Node.js v4.3.1-rc.3 Documentation
Table of Contents
- Child Process
- Class: ChildProcess
- Asynchronous Process Creation
- Synchronous Process Creation
Child Process#
Stability: 2 - Stable
Node.js 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 may not be immediately consumed.)
To create a child process, use require('child_process').spawn()
or
require('child_process').fork()
. The semantics of each are slightly
different as explained below.
For scripting purposes you may find the synchronous counterparts more convenient.
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()
, exec()
, execFile()
, or fork()
methods to create
an instance of ChildProcess
.
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: '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.
Note that the 'exit'
event may or may not fire after an error has occurred.
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.js establishes signal handlers for SIGINT
and
SIGTERM
. It will not terminate due to receipt of those signals. It will exit.
See waitpid(2)
.
Event: 'message'#
message
Object a parsed JSON object or primitive value.sendHandle
Handle object anet.Socket
ornet.Server
object, or undefined.
Messages sent by .send(message, [sendHandle])
are obtained using the
'message'
event.
child.connected#
- Boolean Set to false after
.disconnect
is called
If .connected
is false, it is no longer possible to send messages.
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 when the
child process has any open IPC channels with the parent (i.e fork()
).
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.
const spawn = require('child_process').spawn;
const grep = spawn('grep', ['ssh']);
grep.on('close', (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. Specifically, if the process identifier (PID) 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.pid#
- Integer
The process identifier (PID) of the child process.
Example:
const spawn = require('child_process').spawn;
const grep = spawn('grep', ['ssh']);
console.log(`Spawned child pid: ${grep.pid}`);
grep.stdin.end();
child.send(message[, sendHandle][, callback])#
message
ObjectsendHandle
Handle objectcallback
Function- Return: Boolean
When using child_process.fork()
you can write to the child using
child.send(message[, sendHandle][, callback])
and messages are received by
a 'message'
event on the child.
For example:
const cp = require('child_process');
const n = cp.fork(`${__dirname}/sub.js`);
n.on('message', (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', (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.
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.js core.
Messages containing the prefix are emitted in the 'internalMessage'
event.
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.
The callback
option is a function that is invoked after the message is
sent but before the target may have received it. It is called with a single
argument: null
on success, or an Error
object on failure.
child.send()
emits an 'error'
event if no callback was given and the message
cannot be sent, for example because the child process has already exited.
Returns true
under normal circumstances or false
when the backlog of
unsent messages exceeds a threshold that makes it unwise to send more.
Use the callback mechanism to implement flow control.
Example: sending server object#
Here is an example of sending a server:
const child = require('child_process').fork('child.js');
// Open up the server object and send the handle.
const server = require('net').createServer();
server.on('connection', (socket) => {
socket.end('handled by parent');
});
server.listen(1337, () => {
child.send('server', server);
});
And the child would then receive the server object as:
process.on('message', (m, server) => {
if (m === 'server') {
server.on('connection', (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.
const normal = require('child_process').fork('child.js', ['normal']);
const special = require('child_process').fork('child.js', ['special']);
// Open up the server and send sockets to child
const server = require('net').createServer();
server.on('connection', (socket) => {
// if this is a VIP
if (socket.remoteAddress === '74.125.127.100') {
special.send('socket', socket);
return;
}
// just the usual...
normal.send('socket', socket);
});
server.listen(1337);
The child.js
could look like this:
process.on('message', (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.stderr#
- Stream object
A Readable Stream
that represents the child process's stderr
.
If the child was not spawned with stdio[2]
set to 'pipe'
, then this will
not be set.
child.stderr
is shorthand for child.stdio[2]
. Both properties will refer
to the same object, or null.
child.stdin#
- Stream object
A Writable Stream
that represents the child process's stdin
.
If the child is waiting to read all its input, it will not continue until this
stream has been closed via end()
.
If the child was not spawned with stdio[0]
set to 'pipe'
, then this will
not be set.
child.stdin
is shorthand for child.stdio[0]
. Both properties will refer
to the same object, or null.
child.stdio#
- Array
A sparse array of pipes to the child process, corresponding with positions in
the stdio
option to spawn()
that have been set to 'pipe'
.
Note that streams 0-2 are also available as ChildProcess.stdin,
ChildProcess.stdout, and ChildProcess.stderr, respectively.
In the following example, only the child's fd 1
is setup as a pipe, so only
the parent's child.stdio[1]
is a stream, all other values in the array are
null
.
const assert = require('assert');
const fs = require('fs');
const child_process = require('child_process');
const child = child_process.spawn('ls', {
stdio: [
0, // use parents stdin for child
'pipe', // pipe child's stdout to parent
fs.openSync('err.out', 'w') // direct child's stderr to a file
]
});
assert.equal(child.stdio[0], null);
assert.equal(child.stdio[0], child.stdin);
assert(child.stdout);
assert.equal(child.stdio[1], child.stdout);
assert.equal(child.stdio[2], null);
assert.equal(child.stdio[2], child.stderr);
child.stdout#
- Stream object
A Readable Stream
that represents the child process's stdout
.
If the child was not spawned with stdio[1]
set to 'pipe'
, then this will
not be set.
child.stdout
is shorthand for child.stdio[1]
. Both properties will refer
to the same object, or null.
