Table of Contents

• Child Process
  • Asynchronous Process Creation
    • Spawning .bat and .cmd files on Windows
    • child_process.exec(command[, options][, callback])
    • child_process.execFile(file[, args][, options][, callback])
    • child_process.fork(modulePath[, args][, options])
    • child_process.spawn(command[, args][, options])
      • options.detached
      • options.stdio
  • Synchronous Process Creation
    • child_process.execFileSync(file[, args][, options])
    • child_process.execSync(command[, options])
    • child_process.spawnSync(command[, args][, options])
  • Class: ChildProcess
    • Event: 'close'
    • Event: 'disconnect'
    • Event: 'error'
    • Event: 'exit'
    • Event: 'message'
    • child.connected
    • child.disconnect()
    • child.kill([signal])
    • child.pid
    • child.send(message[, sendHandle][, callback])
      • Example: sending a server object
      • Example: sending a socket object
    • child.stderr
    • child.stdin
    • child.stdio
    • child.stdout

Child Process#

Stability: 2 - Stable

The child_process module provides the ability to spawn child processes in a manner that is similar, but not identical, to popen(3). This capability is primarily provided by the child_process.spawn() function:

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}`);
});

By default, pipes for stdin, stdout and stderr are established between the parent Node.js process and the spawned child. It is possible to stream data through these pipes in a non-blocking way. Note, however, that some programs use line-buffered I/O internally. While that does not affect Node.js, it can mean that data sent to the child process may not be immediately consumed.

The child_process.spawn() method spawns the child process asynchronously, without blocking the Node.js event loop. The child_process.spawnSync() function provides equivalent functionality in a synchronous manner that blocks the event loop until the spawned process either exits or is terminated.

For convenience, the child_process module provides a handful of synchronous and asynchronous alternatives to child_process.spawn() and child_process.spawnSync(). Note that each of these alternatives are implemented on top of child_process.spawn() or child_process.spawnSync().

• child_process.exec(): spawns a shell and runs a command within that shell, passing the stdout and stderr to a callback function when complete.
• child_process.execFile(): similar to child_process.exec() except that it spawns the command directly without first spawning a shell.
• child_process.fork(): spawns a new Node.js process and invokes a specified module with an IPC communication channel established that allows sending messages between parent and child.
• child_process.execSync(): a synchronous version of child_process.exec() that will block the Node.js event loop.
• child_process.execFileSync(): a synchronous version of child_process.execFile() that will block the Node.js event loop.

For certain use cases, such as automating shell scripts, the synchronous counterparts may be more convenient. In many cases, however, the synchronous methods can have significant impact on performance due to stalling the event loop while spawned processes complete.

Asynchronous Process Creation#

The child_process.spawn(), child_process.fork(), child_process.exec(), and child_process.execFile() methods all follow the idiomatic asynchronous programming pattern typical of other Node.js APIs.

Each of the methods returns a ChildProcess instance. These objects implement the Node.js EventEmitter API, allowing the parent process to register listener functions that are called when certain events occur during the life cycle of the child process.

The child_process.exec() and child_process.execFile() methods additionally allow for an optional callback function to be specified that is invoked when the child process terminates.

Spawning .bat and .cmd files on Windows#

The importance of the distinction between child_process.exec() and child_process.execFile() can vary based on platform. On Unix-type operating systems (Unix, Linux, OSX) child_process.execFile() can be more efficient because it does not spawn a shell. On Windows, however, .bat and .cmd files are not executable on their own without a terminal and therefore cannot be launched using child_process.execFile() (or even child_process.spawn()). When running on Windows, .bat and .cmd files can only be invoked using either child_process.exec() or by spawning cmd.exe and passing the .bat or .cmd file as an argument (which is what child_process.exec() does).

// On Windows Only ...
const spawn = require('child_process').spawn;
const bat = spawn('cmd.exe', ['/c', 'my.bat']);

bat.stdout.on('data', (data) => {
  console.log(data);
});

bat.stderr.on('data', (data) => {
  console.log(data);
});

bat.on('exit', (code) => {
  console.log(`Child exited with code ${code}`);
});

// OR...
const exec = require('child_process').exec;
exec('my.bat', (err, stdout, stderr) => {
  if (err) {
    console.error(err);
    return;
  }
  console.log(stdout);
});

child_process.exec(command[, options][, callback])#

Spawns a shell then executes the command within that shell, buffering any generated 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}`);
    }
});

If a callback function is provided, it is called with the arguments (error, stdout, stderr). On success, error will be null. On error, error will be an instance of Error. The error.code property will be the exit code of the child process while error.signal will be set to the signal that terminated the process. Any exit code other than 0 is considered to be an error.

