internalMessage` event, you
197 should by all means avoid using this feature, it is subject to change without notice.
199 The `sendHandle` option to `child.send()` is for sending a TCP server or
200 socket object to another process. The child will receive the object as its
201 second argument to the `message` event.
203 Emits an `'error'` event if the message cannot be sent, for example because
204 the child process has already exited.
206 #### Example: sending server object
208 Here is an example of sending a server:
210 var child = require('child_process').fork('child.js');
212 // Open up the server object and send the handle.
213 var server = require('net').createServer();
214 server.on('connection', function (socket) {
215 socket.end('handled by parent');
217 server.listen(1337, function() {
218 child.send('server', server);
221 And the child would the receive the server object as:
223 process.on('message', function(m, server) {
224 if (m === 'server') {
225 server.on('connection', function (socket) {
226 socket.end('handled by child');
231 Note that the server is now shared between the parent and child, this means
232 that some connections will be handled by the parent and some by the child.
234 For `dgram` servers the workflow is exactly the same. Here you listen on
235 a `message` event instead of `connection` and use `server.bind` instead of
236 `server.listen`. (Currently only supported on UNIX platforms.)
238 #### Example: sending socket object
240 Here is an example of sending a socket. It will spawn two children and handle
241 connections with the remote address `74.125.127.100` as VIP by sending the
242 socket to a "special" child process. Other sockets will go to a "normal" process.
244 var normal = require('child_process').fork('child.js', ['normal']);
245 var special = require('child_process').fork('child.js', ['special']);
247 // Open up the server and send sockets to child
248 var server = require('net').createServer();
249 server.on('connection', function (socket) {
252 if (socket.remoteAddress === '74.125.127.100') {
253 special.send('socket', socket);
256 // just the usual dudes
257 normal.send('socket', socket);
261 The `child.js` could look like this:
263 process.on('message', function(m, socket) {
264 if (m === 'socket') {
265 socket.end('You were handled as a ' + process.argv[2] + ' person');
269 Note that once a single socket has been sent to a child the parent can no
270 longer keep track of when the socket is destroyed. To indicate this condition
271 the `.connections` property becomes `null`.
272 It is also recommended not to use `.maxConnections` in this condition.
274 ### child.disconnect()
276 Close the IPC channel between parent and child, allowing the child to exit
277 gracefully once there are no other connections keeping it alive. After calling
278 this method the `.connected` flag will be set to `false` in both the parent and
279 child, and it is no longer possible to send messages.
281 The 'disconnect' event will be emitted when there are no messages in the process
282 of being received, most likely immediately.
284 Note that you can also call `process.disconnect()` in the child process.
286 ## child_process.spawn(command, [args], [options])
288 * `command` {String} The command to run
289 * `args` {Array} List of string arguments
291 * `cwd` {String} Current working directory of the child process
292 * `stdio` {Array|String} Child's stdio configuration. (See below)
293 * `customFds` {Array} **Deprecated** File descriptors for the child to use
294 for stdio. (See below)
295 * `env` {Object} Environment key-value pairs
296 * `detached` {Boolean} The child will be a process group leader. (See below)
297 * `uid` {Number} Sets the user identity of the process. (See setuid(2).)
298 * `gid` {Number} Sets the group identity of the process. (See setgid(2).)
299 * return: {ChildProcess object}
301 Launches a new process with the given `command`, with command line arguments in `args`.
302 If omitted, `args` defaults to an empty Array.
304 The third argument is used to specify additional options, which defaults to:
310 `cwd` allows you to specify the working directory from which the process is spawned.
