Node.js v6.10.3 Documentation

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().

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(). When running on Windows, .bat and .cmd files can be invoked using child_process.spawn() with the shell option set, with child_process.exec(), or by spawning cmd.exe and passing the .bat or .cmd file as an argument (which is what the shell option and child_process.exec() do). In any case, if the script filename contains spaces it needs to be quoted.

// 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.toString());
});

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

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);
});

// Script with spaces in the filename:
const bat = spawn('"my script.cmd"', ['a', 'b'], { shell: true });
// or:
exec('"my script.cmd" a b', (err, stdout, stderr) => {
  // ...
});

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

Added in: v0.1.90
  • command <String> The command to run, with space-separated arguments
  • options <Object>
    • cwd <String> Current working directory of the child process
    • env <Object> Environment key-value pairs
    • 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> largest amount of data (in bytes) allowed on stdout or stderr - if exceeded child process is killed (Default: 200*1024)
    • killSignal <String> | <Integer> (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 terminates
  • Returns: <ChildProcess>

Spawns a shell then executes the command within that shell, buffering any generated output.

Note: Never pass unsanitised user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.

const exec = require('child_process').exec;
exec('cat *.js bad_file | wc -l', (error, stdout, stderr) => {
  if (error) {
    console.error(`exec error: ${error}`);
    return;
  }
  console.log(`stdout: ${stdout}`);
  console.log(`stderr: ${stderr}`);
});

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', or an unrecognized character encoding, 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 signal identified by the killSignal property (the default is 'SIGTERM') if the child runs longer than timeout milliseconds.

Note: Unlike the exec(3) 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])#

Added in: v0.1.91

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', or an unrecognized character encoding, Buffer objects will be passed to the callback instead.

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

Added in: v0.5.0
  • modulePath <String> The module to run in the child
  • args <Array> List of string arguments
  • options <Object>
    • cwd <String> Current working directory of the child process
    • env <Object> Environment key-value pairs
    • 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, 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 for child_process.spawn()'s stdio for more details (Default: false)
    • stdio <Array> Supports the array version of child_process.spawn()'s stdio option. When this option is provided, it overrides silent. The array must contain exactly one item with value 'ipc' or an error will be thrown. For instance [0, 1, 2, 'ipc'].
    • uid <Number> Sets the user identity of the process. (See setuid(2).)
    • gid <Number> Sets the group identity of the process. (See setgid(2).)
  • Returns: <ChildProcess>

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 child.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 its 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(2) POSIX system call, child_process.fork() does not clone the current process.

child_process.spawn(command[, args][, options])#

Added in: v0.1.90
  • command <String> The command to run
  • args <Array> List of string arguments
  • options <Object>
    • cwd <String> Current working directory of the child process
    • env <Object> Environment key-value pairs
    • argv0 <String> Explicitly set the value of argv[0] sent to the child process. This will be set to command if not specified.
    • stdio <Array> | <String> Child's stdio configuration. (See options.stdio)
    • detached <Boolean> Prepare child to run independently of its parent process. Specific behavior depends on the platform, see options.detached)
    • uid <Number> Sets the user identity of the process. (See setuid(2).)
    • gid <Number> Sets the group identity of the process. (See setgid(2).)
    • shell <Boolean> | <String> If true, runs command inside of a shell. Uses '/bin/sh' on UNIX, and 'cmd.exe' on Windows. A different shell can be specified as a string. The shell should understand the -c switch on UNIX, or /s /c on Windows. Defaults to false (no shell).
  • Returns: <ChildProcess>

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.

Note: If the shell option is enabled, do not pass unsanitised user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.

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.toString());
});

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.');
});

Note: Certain platforms (OS X, Linux) will use the value of argv[0] for the process title while others (Windows, SunOS) will use command.

Note: Node.js currently overwrites argv[0] with process.execPath on startup, so process.argv[0] in a Node.js child process will not match the argv0 parameter passed to spawn from the parent, retrieve it with the process.argv0 property instead.

options.detached#

Added in: v0.7.10

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#

Added in: v0.7.10

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 options.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 child.stdio[fd]. Pipes created for fds 0 - 2 are also available as child.stdin, child.stdout and child.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 child.send() method. If the child writes JSON messages to this file descriptor, the child.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('disconnect') event or the process.on('message') 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])#

Added in: v0.11.12
  • file <String> The name or path of the executable file to run
  • args <Array> List of string arguments
  • options <Object>
    • cwd <String> Current working directory of the child process
    • input <String> | <Buffer> The value which will be passed as stdin to the spawned process
      • supplying this value will override stdio[0]
    • stdio <String> | <Array> Child's stdio configuration. (Default: 'pipe')
      • stderr by default will be output to the parent process' stderr unless stdio is specified
    • env <Object> Environment key-value pairs
    • 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> | <Integer> 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 killed
    • encoding <String> The encoding used for all stdio inputs and outputs. (Default: 'buffer')
  • Returns: <Buffer> | <String> The stdout from the command

The child_process.execFileSync() method is generally identical to