The stdout and stderr arguments passed to the callback will contain the stdout and stderr output of the child process. By default, Node.js will decode the output as UTF-8 and pass strings to the callback. The encoding option can be used to specify the character encoding used to decode the stdout and stderr output. If encoding is 'buffer', Buffer objects will be passed to the callback instead.

The options argument may be passed as the second argument to customize how the process is spawned. The default options are:

{
  encoding: 'utf8',
  timeout: 0,
  maxBuffer: 200*1024,
  killSignal: 'SIGTERM',
  cwd: null,
  env: null
}

If timeout is greater than 0, the parent will send the the signal identified by the killSignal property (the default is 'SIGTERM') if the child runs longer than timeout milliseconds.

The maxBuffer option specifies the largest amount of data (in bytes) allowed on stdout or stderr - if this value is exceeded then the child process is terminated.

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])#

The child_process.execFile() function is similar to child_process.exec() except that it does not spawn a shell. Rather, the specified executable file is spawned directly as a new process making it slightly more efficient than child_process.exec().

The same options as child_process.exec() are supported. Since a shell is not spawned, behaviors such as I/O redirection and file globbing are not supported.

const execFile = require('child_process').execFile;
const child = execFile('node', ['--version'], (error, stdout, stderr) => {
  if (error) {
    throw error;
  }
  console.log(stdout);
});

The stdout and stderr arguments passed to the callback will contain the stdout and stderr output of the child process. By default, Node.js will decode the output as UTF-8 and pass strings to the callback. The encoding option can be used to specify the character encoding used to decode the stdout and stderr output. If encoding is 'buffer', Buffer objects will be passed to the callback instead.

child_process.fork(modulePath[, args][, options])#

The child_process.fork() method is a special case of child_process.spawn() used specifically to spawn new Node.js processes. Like child_process.spawn(), a ChildProcess object is returned. The returned ChildProcess will have an additional communication channel built-in that allows messages to be passed back and forth between the parent and child. See ChildProcess#send() for details.

It is important to keep in mind that spawned Node.js child processes are independent of the parent with exception of the IPC communication channel that is established between the two. Each process has it's own memory, with their own V8 instances. Because of the additional resource allocations required, spawning a large number of child Node.js processes is not recommended.

By default, child_process.fork() will spawn new Node.js instances using the process.execPath of the parent process. The execPath property in the options object allows for an alternative execution path to be used.

Node.js processes launched with a custom execPath will communicate with the parent process using the file descriptor (fd) identified using the environment 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])#

The child_process.spawn() method spawns a new process using the given command, with command line arguments in args. If omitted, args defaults to an empty array.

A third argument may be 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, setting options.detached to true makes it possible for the child process to continue running after the parent exits. The child will have its own console window. Once enabled for a child process, it cannot be disabled.

On non-Windows platforms, if options.detached is set to true, 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 regardless of 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. Doing so will cause the parent's event loop to not include the child in its reference count, allowing the parent to exit independently of the child, unless there is an established IPC channel between the child and parent.

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.

Example of a long-running process, by detaching and also ignoring its parent stdio file descriptors, in order to ignore the parent's termination:

const spawn = require('child_process').spawn;

const child = spawn(process.argv[0], ['child_program.js'], {
  detached: true,
  stdio: ['ignore']
});

child.unref();

Alternatively one can redirect the child process' output into files:

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();

options.stdio#

The options.stdio option is used to configure the pipes that are established between the parent and child process. By default, the child's stdin, stdout, and stderr are redirected to corresponding child.stdin, child.stdout, and child.stderr streams on the ChildProcess object. This is equivalent to setting the options.stdio equal to ['pipe', 'pipe', 'pipe'].

