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:

var spawn = require('child_process').spawn;
var ls = spawn('ls', ['-lh', '/usr']);

ls.stdout.on('data', function(data) {
  console.log('stdout: ' + data);
});

ls.stderr.on('data', function(data) {
  console.log('stderr: ' + data);
});

ls.on('close', function(code) {
  console.log('child process exited with code ' + code);
});

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.

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

• 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, process.env.ComSpec on Windows. See Shell Requirements and Default Windows Shell.
  • 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)).
  • windowsHide {boolean} Hide the subprocess console window that would normally be created on Windows systems. Default: false.
• 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. The command string passed to the exec function is processed directly by the shell and special characters (vary based on shell) need to be dealt with accordingly:

exec('"/path/to/test file/test.sh" arg1 arg2');
//Double quotes are used so that the space in the path is not interpreted as
//multiple arguments

exec('echo "The \\$HOME variable is $HOME"');
//The $HOME variable is escaped in the first instance, but not in the second

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

var 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:

var defaults = {
  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])

• file {string} The name or path of the executable file to run.
• args {string[]} 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)).
  • windowsHide {boolean} Hide the subprocess console window that would normally be created on Windows systems. Default: false.
  • windowsVerbatimArguments {boolean} No quoting or escaping of arguments is done on Windows. Ignored on Unix. Default: false.
• 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.

var execFile = require('child_process');
var child = execFile('node', ['--version'], function(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])

• 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|string} See child_process.spawn()’s stdio. When this option is provided, it overrides silent. If the array variant is used, it must contain exactly one item with value 'ipc' or an error will be thrown. For instance [0, 1, 2, 'ipc'].
  • windowsVerbatimArguments {boolean} No quoting or escaping of arguments is done on Windows. Ignored on Unix. Default: false.
  • uid {number} Sets the user identity of the process (see setuid(2)).
  • gid {number} Sets the group identity of the process (see setgid(2)).
• 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 subprocess.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.

Note: Unlike the fork(2) POSIX system call, child_process.fork() does not clone the current process.

Note: The shell option available in child_process.spawn() is not supported by child_process.fork() and will be ignored if set.

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

• 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 process.env.ComSpec on Windows. A different shell can be specified as a string. See Shell Requirements and Default Windows Shell. Default: false (no shell).
  • windowsVerbatimArguments {boolean} No quoting or escaping of arguments is done on Windows. Ignored on Unix. This is set to true automatically when shell is specified. Default: false.
  • windowsHide {boolean} Hide the subprocess console window that would normally be created on Windows systems. Default: false.
• 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 unsanitized 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:

var 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.

undefined values in env will be ignored.

Example of running ls -lh /usr, capturing stdout, stderr, and the exit code:

var spawn = require('child_process').spawn;
var ls = spawn('ls', ['-lh', '/usr']);

ls.stdout.on('data', function(data) {
  console.log('stdout: ' + data);
});

ls.stderr.on('data', function(data) {
  console.log('stderr: ' + data);
});

ls.on('close', function(code) {
  console.log('child process exited with code ' + code);
});

Example: A very elaborate way to run ps ax | grep ssh

var spawn = require('child_process').spawn;
var ps = spawn('ps', ['ax']);
var grep = spawn('grep', ['ssh']);

ps.stdout.on('data', function(data) {
  grep.stdin.write(data);
});

ps.stderr.on('data', function(data) {
  console.log('ps stderr: ' + data);
});

ps.on('close', function(code) {
  if (code !== 0) {
    console.log('ps process exited with code ' + code);
  }
  grep.stdin.end();
});

grep.stdout.on('data', function(data) {
  console.log(data.toString());
});

grep.stderr.on('data', function(data) {
  console.log('grep stderr: ' + data);
});

grep.on('close', function(code) {
  if (code !== 0) {
    console.log('grep process exited with code ' + code);
  }
});

Example of checking for failed spawn:

var spawn = require('child_process').spawn;
var subprocess = spawn('bad_command');

subprocess.on('error', function(err) {
  console.log('Failed to start subprocess.');
});

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

options.detached

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 subprocess, use the subprocess.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.

