`child_process` module.
It is possible to stream data through a child's `stdin`, `stdout`, and
-`stderr` in a fully non-blocking way.
+`stderr` in a fully non-blocking way. (Note that some programs use
+line-buffered I/O internally. That doesn't affect node.js but it means
+data you send to the child process is not immediately consumed.)
To create a child process use `require('child_process').spawn()` or
`require('child_process').fork()`. The semantics of each are slightly
## Class: ChildProcess
-`ChildProcess` is an `EventEmitter`.
+`ChildProcess` is an [EventEmitter][].
Child processes always have three streams associated with them. `child.stdin`,
`child.stdout`, and `child.stderr`. These may be shared with the stdio
The ChildProcess class is not intended to be used directly. Use the
`spawn()` or `fork()` methods to create a Child Process instance.
+### Event: 'error'
+
+* `err` {Error Object} the error.
+
+Emitted when:
+
+1. The process could not be spawned, or
+2. The process could not be killed, or
+3. Sending a message to the child process failed for whatever reason.
+
+Note that the `exit`-event may or may not fire after an error has occured. If
+you are listening on both events to fire a function, remember to guard against
+calling your function twice.
+
+See also [`ChildProcess#kill()`](#child_process_child_kill_signal) and
+[`ChildProcess#send()`](#child_process_child_send_message_sendhandle).
+
### Event: 'exit'
* `code` {Number} the exit code, if it exited normally.
Note that the child process stdio streams might still be open.
+Also, note that node establishes signal handlers for `'SIGINT'` and `'SIGTERM`',
+so it will not terminate due to receipt of those signals, it will exit.
+
See `waitpid(2)`.
### Event: 'close'
+* `code` {Number} the exit code, if it exited normally.
+* `signal` {String} the signal passed to kill the child process, if it
+ was killed by the parent.
+
This event is emitted when the stdio streams of a child process have all
terminated. This is distinct from 'exit', since multiple processes
might share the same stdio streams.
### Event: 'disconnect'
-This event is emitted after using the `.disconnect()` method in the parent or
-in the child. After disconnecting it is no longer possible to send messages.
-An alternative way to check if you can send messages is to see if the
-`child.connected` property is `true`.
+This event is emitted after calling the `.disconnect()` method in the parent
+or in the child. After disconnecting it is no longer possible to send messages,
+and the `.connected` property is false.
### Event: 'message'
* `message` {Object} a parsed JSON object or primitive value
-* `sendHandle` {Handle object} a handle object
+* `sendHandle` {Handle object} a Socket or Server object
Messages send by `.send(message, [sendHandle])` are obtained using the
`message` event.
console.log('Spawned child pid: ' + grep.pid);
grep.stdin.end();
+### child.connected
+
+* {Boolean} Set to false after `.disconnect' is called
+
+If `.connected` is false, it is no longer possible to send messages.
+
### child.kill([signal])
* `signal` {String}
var spawn = require('child_process').spawn,
grep = spawn('grep', ['ssh']);
- grep.on('exit', function (code, signal) {
+ grep.on('close', function (code, signal) {
console.log('child process terminated due to receipt of signal '+signal);
});
// send SIGHUP to process
grep.kill('SIGHUP');
-Note that while the function is called `kill`, the signal delivered to the child
-process may not actually kill it. `kill` really just sends a signal to a process.
+May emit an `'error'` event when the signal cannot be delivered. Sending a
+signal to a child process that has already exited is not an error but may
+have unforeseen consequences: if the PID (the process ID) has been reassigned
+to another process, the signal will be delivered to that process instead.
+What happens next is anyone's guess.
+
+Note that while the function is called `kill`, the signal delivered to the
+child process may not actually kill it. `kill` really just sends a signal
+to a process.
See `kill(2)`
* `message` {Object}
* `sendHandle` {Handle object}
-When useing `child_process.fork()` an you can write to the child using
+When using `child_process.fork()` you can write to the child using
`child.send(message, [sendHandle])` and messages are received by
a `'message'` event on the child.
In the child the `process` object will have a `send()` method, and `process`
will emit objects each time it receives a message on its channel.
+Please note that the `send()` method on both the parent and child are
+synchronous - sending large chunks of data is not advised (pipes can be used
+instead, see
+[`child_process.spawn`](#child_process_child_process_spawn_command_args_options)).
