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 Please note that the `send()` method on both the parent and child are
198 synchronous - sending large chunks of data is not advised (pipes can be used
200 [`child_process.spawn`](#child_process_child_process_spawn_command_args_options)).
202 There is a special case when sending a `{cmd: 'NODE_foo'}` message. All messages
203 containing a `NODE_` prefix in its `cmd` property will not be emitted in
204 the `message` event, since they are internal messages used by node core.
205 Messages containing the prefix are emitted in the `internalMessage` event, you
206 should by all means avoid using this feature, it is subject to change without notice.
208 The `sendHandle` option to `child.send()` is for sending a TCP server or
209 socket object to another process. The child will receive the object as its
210 second argument to the `message` event.
212 Emits an `'error'` event if the message cannot be sent, for example because
213 the child process has already exited.
215 #### Example: sending server object
217 Here is an example of sending a server:
219 var child = require('child_process').fork('child.js');
221 // Open up the server object and send the handle.
222 var server = require('net').createServer();
223 server.on('connection', function (socket) {
224 socket.end('handled by parent');
226 server.listen(1337, function() {
227 child.send('server', server);
230 And the child would the receive the server object as:
232 process.on('message', function(m, server) {
233 if (m === 'server') {
234 server.on('connection', function (socket) {
235 socket.end('handled by child');
240 Note that the server is now shared between the parent and child, this means
241 that some connections will be handled by the parent and some by the child.
243 For `dgram` servers the workflow is exactly the same. Here you listen on
244 a `message` event instead of `connection` and use `server.bind` instead of
245 `server.listen`. (Currently only supported on UNIX platforms.)
247 #### Example: sending socket object
249 Here is an example of sending a socket. It will spawn two children and handle
250 connections with the remote address `74.125.127.100` as VIP by sending the
251 socket to a "special" child process. Other sockets will go to a "normal" process.
253 var normal = require('child_process').fork('child.js', ['normal']);
254 var special = require('child_process').fork('child.js', ['special']);
256 // Open up the server and send sockets to child
257 var server = require('net').createServer();
258 server.on('connection', function (socket) {
261 if (socket.remoteAddress === '74.125.127.100') {
262 special.send('socket', socket);
265 // just the usual dudes
266 normal.send('socket', socket);
270 The `child.js` could look like this:
272 process.on('message', function(m, socket) {
273 if (m === 'socket') {
274 socket.end('You were handled as a ' + process.argv[2] + ' person');
278 Note that once a single socket has been sent to a child the parent can no
279 longer keep track of when the socket is destroyed. To indicate this condition
280 the `.connections` property becomes `null`.
281 It is also recommended not to use `.maxConnections` in this condition.
283 ### child.disconnect()
285 Close the IPC channel between parent and child, allowing the child to exit
286 gracefully once there are no other connections keeping it alive. After calling
287 this method the `.connected` flag will be set to `false` in both the parent and
288 child, and it is no longer possible to send messages.
290 The 'disconnect' event will be emitted when there are no messages in the process
291 of being received, most likely immediately.
293 Note that you can also call `process.disconnect()` in the child process.
295 ## child_process.spawn(command, [args], [options])
297 * `command` {String} The command to run
298 * `args` {Array} List of string arguments
300 * `cwd` {String} Current working directory of the child process
301 * `stdio` {Array|String} Child's stdio configuration. (See below)
302 * `customFds` {Array} **Deprecated** File descriptors for the child to use
303 for stdio. (See below)
304 * `env` {Object} Environment key-value pairs
305 * `detached` {Boolean} The child will be a process group leader. (See below)
306 * `uid` {Number} Sets the user identity of the process. (See setuid(2).)
307 * `gid` {Number} Sets the group identity of the process. (See setgid(2).)
308 * return: {ChildProcess object}
310 Launches a new process with the given `command`, with command line arguments in `args`.
311 If omitted, `args` defaults to an empty Array.
313 The third argument is used to specify additional options, which defaults to:
319 `cwd` allows you to specify the working directory from which the process is spawned.