For convenience, options.stdio may be one of the following strings:

• 'pipe' - equivalent to ['pipe', 'pipe', 'pipe'] (the default)
• 'ignore' - equivalent to ['ignore', 'ignore', 'ignore']
• 'inherit' - equivalent to [process.stdin, process.stdout, process.stderr] or [0,1,2]

Otherwise, the value of option.stdio is an array where each index corresponds to an fd in the child. The fds 0, 1, and 2 correspond to stdin, stdout, and stderr, respectively. Additional fds can be specified to create additional pipes between the parent and 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, the ChildProcess.on('message') event handler will be triggered in the parent. If the child is a Node.js process, the presence of an IPC channel will enable process.send(), process.disconnect(), process.on('disconnect'), and process.on('message') within the child.
3. 'ignore' - Instructs Node.js to ignore the fd in the child. While Node.js will always open fds 0 - 2 for the processes it spawns, setting the fd to 'ignore' will cause Node.js to 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. Note that the stream must have an underlying descriptor (file streams do not until the 'open' event has occurred).
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'.

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 presenting a
// startd-style interface.
spawn('prg', [], { stdio: ['pipe', null, null, null, 'pipe'] });

It is worth noting that when an IPC channel is established between the parent and child processes, and the child is a Node.js process, the child is launched with the IPC channel unreferenced (using unref()) until the child registers an event handler for the process.on('disconnected') event. This allows the child to exit normally without the process being held open by the open IPC channel.

See also: child_process.exec() and child_process.fork()

Synchronous Process Creation#

The child_process.spawnSync(), child_process.execSync(), and child_process.execFileSync() methods are synchronous and WILL block the Node.js event loop, pausing execution of any additional 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])#

The child_process.execFileSync() method is generally identical to child_process.execFile() with the exception that the method 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. Note that if the child process intercepts and handles the SIGTERM signal and does not exit, the parent process will still 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])#

The child_process.execSync() method is generally identical to child_process.exec() with the exception that the method 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. Note that if the child process intercepts and handles the SIGTERM signal and doesn't exit, the parent 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])#

The child_process.spawnSync() method is generally identical to child_process.spawn() with the exception that the function 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. Note that if the process intercepts and handles the SIGTERM signal and doesn't exit, the parent process will wait until the child process has exited.

Class: ChildProcess#

Instances of the ChildProcess class are EventEmitters that represent spawned child processes.

Instances of ChildProcess are not intended to be created directly. Rather, use the child_process.spawn(), child_process.exec(), child_process.execFile(), or child_process.fork() methods to create instances of ChildProcess.

Event: 'close'#

The 'close' event is emitted when the stdio streams of a child process have been closed. This is distinct from the 'exit' event, since multiple processes might share the same stdio streams.

Event: 'disconnect'#

The 'disconnect' event is emitted after calling the ChildProcess.disconnect() method in the parent or child process. After disconnecting it is no longer possible to send or receive messages, and the ChildProcess.connected property is false.

Event: 'error'#

The 'error' event is emitted whenever:

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.

Note that the 'exit' event may or may not fire after an error has occurred. If you are listening to both the 'exit' and 'error' events, it is important to guard against accidentally invoking handler functions multiple times.

See also ChildProcess#kill() and ChildProcess#send().

Event: 'exit'#

The 'exit' event is emitted after the child process ends. If the process exited, 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. One of the two will always be non-null.

Note that when the 'exit' event is triggered, child process stdio streams might still be open.

Also, note that Node.js establishes signal handlers for SIGINT and SIGTERM and Node.js processes will not terminate immediately due to receipt of those signals. Rather, Node.js will perform a sequence of cleanup actions and then will re-raise the handled signal.

See waitpid(2).

Event: 'message'#

The 'message' event is triggered when a child process uses process.send() to send messages.

child.connected#

The child.connected property indicates whether it is still possible to send and receive messages from a child process. When child.connected is false, it is no longer possible to send or receive messages.

child.disconnect()#

Closes 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 child.connected and process.connected properties in both the parent and child (respectively) will be set to false, and it will be no longer possible to pass messages between the processes.

The 'disconnect' event will be emitted when there are no messages in the process of being received. This will most often be triggered immediately after calling child.disconnect().