Asynchronous Process Creation#
These methods follow the common async programming patterns (accepting a callback or returning an EventEmitter).
child_process.exec(command[, options], callback)#
command
String The command to run, with space-separated argumentsoptions
Objectcwd
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 largest amount of data (in bytes) allowed on stdout or stderr - if exceeded child process is killed (Default:200*1024
)killSignal
String (Default: 'SIGTERM')uid
Number Sets the user identity of the process. (See setuid(2).)gid
Number Sets the group identity of the process. (See setgid(2).)
callback
Function called with the output when process terminateserror
Errorstdout
Bufferstderr
Buffer
- Return: ChildProcess object
Runs a command in a shell and buffers the output.
const exec = require('child_process').exec;
const child = exec('cat *.js bad_file | wc -l',
(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 (in bytes) allowed on stdout or stderr - if this value is
exceeded then the child process is killed.
Note: Unlike the exec()
POSIX system call, child_process.exec()
does not replace
the existing process and uses a shell to execute the command.
child_process.execFile(file[, args][, options][, callback])#
file
String The filename of the program to runargs
Array List of string argumentsoptions
Objectcwd
String Current working directory of the child processenv
Object Environment key-value pairsencoding
String (Default: 'utf8')timeout
Number (Default: 0)maxBuffer
Number largest amount of data (in bytes) allowed on stdout or stderr - if exceeded child process is killed (Default: 200*1024)killSignal
String (Default: 'SIGTERM')uid
Number Sets the user identity of the process. (See setuid(2).)gid
Number Sets the group identity of the process. (See setgid(2).)
callback
Function called with the output when process terminateserror
Errorstdout
Bufferstderr
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
Objectcwd
String Current working directory of the child processenv
Object Environment key-value pairsexecPath
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)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
This is a special case of the child_process.spawn()
functionality for
spawning Node.js processes. In addition to having all the methods in a normal
ChildProcess instance, the returned object has a communication channel built-in.
See ChildProcess#send()
for details.
These child Node.js processes are still whole new instances of V8. Assume at least 30ms startup and 10mb memory for each new Node.js. 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.
Note: Unlike the fork()
POSIX system call, child_process.fork()
does not clone the
current process.
child_process.spawn(command[, args][, options])#
command
String The command to runargs
Array List of string argumentsoptions
Objectcwd
String Current working directory of the child processenv
Object Environment key-value pairsstdio
Array|String Child's stdio configuration. (See below)detached
Boolean Prepare child to run independently of its parent process. Specific behavior depends on the platform, 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, with these defaults:
{ cwd: undefined,
env: process.env
}
Use cwd
to specify the working directory from which the process is spawned.
If not given, the default is to inherit the current working directory.
Use env
to specify environment variables that will be visible to the new
process, the default is process.env
.
Example of running ls -lh /usr
, capturing stdout
, stderr
, and the exit code:
const spawn = require('child_process').spawn;
const ls = spawn('ls', ['-lh', '/usr']);
ls.stdout.on('data', (data) => {
console.log(`stdout: ${data}`);
});
ls.stderr.on('data', (data) => {
console.log(`stderr: ${data}`);
});
ls.on('close', (code) => {
console.log(`child process exited with code ${code}`);
});
Example: A very elaborate way to run 'ps ax | grep ssh'
const spawn = require('child_process').spawn;
const ps = spawn('ps', ['ax']);
const grep = spawn('grep', ['ssh']);
ps.stdout.on('data', (data) => {
grep.stdin.write(data);
});
ps.stderr.on('data', (data) => {
console.log(`ps stderr: ${data}`);
});
ps.on('close', (code) => {
if (code !== 0) {
console.log(`ps process exited with code ${code}`);
}
grep.stdin.end();
});
grep.stdout.on('data', (data) => {
console.log(`${data}`);
});
grep.stderr.on('data', (data) => {
console.log(`grep stderr: ${data}`);
});
grep.on('close', (code) => {
if (code !== 0) {
console.log(`grep process exited with code ${code}`);
}
});
Example of checking for failed exec:
const spawn = require('child_process').spawn;
const child = spawn('bad_command');
child.on('error', (err) => {
console.log('Failed to start child process.');
});
options.detached#
On Windows, this makes it possible for the child to continue running after the parent exits. The child will have a new console window (this cannot be disabled).
On non-Windows, if the detached
option is set, the child process will be made
the leader of a new process group and session. Note that child processes may
continue running after the parent exits whether they are detached or not. See
setsid(2)
for more information.
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:
const fs = require('fs');
const spawn = require('child_process').spawn;
const out = fs.openSync('./out.log', 'a');
const err = fs.openSync('./out.log', 'a');
const 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 after the parent exits 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.
options.stdio#
As a shorthand, the stdio
argument may be one of the following strings:
'pipe'
-['pipe', 'pipe', 'pipe']
, this is the default value'ignore'
-['ignore', 'ignore', 'ignore']
'inherit'
-[process.stdin, process.stdout, process.stderr]
or[0,1,2]
Otherwise, 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 an 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.js will always open fd 0 - 2 for the processes it spawns. When any of these is ignored Node.js 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'
.
Example:
const 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'] });
See also: child_process.exec()
and child_process.fork()
Synchronous Process Creation#
These methods are synchronous, meaning they WILL block the event loop, pausing execution of your code until the spawned process exits.
Blocking calls like these are mostly useful for simplifying general purpose scripting tasks and for simplifying the loading/processing of application configuration at startup.
child_process.execFileSync(file[, args][, options])#
file
String The filename of the program to runargs
Array List of string argumentsoptions
Objectcwd
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 largest amount of data (in bytes) allowed on stdout or stderr - if exceeded child process is killedencoding
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
Objectcwd
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 pairsshell
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
.)uid
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 largest amount of data (in bytes) allowed on stdout or stderr - if exceeded child process is killedencoding
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()
child_process.spawnSync(command[, args][, options])#
command
String The command to runargs
Array List of string argumentsoptions
Objectcwd
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 largest amount of data (in bytes) allowed on stdout or stderr - if exceeded child process is killedencoding
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 timed out
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.