+
There is a special case when sending a `{cmd: 'NODE_foo'}` message. All messages
containing a `NODE_` prefix in its `cmd` property will not be emitted in
the `message` event, since they are internal messages used by node core.
Messages containing the prefix are emitted in the `internalMessage` event, you
should by all means avoid using this feature, it is subject to change without notice.
-The `sendHandle` option to `child.send()` is for sending a handle object to
-another process. The child will receive the object as its second argument to
-the `message` event.
+The `sendHandle` option to `child.send()` is for sending a TCP server or
+socket object to another process. The child will receive the object as its
+second argument to the `message` event.
+
+Emits an `'error'` event if the message cannot be sent, for example because
+the child process has already exited.
+
+#### Example: sending server object
+
+Here is an example of sending a server:
+
+ var child = require('child_process').fork('child.js');
+
+ // Open up the server object and send the handle.
+ var server = require('net').createServer();
+ server.on('connection', function (socket) {
+ socket.end('handled by parent');
+ });
+ server.listen(1337, function() {
+ child.send('server', server);
+ });
+
+And the child would the receive the server object as:
+
+ process.on('message', function(m, server) {
+ if (m === 'server') {
+ server.on('connection', function (socket) {
+ socket.end('handled by child');
+ });
+ }
+ });
+
+Note that the server is now shared between the parent and child, this means
+that some connections will be handled by the parent and some by the child.
+
+For `dgram` servers the workflow is exactly the same. Here you listen on
+a `message` event instead of `connection` and use `server.bind` instead of
+`server.listen`. (Currently only supported on UNIX platforms.)
+
+#### Example: sending socket object
+
+Here is an example of sending a socket. It will spawn two children and handle
+connections with the remote address `74.125.127.100` as VIP by sending the
+socket to a "special" child process. Other sockets will go to a "normal" process.
+
+ var normal = require('child_process').fork('child.js', ['normal']);
+ var special = require('child_process').fork('child.js', ['special']);
+
+ // Open up the server and send sockets to child
+ var server = require('net').createServer();
+ server.on('connection', function (socket) {
+
+ // if this is a VIP
+ if (socket.remoteAddress === '74.125.127.100') {
+ special.send('socket', socket);
+ return;
+ }
+ // just the usual dudes
+ normal.send('socket', socket);
+ });
+ server.listen(1337);
+
+The `child.js` could look like this:
+
+ process.on('message', function(m, socket) {
+ if (m === 'socket') {
+ socket.end('You were handled as a ' + process.argv[2] + ' person');
+ }
+ });
+
+Note that once a single socket has been sent to a child the parent can no
+longer keep track of when the socket is destroyed. To indicate this condition
+the `.connections` property becomes `null`.
+It is also recommended not to use `.maxConnections` in this condition.
### child.disconnect()
-To close the IPC connection between parent and child use the
-`child.disconnect()` method. This allows the child to exit gracefully since
-there is no IPC channel keeping it alive. When calling this method the
-`disconnect` event will be emitted in both parent and child, and the
-`connected` flag will be set to `false`. Please note that you can also call
-`process.disconnect()` in the child process.
+Close the IPC channel between parent and child, allowing the child to exit
+gracefully once there are no other connections keeping it alive. After calling
+this method the `.connected` flag will be set to `false` in both the parent and
+child, and it is no longer possible to send messages.
+
+The 'disconnect' event will be emitted when there are no messages in the process
+of being received, most likely immediately.
+
+Note that you can also call `process.disconnect()` in the child process.
## child_process.spawn(command, [args], [options])
* `args` {Array} List of string arguments
* `options` {Object}
* `cwd` {String} Current working directory of the child process
+ * `stdio` {Array|String} Child's stdio configuration. (See below)
* `customFds` {Array} **Deprecated** File descriptors for the child to use
for stdio. (See below)
* `env` {Object} Environment key-value pairs
- * `setsid` {Boolean}
+ * `detached` {Boolean} The child will be a process group leader. (See below)
+ * `uid` {Number} Sets the user identity of the process. (See setuid(2).)
+ * `gid` {Number} Sets the group identity of the process. (See setgid(2).)
* return: {ChildProcess object}
Launches a new process with the given `command`, with command line arguments in `args`.
