26 KiB
Child Process
Stability: 3 - Stable
Node provides a tri-directional popen(3)
facility through the
child_process
module.
It is possible to stream data through a child's stdin
, stdout
, and
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
different, and explained below.
Class: ChildProcess
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
streams of the parent process, or they may be separate stream objects
which can be piped to and from.
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:
- The process could not be spawned, or
- The process could not be killed, or
- 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()
and
ChildProcess#send()
.
Event: 'exit'
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 after the child process ends. If the process terminated
normally, code
is the final exit code of the process, otherwise null
. If
the process terminated due to receipt of a signal, signal
is the string name
of the signal, otherwise null
.
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 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 valuesendHandle
{Handle object} a Socket or Server object
Messages send by .send(message, [sendHandle])
are obtained using the
message
event.
child.stdin
- {Stream object}
A Writable Stream
that represents the child process's stdin
.
Closing this stream via end()
often causes the child process to terminate.
If the child stdio streams are shared with the parent, then this will not be set.
child.stdout
- {Stream object}
A Readable Stream
that represents the child process's stdout
.
If the child stdio streams are shared with the parent, then this will not be set.
child.stderr
- {Stream object}
A Readable Stream
that represents the child process's stderr
.
If the child stdio streams are shared with the parent, then this will not be set.
child.pid
- {Integer}
The PID of the child process.
Example:
var spawn = require('child_process').spawn,
grep = spawn('grep', ['ssh']);
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}
Send a signal to the child process. If no argument is given, the process will
be sent 'SIGTERM'
. See signal(7)
for a list of available signals.
var spawn = require('child_process').spawn,
grep = spawn('grep', ['ssh']);
grep.on('close', function (code, signal) {
console.log('child process terminated due to receipt of signal '+signal);
});
// send SIGHUP to process
grep.kill('SIGHUP');
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)
child.send(message, [sendHandle])
message
{Object}sendHandle
{Handle object}
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.
For example:
var cp = require('child_process');
var n = cp.fork(__dirname + '/sub.js');
n.on('message', function(m) {
console.log('PARENT got message:', m);
});
n.send({ hello: 'world' });
And then the child script, 'sub.js'
might look like this:
process.on('message', function(m) {
console.log('CHILD got message:', m);
});
process.send({ foo: 'bar' });
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
).
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 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()
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])
command
{String} The command to runargs
{Array} List of string argumentsoptions
{Object}cwd
{String} Current working directory of the child processstdio
{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 pairsdetached
{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
.
If omitted, args
defaults to an empty Array.
The third argument is used to specify additional options, which defaults to:
{ cwd: undefined,
env: process.env
}
cwd
allows you to specify the working directory from which the process is spawned.
Use env
to specify environment variables that will be visible to the new process.
Example of running ls -lh /usr
, capturing stdout
, stderr
, and the exit code:
var spawn = require('child_process').spawn,
ls = spawn('ls', ['-lh', '/usr']);
ls.stdout.on('data', function (data) {
console.log('stdout: ' + data);
});
ls.stderr.on('data', function (data) {
console.log('stderr: ' + data);
});
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 spawn = require('child_process').spawn,
ps = spawn('ps', ['ax']),
grep = spawn('grep', ['ssh']);
ps.stdout.on('data', function (data) {
grep.stdin.write(data);
});
ps.stderr.on('data', function (data) {
console.log('ps stderr: ' + data);
});
ps.on('close', function (code) {
if (code !== 0) {
console.log('ps process exited with code ' + code);
}
grep.stdin.end();
});
grep.stdout.on('data', function (data) {
console.log('' + data);
});
grep.stderr.on('data', function (data) {
console.log('grep stderr: ' + data);
});
grep.on('close', function (code) {
if (code !== 0) {
console.log('grep process exited with code ' + code);
}
});
Example of checking for failed exec:
var spawn = require('child_process').spawn,
child = spawn('bad_command');
child.on('error', function (err) {
console.log('Failed to start child process.');
});
Note that if spawn receives an empty options object, it will result in
spawning the process with an empty environment rather than using
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:
'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 thechild_process
object asChildProcess.stdio[fd]
. Pipes created for fds 0 - 2 are also available as ChildProcess.stdin, ChildProcess.stdout and ChildProcess.stderr, respectively.'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').'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.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 thestdio
array. Note that the stream must have an underlying descriptor (file streams do not until the'open'
event has occurred).- 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. 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.
With customFds
it was possible to hook up the new process' [stdin, stdout, stderr]
to existing streams; -1
meant that a new stream should be created.
Use at your own risk.
See also: child_process.exec()
and child_process.fork()
child_process.exec(command, [options], callback)
command
{String} The command to run, with space-separated argumentsoptions
{Object}cwd
{String} Current working directory of the child processenv
{Object} Environment key-value pairsencoding
{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 withcmd.exe
.)timeout
{Number} (Default: 0)maxBuffer
{Number} (Default:200*1024
)killSignal
{String} (Default: 'SIGTERM')
callback
{Function} called with the output when process terminateserror
{Error}stdout
{Buffer}stderr
{Buffer}
- Return: ChildProcess object
Runs a command in a shell and buffers the output.
var exec = require('child_process').exec,
child;
child = exec('cat *.js bad_file | wc -l',
function (error, stdout, stderr) {
console.log('stdout: ' + stdout);
console.log('stderr: ' + stderr);
if (error !== null) {
console.log('exec error: ' + error);
}
});
The callback gets the arguments (error, stdout, stderr)
. On success, error
will be null
. On error, error
will be an instance of Error
and error.code
will be the exit code of the child process, and error.signal
will be set to the
signal that terminated the process.