320 Use `env` to specify environment variables that will be visible to the new process.
322 Example of running `ls -lh /usr`, capturing `stdout`, `stderr`, and the exit code:
324 var spawn = require('child_process').spawn,
325 ls = spawn('ls', ['-lh', '/usr']);
327 ls.stdout.on('data', function (data) {
328 console.log('stdout: ' + data);
331 ls.stderr.on('data', function (data) {
332 console.log('stderr: ' + data);
335 ls.on('close', function (code) {
336 console.log('child process exited with code ' + code);
340 Example: A very elaborate way to run 'ps ax | grep ssh'
342 var spawn = require('child_process').spawn,
343 ps = spawn('ps', ['ax']),
344 grep = spawn('grep', ['ssh']);
346 ps.stdout.on('data', function (data) {
347 grep.stdin.write(data);
350 ps.stderr.on('data', function (data) {
351 console.log('ps stderr: ' + data);
354 ps.on('close', function (code) {
356 console.log('ps process exited with code ' + code);
361 grep.stdout.on('data', function (data) {
362 console.log('' + data);
365 grep.stderr.on('data', function (data) {
366 console.log('grep stderr: ' + data);
369 grep.on('close', function (code) {
371 console.log('grep process exited with code ' + code);
376 Example of checking for failed exec:
378 var spawn = require('child_process').spawn,
379 child = spawn('bad_command');
381 child.stderr.setEncoding('utf8');
382 child.stderr.on('data', function (data) {
383 if (/^execvp\(\)/.test(data)) {
384 console.log('Failed to start child process.');
388 Note that if spawn receives an empty options object, it will result in
389 spawning the process with an empty environment rather than using
390 `process.env`. This due to backwards compatibility issues with a deprecated
393 The 'stdio' option to `child_process.spawn()` is an array where each
394 index corresponds to a fd in the child. The value is one of the following:
396 1. `'pipe'` - Create a pipe between the child process and the parent process.
397 The parent end of the pipe is exposed to the parent as a property on the
398 `child_process` object as `ChildProcess.stdio[fd]`. Pipes created for
399 fds 0 - 2 are also available as ChildProcess.stdin, ChildProcess.stdout
400 and ChildProcess.stderr, respectively.
401 2. `'ipc'` - Create an IPC channel for passing messages/file descriptors
402 between parent and child. A ChildProcess may have at most *one* IPC stdio
403 file descriptor. Setting this option enables the ChildProcess.send() method.
404 If the child writes JSON messages to this file descriptor, then this will
405 trigger ChildProcess.on('message'). If the child is a Node.js program, then
406 the presence of an IPC channel will enable process.send() and
407 process.on('message').
408 3. `'ignore'` - Do not set this file descriptor in the child. Note that Node
409 will always open fd 0 - 2 for the processes it spawns. When any of these is
410 ignored node will open `/dev/null` and attach it to the child's fd.
411 4. `Stream` object - Share a readable or writable stream that refers to a tty,
412 file, socket, or a pipe with the child process. The stream's underlying
413 file descriptor is duplicated in the child process to the fd that
414 corresponds to the index in the `stdio` array.
415 5. Positive integer - The integer value is interpreted as a file descriptor
416 that is is currently open in the parent process. It is shared with the child
417 process, similar to how `Stream` objects can be shared.
418 6. `null`, `undefined` - Use default value. For stdio fds 0, 1 and 2 (in other
419 words, stdin, stdout, and stderr) a pipe is created. For fd 3 and up, the
420 default is `'ignore'`.
422 As a shorthand, the `stdio` argument may also be one of the following
423 strings, rather than an array:
425 * `ignore` - `['ignore', 'ignore', 'ignore']`
426 * `pipe` - `['pipe', 'pipe', 'pipe']`
427 * `inherit` - `[process.stdin, process.stdout, process.stderr]` or `[0,1,2]`
431 var spawn = require('child_process').spawn;
433 // Child will use parent's stdios
434 spawn('prg', [], { stdio: 'inherit' });
436 // Spawn child sharing only stderr
437 spawn('prg', [], { stdio: ['pipe', 'pipe', process.stderr] });
439 // Open an extra fd=4, to interact with programs present a
440 // startd-style interface.
441 spawn('prg', [], { stdio: ['pipe', null, null, null, 'pipe'] });
443 If the `detached` option is set, the child process will be made the leader of a
444 new process group. This makes it possible for the child to continue running
445 after the parent exits.
447 By default, the parent will wait for the detached child to exit. To prevent
448 the parent from waiting for a given `child`, use the `child.unref()` method,
449 and the parent's event loop will not include the child in its reference count.
451 Example of detaching a long-running process and redirecting its output to a
454 var fs = require('fs'),
455 spawn = require('child_process').spawn,
456 out = fs.openSync('./out.log', 'a'),
457 err = fs.openSync('./out.log', 'a');
459 var child = spawn('prg', [], {
461 stdio: [ 'ignore', out, err ]
466 When using the `detached` option to start a long-running process, the process
467 will not stay running in the background unless it is provided with a `stdio`
468 configuration that is not connected to the parent. If the parent's `stdio` is
469 inherited, the child will remain attached to the controlling terminal.