311 Use `env` to specify environment variables that will be visible to the new process.
313 Example of running `ls -lh /usr`, capturing `stdout`, `stderr`, and the exit code:
315 var spawn = require('child_process').spawn,
316 ls = spawn('ls', ['-lh', '/usr']);
318 ls.stdout.on('data', function (data) {
319 console.log('stdout: ' + data);
322 ls.stderr.on('data', function (data) {
323 console.log('stderr: ' + data);
326 ls.on('close', function (code) {
327 console.log('child process exited with code ' + code);
331 Example: A very elaborate way to run 'ps ax | grep ssh'
333 var spawn = require('child_process').spawn,
334 ps = spawn('ps', ['ax']),
335 grep = spawn('grep', ['ssh']);
337 ps.stdout.on('data', function (data) {
338 grep.stdin.write(data);
341 ps.stderr.on('data', function (data) {
342 console.log('ps stderr: ' + data);
345 ps.on('close', function (code) {
347 console.log('ps process exited with code ' + code);
352 grep.stdout.on('data', function (data) {
353 console.log('' + data);
356 grep.stderr.on('data', function (data) {
357 console.log('grep stderr: ' + data);
360 grep.on('close', function (code) {
362 console.log('grep process exited with code ' + code);
367 Example of checking for failed exec:
369 var spawn = require('child_process').spawn,
370 child = spawn('bad_command');
372 child.stderr.setEncoding('utf8');
373 child.stderr.on('data', function (data) {
374 if (/^execvp\(\)/.test(data)) {
375 console.log('Failed to start child process.');
379 Note that if spawn receives an empty options object, it will result in
380 spawning the process with an empty environment rather than using
381 `process.env`. This due to backwards compatibility issues with a deprecated
384 The 'stdio' option to `child_process.spawn()` is an array where each
385 index corresponds to a fd in the child. The value is one of the following:
387 1. `'pipe'` - Create a pipe between the child process and the parent process.
388 The parent end of the pipe is exposed to the parent as a property on the
389 `child_process` object as `ChildProcess.stdio[fd]`. Pipes created for
390 fds 0 - 2 are also available as ChildProcess.stdin, ChildProcess.stdout
391 and ChildProcess.stderr, respectively.
392 2. `'ipc'` - Create an IPC channel for passing messages/file descriptors
393 between parent and child. A ChildProcess may have at most *one* IPC stdio
394 file descriptor. Setting this option enables the ChildProcess.send() method.
395 If the child writes JSON messages to this file descriptor, then this will
396 trigger ChildProcess.on('message'). If the child is a Node.js program, then
397 the presence of an IPC channel will enable process.send() and
398 process.on('message').
399 3. `'ignore'` - Do not set this file descriptor in the child. Note that Node
400 will always open fd 0 - 2 for the processes it spawns. When any of these is
401 ignored node will open `/dev/null` and attach it to the child's fd.
402 4. `Stream` object - Share a readable or writable stream that refers to a tty,
403 file, socket, or a pipe with the child process. The stream's underlying
404 file descriptor is duplicated in the child process to the fd that
405 corresponds to the index in the `stdio` array.
406 5. Positive integer - The integer value is interpreted as a file descriptor
407 that is is currently open in the parent process. It is shared with the child
408 process, similar to how `Stream` objects can be shared.
409 6. `null`, `undefined` - Use default value. For stdio fds 0, 1 and 2 (in other
410 words, stdin, stdout, and stderr) a pipe is created. For fd 3 and up, the
411 default is `'ignore'`.
413 As a shorthand, the `stdio` argument may also be one of the following
414 strings, rather than an array:
416 * `ignore` - `['ignore', 'ignore', 'ignore']`
417 * `pipe` - `['pipe', 'pipe', 'pipe']`
418 * `inherit` - `[process.stdin, process.stdout, process.stderr]` or `[0,1,2]`
422 var spawn = require('child_process').spawn;
424 // Child will use parent's stdios
425 spawn('prg', [], { stdio: 'inherit' });
427 // Spawn child sharing only stderr
428 spawn('prg', [], { stdio: ['pipe', 'pipe', process.stderr] });
430 // Open an extra fd=4, to interact with programs present a
431 // startd-style interface.
432 spawn('prg', [], { stdio: ['pipe', null, null, null, 'pipe'] });
434 If the `detached` option is set, the child process will be made the leader of a
435 new process group. This makes it possible for the child to continue running
436 after the parent exits.
438 By default, the parent will wait for the detached child to exit. To prevent
439 the parent from waiting for a given `child`, use the `child.unref()` method,
440 and the parent's event loop will not include the child in its reference count.
442 Example of detaching a long-running process and redirecting its output to a
445 var fs = require('fs'),
446 spawn = require('child_process').spawn,
447 out = fs.openSync('./out.log', 'a'),
448 err = fs.openSync('./out.log', 'a');
450 var child = spawn('prg', [], {
452 stdio: [ 'ignore', out, err ]
457 When using the `detached` option to start a long-running process, the process
458 will not stay running in the background unless it is provided with a `stdio`
459 configuration that is not connected to the parent. If the parent's `stdio` is
460 inherited, the child will remain attached to the controlling terminal.
