Node.js Child Process — Создание и работа с дочерними процессами в Node

Child Process — это модуль в Node.js, который можно подключить с помощью функции require(). Используя модуль child_process мы можем полноценно использовать функционал процессов. Ниже перечислены некоторые возможности модуля:

• С помощью модуля child_process мы можем запускать команды shell. Shell — это командный язык высокого уровня, который используется в Unix как интерпретатор. Он совместим с Bash. С помощью простых Shell команд мы можем управлять сервером и написать сложные сценарий.
• Модуль child_process позволяет запускать параллельные дочерние процессы.
• Модуль child_process позволяет обмениваться сообщениями между собой.

Модуль имеет несколько простых но очень важных методов, которые облегчают жизнь.

Метод child_process.exec()

С помощью метода child_process.exec() мы можем запускать команды на выполнения. После выполнения результат будет сохранен в буфере.

Пример использование метода child_process.exec():

Тут мы с помощью метода child_process.exec() запускаем shell-команду cat, выводящую содержимое нескольких файлов и через пайп передающую их команде wc, считывающей количество строк. Вторым аргументом передается массив параметров в данном случае мы изменяем рабочую директорию. Метод в качестве третьего аргумента принимает функцию, который принимает 3 параметра. Первый параметр — это объект ошибки, а второй и третий — это потоки stdout и stderr. Ценность команды состоит в возможности действий с полученными результатами.

Метод child_process.spawn()

Следующий метод — это child_process.spawn(), который запускает команду в новом процессе, дескриптор которого становится доступным:

Здесь все просто — любой вывод исполняемой команды передается в поток stdout дочернего процесса (или stderr, если случилось ошибка), и мы связали обработчики именно с этими объектами. Мы видим, что поток выполняется параллельно с основным процессом и завершается с кодом 0, что свидетельствует об успехе.

Метод child_process.fork()

Следующий метод — это child_process.fork(), который запускает дочерним процессом новый процесс, порожденный самой Node.js. Ниже представлен простой пример работы данного метода:

Тут мы запускаем два дочерних процесса, исполняющих код, характеризующихся в файлах sub1.js и sub2.js, и создаем бесконечный цикл вывода сообщений. Ниже представлен содержимое дочерних процессов:

sub1.js:

sub2.js:

После запуска основного скрипта мы увидим, что все работает параллельно.

Метод child_process.send()

Последний метод, который будет рассмотрен в этом статье — это метод send() модуля Child Process. Он отсылает сообщения от одного процесса к другому:

main.js:

sub.js:

В виде сообщение также можно посылать команды управления.

Node.js v15.14.0 documentation

Child process #

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

By default, pipes for stdin , stdout , and stderr are established between the parent Node.js process and the spawned subprocess. These pipes have limited (and platform-specific) capacity. If the subprocess writes to stdout in excess of that limit without the output being captured, the subprocess blocks waiting for the pipe buffer to accept more data. This is identical to the behavior of pipes in the shell. Use the < stdio: 'ignore' >option if the output will not be consumed.

The command lookup is performed using the options.env.PATH environment variable if it is in the options object. Otherwise, process.env.PATH is used.

On Windows, environment variables are case-insensitive. Node.js lexicographically sorts the env keys and uses the first one that case-insensitively matches. Only first (in lexicographic order) entry will be passed to the subprocess. This might lead to issues on Windows when passing objects to the env option that have multiple variants of the same key, such as PATH and Path .

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() . 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 by default.
• 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, macOS) child_process.execFile() can be more efficient because it does not spawn a shell by default. 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.

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

History

AbortSignal support was added.

The windowsHide option is supported now.

• command The command to run, with space-separated arguments.
• options
  • cwd Current working directory of the child process. Default: process.cwd() .
  • env Environment key-value pairs. Default: process.env .
  • encoding Default: ‘utf8’
  • shell Shell to execute the command with. See Shell requirements and Default Windows shell. Default: ‘/bin/sh’ on Unix, process.env.ComSpec on Windows.
  • signal allows aborting the child process using an AbortSignal.
  • timeout Default: 0
  • maxBuffer Largest amount of data in bytes allowed on stdout or stderr. If exceeded, the child process is terminated and any output is truncated. See caveat at maxBuffer and Unicode. Default: 1024 * 1024 .
  • killSignal | Default: ‘SIGTERM’
  • uid Sets the user identity of the process (see setuid(2) ).
  • gid Sets the group identity of the process (see setgid(2) ).
  • windowsHide Hide the subprocess console window that would normally be created on Windows systems. Default: false .
• callback called with the output when process terminates.
  • error
  • stdout |
  • stderr |
• Returns:

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:

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

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 process. By convention, any exit code other than 0 indicates an error. error.signal will be the signal that terminated the process.