There is a second optional argument to specify several options. The default options are
{ encoding: 'utf8',
timeout: 0,
maxBuffer: 200*1024,
killSignal: 'SIGTERM',
cwd: null,
env: null }
If timeout
is greater than 0, then it will kill the child process
if it runs longer than timeout
milliseconds. The child process is killed with
killSignal
(default: 'SIGTERM'
). maxBuffer
specifies the largest
amount of data allowed on stdout or stderr - if this value is exceeded then
the child process is killed.
child_process.execFile(file, [args], [options], [callback])
file
{String} The filename of the program to runargs
{Array} List of string argumentsoptions
{Object}cwd
{String} Current working directory of the child processenv
{Object} Environment key-value pairsencoding
{String} (Default: 'utf8')timeout
{Number} (Default: 0)maxBuffer
{Number} (Default: 200*1024)killSignal
{String} (Default: 'SIGTERM')
callback
{Function} called with the output when process terminateserror
{Error}stdout
{Buffer}stderr
{Buffer}
- Return: ChildProcess object
This is similar to child_process.exec()
except it does not execute a
subshell but rather the specified file directly. This makes it slightly
leaner than child_process.exec
. It has the same options.
child_process.fork(modulePath, [args], [options])
modulePath
{String} The module to run in the childargs
{Array} List of string argumentsoptions
{Object}cwd
{String} Current working directory of the child processenv
{Object} Environment key-value pairsencoding
{String} (Default: 'utf8')execPath
{String} Executable used to create the child processexecArgv
{Array} List of string arguments passed to the executable (Default:process.execArgv
)silent
{Boolean} If true, stdin, stdout, and stderr of the child will be piped to the parent, otherwise they will be inherited from the parent, see the "pipe" and "inherit" options forspawn()
'sstdio
for more details (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. See
child.send(message, [sendHandle])
for details.
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.
child_process.spawnSync(command, [args], [options])
command
{String} The command to runargs
{Array} List of string argumentsoptions
{Object}cwd
{String} Current working directory of the child processinput
{String|Buffer} The value which will be passed as stdin to the spawned process- supplying this value will override
stdio[0]
- supplying this value will override
stdio
{Array} Child's stdio configuration.env
{Object} Environment key-value pairsuid
{Number} Sets the user identity of the process. (See setuid(2).)gid
{Number} Sets the group identity of the process. (See setgid(2).)timeout
{Number} In milliseconds the maximum amount of time the process is allowed to run. (Default: undefined)killSignal
{String} The signal value to be used when the spawned process will be killed. (Default: 'SIGTERM')maxBuffer
{Number}encoding
{String} The encoding used for all stdio inputs and outputs. (Default: 'buffer')
- return: {Object}
pid
{Number} Pid of the child processoutput
{Array} Array of results from stdio outputstdout
{Buffer|String} The contents ofoutput[1]
stderr
{Buffer|String} The contents ofoutput[2]
status
{Number} The exit code of the child processsignal
{String} The signal used to kill the child processerror
{Error} The error object if the child process failed or timedout
spawnSync
will not return until the child process has fully closed. When a
timeout has been encountered and killSignal
is sent, the method won't return
until the process has completely exited. That is to say, if the process handles
the SIGTERM
signal and doesn't exit, your process will wait until the child
process has exited.
child_process.execFileSync(command, [args], [options])
command
{String} The command to runargs
{Array} List of string argumentsoptions
{Object}cwd
{String} Current working directory of the child processinput
{String|Buffer} The value which will be passed as stdin to the spawned process- supplying this value will override
stdio[0]
- supplying this value will override
stdio
{Array} Child's stdio configuration. (Default: 'pipe')stderr
by default will be output to the parent process' stderr unlessstdio
is specified
env
{Object} Environment key-value pairsuid
{Number} Sets the user identity of the process. (See setuid(2).)gid
{Number} Sets the group identity of the process. (See setgid(2).)timeout
{Number} In milliseconds the maximum amount of time the process is allowed to run. (Default: undefined)killSignal
{String} The signal value to be used when the spawned process will be killed. (Default: 'SIGTERM')maxBuffer
{Number}encoding
{String} The encoding used for all stdio inputs and outputs. (Default: 'buffer')
- return: {Buffer|String} The stdout from the command
execFileSync
will not return until the child process has fully closed. When a
timeout has been encountered and killSignal
is sent, the method won't return
until the process has completely exited. That is to say, if the process handles
the SIGTERM
signal and doesn't exit, your process will wait until the child
process has exited.
If the process times out, or has a non-zero exit code, this method will
throw. The Error
object will contain the entire result from
child_process.spawnSync
child_process.execSync(command, [options])
command
{String} The command to runoptions
{Object}cwd
{String} Current working directory of the child processinput
{String|Buffer} The value which will be passed as stdin to the spawned process- supplying this value will override
stdio[0]
- supplying this value will override
stdio
{Array} Child's stdio configuration. (Default: 'pipe')stderr
by default will be output to the parent process' stderr unlessstdio
is specified
env
{Object} Environment key-value pairsuid
{Number} Sets the user identity of the process. (See setuid(2).)gid
{Number} Sets the group identity of the process. (See setgid(2).)timeout
{Number} In milliseconds the maximum amount of time the process is allowed to run. (Default: undefined)killSignal
{String} The signal value to be used when the spawned process will be killed. (Default: 'SIGTERM')maxBuffer
{Number}encoding
{String} The encoding used for all stdio inputs and outputs. (Default: 'buffer')
- return: {Buffer|String} The stdout from the command
execSync
will not return until the child process has fully closed. When a
timeout has been encountered and killSignal
is sent, the method won't return
until the process has completely exited. That is to say, if the process handles
the SIGTERM
signal and doesn't exit, your process will wait until the child
process has exited.
If the process times out, or has a non-zero exit code, this method will
throw. The Error
object will contain the entire result from
child_process.spawnSync