462 There is a deprecated option called `customFds` which allows one to specify
463 specific file descriptors for the stdio of the child process. This API was
464 not portable to all platforms and therefore removed.
465 With `customFds` it was possible to hook up the new process' `[stdin, stdout,
466 stderr]` to existing streams; `-1` meant that a new stream should be created.
467 Use at your own risk.
469 See also: `child_process.exec()` and `child_process.fork()`
471 ## child_process.exec(command, [options], callback)
473 * `command` {String} The command to run, with space-separated arguments
475 * `cwd` {String} Current working directory of the child process
476 * `env` {Object} Environment key-value pairs
477 * `encoding` {String} (Default: 'utf8')
478 * `shell` {String} Shell to execute the command with
479 (Default: '/bin/sh' on UNIX, 'cmd.exe' on Windows, The shell should
480 understand the `-c` switch on UNIX or `/s /c` on Windows. On Windows,
481 command line parsing should be compatible with `cmd.exe`.)
482 * `timeout` {Number} (Default: 0)
483 * `maxBuffer` {Number} (Default: `200*1024`)
484 * `killSignal` {String} (Default: 'SIGTERM')
485 * `callback` {Function} called with the output when process terminates
489 * Return: ChildProcess object
491 Runs a command in a shell and buffers the output.
493 var exec = require('child_process').exec,
496 child = exec('cat *.js bad_file | wc -l',
497 function (error, stdout, stderr) {
498 console.log('stdout: ' + stdout);
499 console.log('stderr: ' + stderr);
500 if (error !== null) {
501 console.log('exec error: ' + error);
505 The callback gets the arguments `(error, stdout, stderr)`. On success, `error`
506 will be `null`. On error, `error` will be an instance of `Error` and `err.code`
507 will be the exit code of the child process, and `err.signal` will be set to the
508 signal that terminated the process.
510 There is a second optional argument to specify several options. The
516 killSignal: 'SIGTERM',
520 If `timeout` is greater than 0, then it will kill the child process
521 if it runs longer than `timeout` milliseconds. The child process is killed with
522 `killSignal` (default: `'SIGTERM'`). `maxBuffer` specifies the largest
523 amount of data allowed on stdout or stderr - if this value is exceeded then
524 the child process is killed.
527 ## child_process.execFile(file, args, options, callback)
529 * `file` {String} The filename of the program to run
530 * `args` {Array} List of string arguments
532 * `cwd` {String} Current working directory of the child process
533 * `env` {Object} Environment key-value pairs
534 * `encoding` {String} (Default: 'utf8')
535 * `timeout` {Number} (Default: 0)
536 * `maxBuffer` {Number} (Default: 200\*1024)
537 * `killSignal` {String} (Default: 'SIGTERM')
538 * `callback` {Function} called with the output when process terminates
542 * Return: ChildProcess object
544 This is similar to `child_process.exec()` except it does not execute a
545 subshell but rather the specified file directly. This makes it slightly
546 leaner than `child_process.exec`. It has the same options.
549 ## child_process.fork(modulePath, [args], [options])
551 * `modulePath` {String} The module to run in the child
552 * `args` {Array} List of string arguments
554 * `cwd` {String} Current working directory of the child process
555 * `env` {Object} Environment key-value pairs
556 * `encoding` {String} (Default: 'utf8')
557 * `execPath` {String} Executable used to create the child process
558 * `silent` {Boolean} If true, prevent stdout and stderr in the spawned node
559 process from being associated with the parent's (default is false)
560 * Return: ChildProcess object
562 This is a special case of the `spawn()` functionality for spawning Node
563 processes. In addition to having all the methods in a normal ChildProcess
564 instance, the returned object has a communication channel built-in. See
565 `child.send(message, [sendHandle])` for details.
567 These child Nodes are still whole new instances of V8. Assume at least 30ms
568 startup and 10mb memory for each new Node. That is, you cannot create many
571 The `execPath` property in the `options` object allows for a process to be
572 created for the child rather than the current `node` executable. This should be
573 done with care and by default will talk over the fd represented an
574 environmental variable `NODE_CHANNEL_FD` on the child process. The input and
575 output on this fd is expected to be line delimited JSON objects.
577 [EventEmitter]: events.html#events_class_events_eventemitter