471 There is a deprecated option called `customFds` which allows one to specify
472 specific file descriptors for the stdio of the child process. This API was
473 not portable to all platforms and therefore removed.
474 With `customFds` it was possible to hook up the new process' `[stdin, stdout,
475 stderr]` to existing streams; `-1` meant that a new stream should be created.
476 Use at your own risk.
478 See also: `child_process.exec()` and `child_process.fork()`
480 ## child_process.exec(command, [options], callback)
482 * `command` {String} The command to run, with space-separated arguments
484 * `cwd` {String} Current working directory of the child process
485 * `env` {Object} Environment key-value pairs
486 * `encoding` {String} (Default: 'utf8')
487 * `shell` {String} Shell to execute the command with
488 (Default: '/bin/sh' on UNIX, 'cmd.exe' on Windows, The shell should
489 understand the `-c` switch on UNIX or `/s /c` on Windows. On Windows,
490 command line parsing should be compatible with `cmd.exe`.)
491 * `timeout` {Number} (Default: 0)
492 * `maxBuffer` {Number} (Default: `200*1024`)
493 * `killSignal` {String} (Default: 'SIGTERM')
494 * `callback` {Function} called with the output when process terminates
498 * Return: ChildProcess object
500 Runs a command in a shell and buffers the output.
502 var exec = require('child_process').exec,
505 child = exec('cat *.js bad_file | wc -l',
506 function (error, stdout, stderr) {
507 console.log('stdout: ' + stdout);
508 console.log('stderr: ' + stderr);
509 if (error !== null) {
510 console.log('exec error: ' + error);
514 The callback gets the arguments `(error, stdout, stderr)`. On success, `error`
515 will be `null`. On error, `error` will be an instance of `Error` and `err.code`
516 will be the exit code of the child process, and `err.signal` will be set to the
517 signal that terminated the process.
519 There is a second optional argument to specify several options. The
525 killSignal: 'SIGTERM',
529 If `timeout` is greater than 0, then it will kill the child process
530 if it runs longer than `timeout` milliseconds. The child process is killed with
531 `killSignal` (default: `'SIGTERM'`). `maxBuffer` specifies the largest
532 amount of data allowed on stdout or stderr - if this value is exceeded then
533 the child process is killed.
536 ## child_process.execFile(file, args, options, callback)
538 * `file` {String} The filename of the program to run
539 * `args` {Array} List of string arguments
541 * `cwd` {String} Current working directory of the child process
542 * `env` {Object} Environment key-value pairs
543 * `encoding` {String} (Default: 'utf8')
544 * `timeout` {Number} (Default: 0)
545 * `maxBuffer` {Number} (Default: 200\*1024)
546 * `killSignal` {String} (Default: 'SIGTERM')
547 * `callback` {Function} called with the output when process terminates
551 * Return: ChildProcess object
553 This is similar to `child_process.exec()` except it does not execute a
554 subshell but rather the specified file directly. This makes it slightly
555 leaner than `child_process.exec`. It has the same options.
558 ## child_process.fork(modulePath, [args], [options])
560 * `modulePath` {String} The module to run in the child
561 * `args` {Array} List of string arguments
563 * `cwd` {String} Current working directory of the child process
564 * `env` {Object} Environment key-value pairs
565 * `encoding` {String} (Default: 'utf8')
566 * `execPath` {String} Executable used to create the child process
567 * `execArgv` {Array} List of string arguments passed to the executable
568 (Default: `process.execArgv`)
569 * `silent` {Boolean} If true, prevent stdout and stderr in the spawned node
570 process from being associated with the parent's (default is false)
571 * Return: ChildProcess object
573 This is a special case of the `spawn()` functionality for spawning Node
574 processes. In addition to having all the methods in a normal ChildProcess
575 instance, the returned object has a communication channel built-in. See
576 `child.send(message, [sendHandle])` for details.
578 These child Nodes are still whole new instances of V8. Assume at least 30ms
579 startup and 10mb memory for each new Node. That is, you cannot create many
582 The `execPath` property in the `options` object allows for a process to be
583 created for the child rather than the current `node` executable. This should be
584 done with care and by default will talk over the fd represented an
585 environmental variable `NODE_CHANNEL_FD` on the child process. The input and
586 output on this fd is expected to be line delimited JSON objects.
588 [EventEmitter]: events.html#events_class_events_eventemitter