Note that when the child process is a Node.js instance (e.g. spawned using child_process.fork()), the process.disconnect() method can be invoked within the child process to close the IPC channel as well.

child.kill([signal])#

The child.kill() methods sends a signal to the child process. If no argument is given, the process will be sent the 'SIGTERM' signal. 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');

The ChildProcess object may emit an 'error' event if 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 which can have unexpected results.

Note that while the function is called kill, the signal delivered to the child process may not actually terminate the process.

See kill(2)

child.pid#

Returns 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])#

When an IPC channel has been established between the parent and child ( i.e. when using child_process.fork()), the child.send() method can be used to send messages to the child process. When the child process is a Node.js instance, these messages can be received via the process.on('message') event.

For example, in the parent script:

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' });

Child Node.js processes will have a process.send() method of their own that allows the child to send messages back to the parent.

There is a special case when sending a {cmd: 'NODE_foo'} message. All messages containing a NODE_ prefix in its cmd property are considered to be reserved for use within Node.js core and will not be emitted in the child's process.on('message') event. Rather, such messages are emitted using the process.on('internalMessage') event and are consumed internally by Node.js. Applications should avoid using such messages or listening for 'internalMessage' events as it is subject to change without notice.

The optional sendHandle argument that may be passed to child.send() is for passing a TCP server or socket object to the child process. The child will receive the object as the second argument passed to the callback function registered on the process.on('message') event.

The optional callback is a function that is invoked after the message is sent but before the child may have received it. The function is called with a single argument: null on success, or an Error object on failure.

If no callback function is provided and the message cannot be sent, an 'error' event will be emitted by the ChildProcess object. This can happen, for instance, when the child process has already exited.

child.send() will return false if the channel has closed or when the backlog of unsent messages exceeds a threshold that makes it unwise to send more. Otherwise, the method returns true. The callback function can be used to implement flow control.

Example: sending a server object#

The sendHandle argument can be used, for instance, to pass the handle of a TCP server object to the child process as illustrated in the example below:

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);
});

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');
    });
  }
});

Once the server is now shared between the parent and child, some connections can be handled by the parent and some by the child.

While the example above uses a server created using the net module, dgram module servers use exactly the same workflow with the exceptions of listening on a 'message' event instead of 'connection' and using server.bind instead of server.listen. This is, however, currently only supported on UNIX platforms.

Example: sending a socket object#

Similarly, the sendHandler argument can be used to pass the handle of a socket to the child process. The example below spawns two children that each handle connections with "normal" or "special" priority:

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 special priority
  if (socket.remoteAddress === '74.125.127.100') {
    special.send('socket', socket);
    return;
  }
  // This is normal priority
  normal.send('socket', socket);
});
server.listen(1337);

The child.js would receive the socket handle as the second argument passed to the event callback function:

process.on('message', (m, socket) => {
  if (m === 'socket') {
    socket.end(`Request handled with ${process.argv[2]} priority`);
  }
});

Once a socket has been passed to a child, the parent is no longer capable of tracking when the socket is destroyed. To indicate this, the .connections property becomes null. It is recommended not to use .maxConnections when this occurs.

Note: this function uses JSON.stringify() internally to serialize the message.

child.stderr#

A Readable Stream that represents the child process's stderr.

If the child was spawned with stdio[2] set to anything other than 'pipe', then this will be undefined.

child.stderr is an alias for child.stdio[2]. Both properties will refer to the same value.

child.stdin#

A Writable Stream that represents the child process's stdin.

Note that if a child process waits to read all of its input, the child will not continue until this stream has been closed via end().

If the child was spawned with stdio[0] set to anything other than 'pipe', then this will be undefined.

child.stdin is an alias for child.stdio[0]. Both properties will refer to the same value.

child.stdio#

A sparse array of pipes to the child process, corresponding with positions in the stdio option passed to child_process.spawn() that have been set to the value 'pipe'. Note that child.stdio[0], child.stdio[1], and child.stdio[2] are also available as child.stdin, child.stdout, and child.stderr, respectively.

In the following example, only the child's fd 1 (stdout) is configured 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#

A Readable Stream that represents the child process's stdout.

If the child was spawned with stdio[1] set to anything other than 'pipe', then this will be undefined.

child.stdout is an alias for child.stdio[1]. Both properties will refer to the same value.