Example of running `ls -lh /usr`, capturing `stdout`, `stderr`, and the exit code:
- var util = require('util'),
- spawn = require('child_process').spawn,
+ var spawn = require('child_process').spawn,
ls = spawn('ls', ['-lh', '/usr']);
ls.stdout.on('data', function (data) {
console.log('stderr: ' + data);
});
- ls.on('exit', function (code) {
+ ls.on('close', function (code) {
console.log('child process exited with code ' + code);
});
Example: A very elaborate way to run 'ps ax | grep ssh'
- var util = require('util'),
- spawn = require('child_process').spawn,
+ var spawn = require('child_process').spawn,
ps = spawn('ps', ['ax']),
grep = spawn('grep', ['ssh']);
console.log('ps stderr: ' + data);
});
- ps.on('exit', function (code) {
+ ps.on('close', function (code) {
if (code !== 0) {
console.log('ps process exited with code ' + code);
}
});
grep.stdout.on('data', function (data) {
- console.log(data);
+ console.log('' + data);
});
grep.stderr.on('data', function (data) {
console.log('grep stderr: ' + data);
});
- grep.on('exit', function (code) {
+ grep.on('close', function (code) {
if (code !== 0) {
console.log('grep process exited with code ' + code);
}
`process.env`. This due to backwards compatibility issues with a deprecated
API.
+The 'stdio' option to `child_process.spawn()` is an array where each
+index corresponds to a fd in the child. The value is one of the following:
+
+1. `'pipe'` - Create a pipe between the child process and the parent process.
+ The parent end of the pipe is exposed to the parent as a property on the
+ `child_process` object as `ChildProcess.stdio[fd]`. Pipes created for
+ fds 0 - 2 are also available as ChildProcess.stdin, ChildProcess.stdout
+ and ChildProcess.stderr, respectively.
+2. `'ipc'` - Create an IPC channel for passing messages/file descriptors
+ between parent and child. A ChildProcess may have at most *one* IPC stdio
+ file descriptor. Setting this option enables the ChildProcess.send() method.
+ If the child writes JSON messages to this file descriptor, then this will
+ trigger ChildProcess.on('message'). If the child is a Node.js program, then
+ the presence of an IPC channel will enable process.send() and
+ process.on('message').
+3. `'ignore'` - Do not set this file descriptor in the child. Note that Node
+ will always open fd 0 - 2 for the processes it spawns. When any of these is
+ ignored node will open `/dev/null` and attach it to the child's fd.
+4. `Stream` object - Share a readable or writable stream that refers to a tty,
+ file, socket, or a pipe with the child process. The stream's underlying
+ file descriptor is duplicated in the child process to the fd that
+ corresponds to the index in the `stdio` array.
+5. Positive integer - The integer value is interpreted as a file descriptor
+ that is is currently open in the parent process. It is shared with the child
+ process, similar to how `Stream` objects can be shared.
+6. `null`, `undefined` - Use default value. For stdio fds 0, 1 and 2 (in other
+ words, stdin, stdout, and stderr) a pipe is created. For fd 3 and up, the
+ default is `'ignore'`.
+
+As a shorthand, the `stdio` argument may also be one of the following
+strings, rather than an array:
+
+* `ignore` - `['ignore', 'ignore', 'ignore']`
+* `pipe` - `['pipe', 'pipe', 'pipe']`
+* `inherit` - `[process.stdin, process.stdout, process.stderr]` or `[0,1,2]`
+
+Example:
+
+ var spawn = require('child_process').spawn;
+
+ // Child will use parent's stdios
+ spawn('prg', [], { stdio: 'inherit' });
+
+ // Spawn child sharing only stderr
+ spawn('prg', [], { stdio: ['pipe', 'pipe', process.stderr] });
+
+ // Open an extra fd=4, to interact with programs present a
+ // startd-style interface.
+ spawn('prg', [], { stdio: ['pipe', null, null, null, 'pipe'] });
+
+If the `detached` option is set, the child process will be made the leader of a
+new process group. This makes it possible for the child to continue running
+after the parent exits.
+
+By default, the parent will wait for the detached child to exit. To prevent
+the parent from waiting for a given `child`, use the `child.unref()` method,
+and the parent's event loop will not include the child in its reference count.
