Child Process | Node.js v7.9.0 Documentation

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# 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()`. 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 `/d /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. (Default: `200*1024`) If exceeded, the child process is terminated. See caveat at `maxBuffer` and Unicode.
  - `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
  - `error` <Error>
  - `stdout` <string> | <Buffer>
  - `stderr` <string> | <Buffer>
- 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

- `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
  - `env` <Object> Environment key-value pairs
  - `encoding` <string> (Default: `'utf8'`)
  - `timeout` <number> (Default: `0`)
  - `maxBuffer` <number> Largest amount of data in bytes allowed on stdout or stderr. (Default: `200*1024`) If exceeded, the child process is terminated. See caveat at `maxBuffer` and Unicode.
  - `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
  - `error` <Error>
  - `stdout` <string> | <Buffer>
  - `stderr` <string> | <Buffer>
- Returns: <ChildProcess>

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])#

History

Version | Changes
--- | ---
v6.4.0 | The `stdio` option is supported now.
v0.5.0 | 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])#

History

Version | Changes
--- | ---
v6.4.0 | The `argv0` option is supported now.
v5.7.0 | The `shell` option is supported now.
v0.1.90 | 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 `/d /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#

History

Version | Changes
--- | ---
v3.3.1 | The value `0` is now accepted as a file descriptor.
v0.7.10 | 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])#

History

Version | Changes
--- | ---
v6.2.1, v4.5.0 | The `encoding` option can now explicitly be set to `buffer`.
v0.11.12 | 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. (Default: `200*1024`) If exceeded, the child process is terminated. See caveat at `maxBuffer` and Unicode.
  - `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 `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])#

Added in: v0.11.12

- `command` <string> The command to run
- `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
  - `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 `/d /s /c` on Windows. On Windows, command line parsing should be compatible with `cmd.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> | <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. (Default: `200*1024`) If exceeded, the child process is terminated. See caveat at `maxBuffer` and Unicode.
  - `encoding` <string> The encoding used for all stdio inputs and outputs. (Default: `'buffer'`)
- Returns: <Buffer> | <string> The stdout from the command

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()`

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

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

History

Version | Changes
--- | ---
v6.2.1, v4.5.0 | The `encoding` option can now explicitly be set to `buffer`.
v5.7.0 | The `shell` option is supported now.
v0.11.12 | Added in: v0.11.12

- `command` <string> The command 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.
  - `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. (Default: `200*1024`) If exceeded, the child process is terminated. See caveat at `maxBuffer` and Unicode.
  - `encoding` <string> The encoding used for all stdio inputs and outputs. (Default: `'buffer'`)
  - `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 `/d /s /c` on Windows. Defaults to `false` (no shell).
- Returns: <Object>
  - `pid` <number> Pid of the child process
  - `output` <Array> Array of results from stdio output
  - `stdout` <Buffer> | <string> The contents of `output[1]`
  - `stderr` <Buffer> | <string> The contents of `output[2]`
  - `status` <number> The exit code of the child process
  - `signal` <string> The signal used to kill the child process
  - `error` <Error> The error object if the child process failed or timed out

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.

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.

## Class: ChildProcess#

Added in: v2.2.0

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'#

Added in: v0.7.7

- `code` <number> the exit code if the child exited on its own.
- `signal` <string> the signal by which the child process was terminated.

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'#

Added in: v0.7.2

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

### Event: 'error'#

- `err` <Error> the 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 `child.kill()` and `child.send()`.

### Event: 'exit'#

Added in: v0.1.90

- `code` <number> the exit code if the child exited on its own.
- `signal` <string> the signal by which the child process was terminated.

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'#

Added in: v0.5.9

- `message` <Object> a parsed JSON object or primitive value.
- `sendHandle` <Handle> a `net.Socket` or `net.Server` object, or undefined.

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

### child.channel#

Added in: v7.1.0

- <Object> A pipe representing the IPC channel to the child process.

The `child.channel` property is a reference to the child's IPC channel. If no IPC channel currently exists, this property is `undefined`.

### child.connected#

Added in: v0.7.2

- <boolean> Set to `false` after `child.disconnect()` is called

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()#

Added in: v0.7.2

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])#

Added in: v0.1.90

- `signal` <string>

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) for reference.

Also note: on Linux, child processes of child processes will not be terminated when attempting to kill their parent. This is likely to happen when running a new process in a shell or with use of the `shell` option of `ChildProcess`, such as in this example:

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

    const child = spawn('sh', ['-c',
      `node -e "setInterval(() => {
          console.log(process.pid, 'is alive')
        }, 500);"`
      ], {
        stdio: ['inherit', 'inherit', 'inherit']
      });

    setTimeout(() => {
      child.kill(); // does not terminate the node process in the shell
    }, 2000);

### child.pid#

Added in: v0.1.90

- <number> Integer

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[, options]][, callback])#

History

Version | Changes
--- | ---
v5.8.0 | The `options` parameter, and the `keepOpen` option in particular, is supported now.
v5.0.0 | This method returns a boolean for flow control now.
v4.0.0 | The `callback` parameter is supported now.
v0.5.9 | Added in: v0.5.9

- `message` <Object>
- `sendHandle` <Handle>
- `options` <Object>
- `callback` <Function>
- Returns: <boolean>

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. Any data that is received and buffered in the socket will not be sent to the child.

The `options` argument, if present, is an object used to parameterize the sending of certain types of handles. `options` supports the following properties:

- `keepOpen` - A Boolean value that can be used when passing instances of `net.Socket`. When `true`, the socket is kept open in the sending process. Defaults to `false`.

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#

Added in: v0.1.90

- <stream.Readable>

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 `null`.

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

### child.stdin#

Added in: v0.1.90

- <stream.Writable>

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 `null`.

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

### child.stdio#

Added in: v0.7.10

- <Array>

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 parent's stdin for child
        'pipe', // Pipe child's stdout to parent
        fs.openSync('err.out', 'w') // Direct child's stderr to a file
      ]
    });

    assert.strictEqual(child.stdio[0], null);
    assert.strictEqual(child.stdio[0], child.stdin);

    assert(child.stdout);
    assert.strictEqual(child.stdio[1], child.stdout);

    assert.strictEqual(child.stdio[2], null);
    assert.strictEqual(child.stdio[2], child.stderr);

### child.stdout#

Added in: v0.1.90

- <stream.Readable>

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 `null`.

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

## `maxBuffer` and Unicode#

The `maxBuffer` option specifies the largest number of bytes allowed on `stdout` or `stderr`. If this value is exceeded, then the child process is terminated. This impacts output that includes multibyte character encodings such as UTF-8 or UTF-16. For instance, `console.log('中文测试')` will send 13 UTF-8 encoded bytes to `stdout` although there are only 4 characters.