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 subprocess.stdin, subprocess.stdout, and subprocess.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 subprocess.stdio[fd]. Pipes created for fds 0 - 2 are also available as subprocess.stdin, subprocess.stdout and subprocess.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 subprocess.send() method. 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 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:

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

// Child will use parent's stdios
spawn('prg', [], { stdio: 'inherit' });

// Spawn child sharing only stderr
spawn('prg', [], { stdio: ['pipe', 'pipe', process.stderr] });

// Open an extra fd=4, to interact with programs 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()

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’

• 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’

The 'disconnect' event is emitted after calling the subprocess.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 subprocess.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: The 'exit' event may or may not fire after an error has occurred. When listening to both the 'exit' and 'error' events, it is important to guard against accidentally invoking handler functions multiple times.

See also subprocess.kill() and subprocess.send().

Event: ‘exit’

• 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’

• 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.

Note: The message goes through serialization and parsing. The resulting message might not be the same as what is originally sent.

subprocess.channel

• {Object} A pipe representing the IPC channel to the child process.

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

subprocess.connected

• {boolean} Set to false after subprocess.disconnect() is called.

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

subprocess.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 subprocess.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 subprocess.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.

subprocess.kill([signal])

• signal {string}

The subprocess.kill() method 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.

var spawn = require('child_process').spawn;
var grep = spawn('grep', ['ssh']);

grep.on('close', function(code, signal) {
  console.log(
    'child process terminated due to receipt of signal', signal});
});

// Send SIGHUP to process
grep.kill('SIGHUP');

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

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

subprocess.killed

• {boolean} Set to true after subprocess.kill() is used to successfully send a signal to the child process.

The subprocess.killed property indicates whether the child process successfully received a signal from subprocess.kill(). The killed property does not indicate that the child process has been terminated.

subprocess.pid

• {number} Integer

Returns the process identifier (PID) of the child process.

Example:

var spawn = require('child_process').spawn;
var grep = spawn('grep', ['ssh']);

console.log('Spawned child pid:', grep.pid);
grep.stdin.end();

subprocess.send(message[, sendHandle[, options]][, callback])

• 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 subprocess.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.

Note: The message goes through serialization and parsing. The resulting message might not be the same as what is originally sent.

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);
});

// Causes the child to print: CHILD got message: { hello: 'world' }
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);
});

// Causes the parent to print: PARENT got message: { foo: 'bar', baz: null }
process.send({ foo: 'bar', baz: NaN });

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. Messages containing a NODE_ prefix in the cmd property are 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 subprocess.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.

subprocess.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.

subprocess.stderr

• {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.

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

subprocess.stdin

• {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.

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

subprocess.stdio

• {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 subprocess.stdio[0], subprocess.stdio[1], and subprocess.stdio[2] are also available as subprocess.stdin, subprocess.stdout, and subprocess.stderr, respectively.

In the following example, only the child’s fd 1 (stdout) is configured as a pipe, so only the parent’s subprocess.stdio[1] is a stream, all other values in the array are null.

var assert = require('assert');
var fs = require('fs');
var child_process = require('child_process');

const subprocess = 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(subprocess.stdio[0], null);
assert.strictEqual(subprocess.stdio[0], subprocess.stdin);

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

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

subprocess.stdout

• {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.

subprocess.stdout is an alias for subprocess.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.

Shell Requirements

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'.

Default Windows Shell

Although Microsoft specifies %COMSPEC% must contain the path to 'cmd.exe' in the root environment, child processes are not always subject to the same requirement. Thus, in child_process functions where a shell can be spawned, 'cmd.exe' is used as a fallback if process.env.ComSpec is unavailable.