+
+Example of detaching a long-running process and redirecting its output to a
+file:
+
+ var fs = require('fs'),
+ spawn = require('child_process').spawn,
+ out = fs.openSync('./out.log', 'a'),
+ err = fs.openSync('./out.log', 'a');
+
+ var child = spawn('prg', [], {
+ detached: true,
+ stdio: [ 'ignore', out, err ]
+ });
+
+ child.unref();
+
+When using the `detached` option to start a long-running process, the process
+will not stay running in the background unless it is provided with a `stdio`
+configuration that is not connected to the parent. If the parent's `stdio` is
+inherited, the child will remain attached to the controlling terminal.
+
There is a deprecated option called `customFds` which allows one to specify
specific file descriptors for the stdio of the child process. This API was
not portable to all platforms and therefore removed.
stderr]` to existing streams; `-1` meant that a new stream should be created.
Use at your own risk.
-There are several internal options. In particular `stdinStream`,
-`stdoutStream`, `stderrStream`. They are for INTERNAL USE ONLY. As with all
-undocumented APIs in Node, they should not be used.
-
See also: `child_process.exec()` and `child_process.fork()`
## child_process.exec(command, [options], callback)
* `command` {String} The command to run, with space-separated arguments
* `options` {Object}
* `cwd` {String} Current working directory of the child process
- * `customFds` {Array} **Deprecated** File descriptors for the child to use
- for stdio. (See below)
* `env` {Object} Environment key-value pairs
- * `setsid` {Boolean}
* `encoding` {String} (Default: 'utf8')
+ * `shell` {String} Shell to execute the command with
+ (Default: '/bin/sh' on UNIX, 'cmd.exe' on Windows, The shell should
+ understand the `-c` switch on UNIX or `/s /c` on Windows. On Windows,
+ command line parsing should be compatible with `cmd.exe`.)
* `timeout` {Number} (Default: 0)
- * `maxBuffer` {Number} (Default: 200*1024)
+ * `maxBuffer` {Number} (Default: `200*1024`)
* `killSignal` {String} (Default: 'SIGTERM')
* `callback` {Function} called with the output when process terminates
* `error` {Error}
Runs a command in a shell and buffers the output.
- var util = require('util'),
- exec = require('child_process').exec,
+ var exec = require('child_process').exec,
child;
child = exec('cat *.js bad_file | wc -l',
* `args` {Array} List of string arguments
* `options` {Object}
* `cwd` {String} Current working directory of the child process
- * `customFds` {Array} **Deprecated** File descriptors for the child to use
- for stdio. (See below)
* `env` {Object} Environment key-value pairs
- * `setsid` {Boolean}
* `encoding` {String} (Default: 'utf8')
* `timeout` {Number} (Default: 0)
- * `maxBuffer` {Number} (Default: 200*1024)
+ * `maxBuffer` {Number} (Default: 200\*1024)
* `killSignal` {String} (Default: 'SIGTERM')
* `callback` {Function} called with the output when process terminates
* `error` {Error}
* `args` {Array} List of string arguments
* `options` {Object}
* `cwd` {String} Current working directory of the child process
- * `customFds` {Array} **Deprecated** File descriptors for the child to use
- for stdio. (See below)
* `env` {Object} Environment key-value pairs
- * `setsid` {Boolean}
* `encoding` {String} (Default: 'utf8')
- * `timeout` {Number} (Default: 0)
+ * `execPath` {String} Executable used to create the child process
+ * `execArgv` {Array} List of string arguments passed to the executable
+ (Default: `process.execArgv`)
+ * `silent` {Boolean} If true, prevent stdout and stderr in the spawned node
+ process from being associated with the parent's (default is false)
* Return: ChildProcess object
This is a special case of the `spawn()` functionality for spawning Node
processes. In addition to having all the methods in a normal ChildProcess
-instance, the returned object has a communication channel built-in. Se
+instance, the returned object has a communication channel built-in. See
`child.send(message, [sendHandle])` for details.
-By default the spawned Node process will have the stdout, stderr associated
-with the parent's. To change this behavior set the `silent` property in the
-`options` object to `true`.
-
These child Nodes are still whole new instances of V8. Assume at least 30ms
startup and 10mb memory for each new Node. That is, you cannot create many
thousands of them.
+
+The `execPath` property in the `options` object allows for a process to be
+created for the child rather than the current `node` executable. This should be
+done with care and by default will talk over the fd represented an
+environmental variable `NODE_CHANNEL_FD` on the child process. The input and
+output on this fd is expected to be line delimited JSON objects.
+
+[EventEmitter]: events.html#events_class_events_eventemitter
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