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.

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.

Unlike the exec(3) POSIX system call, child_process.exec() does not replace the existing process and uses a shell to execute the command.

If this method is invoked as its util.promisify() ed version, it returns a Promise for an Object with stdout and stderr properties. The returned ChildProcess instance is attached to the Promise as a child property. In case of an error (including any error resulting in an exit code other than 0), a rejected promise is returned, with the same error object given in the callback, but with two additional properties stdout and stderr .

If the signal option is enabled, calling .abort() on the corresponding AbortController is similar to calling .kill() on the child process except the error passed to the callback will be an AbortError :

child_process.execFile(file[, args][, options][, callback]) #

History

AbortSignal support was added.

The windowsHide option is supported now.

• file The name or path of the executable file to run.
• args List of string arguments.
• options
  • cwd Current working directory of the child process.
  • env Environment key-value pairs. Default: process.env .
  • encoding Default: ‘utf8’
  • timeout Default: 0
  • maxBuffer Largest amount of data in bytes allowed on stdout or stderr. If exceeded, the child process is terminated and any output is truncated. See caveat at maxBuffer and Unicode. Default: 1024 * 1024 .
  • killSignal | Default: ‘SIGTERM’
  • uid Sets the user identity of the process (see setuid(2) ).
  • gid Sets the group identity of the process (see setgid(2) ).
  • windowsHide Hide the subprocess console window that would normally be created on Windows systems. Default: false .
  • windowsVerbatimArguments No quoting or escaping of arguments is done on Windows. Ignored on Unix. Default: false .
  • shell | 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).
  • signal allows aborting the child process using an AbortSignal.
• callback Called with the output when process terminates.
  • error
  • stdout |
  • stderr |
• Returns:

The child_process.execFile() function is similar to child_process.exec() except that it does not spawn a shell by default. 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.

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.

If this method is invoked as its util.promisify() ed version, it returns a Promise for an Object with stdout and stderr properties. The returned ChildProcess instance is attached to the Promise as a child property. In case of an error (including any error resulting in an exit code other than 0), a rejected promise is returned, with the same error object given in the callback, but with two additional properties stdout and stderr .

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.

If the signal option is enabled, calling .abort() on the corresponding AbortController is similar to calling .kill() on the child process except the error passed to the callback will be an AbortError :

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

History

timeout was added.

killSignal for AbortSignal was added.

AbortSignal support was added.

The serialization option is supported now.

The stdio option can now be a string.

The stdio option is supported now.

• modulePath The module to run in the child.
• args List of string arguments.
• options
  • cwd Current working directory of the child process.
  • detached Prepare child to run independently of its parent process. Specific behavior depends on the platform, see options.detached ).
  • env Environment key-value pairs. Default: process.env .
  • execPath Executable used to create the child process.
  • execArgv List of string arguments passed to the executable. Default: process.execArgv .
  • gid Sets the group identity of the process (see setgid(2) ).
  • serialization Specify the kind of serialization used for sending messages between processes. Possible values are ‘json’ and ‘advanced’ . See Advanced serialization for more details. Default: ‘json’ .
  • signal Allows closing the child process using an AbortSignal.
  • killSignal | The signal value to be used when the spawned process will be killed by timeout or abort signal. Default: ‘SIGTERM’ .
  • silent 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 | 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’] .
  • uid Sets the user identity of the process (see setuid(2) ).
  • windowsVerbatimArguments No quoting or escaping of arguments is done on Windows. Ignored on Unix. Default: false .
  • timeout In milliseconds the maximum amount of time the process is allowed to run. Default: undefined .
• Returns:

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.

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.

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

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

If the signal option is enabled, calling .abort() on the corresponding AbortController is similar to calling .kill() on the child process except the error passed to the callback will be an AbortError :

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

History

timeout was added.

killSignal for AbortSignal was added.

AbortSignal support was added.

The serialization option is supported now.

The windowsHide option is supported now.

The argv0 option is supported now.

The shell option is supported now.

command The command to run.

args List of string arguments.

• cwd Current working directory of the child process.
• env Environment key-value pairs. Default: process.env .
• argv0 Explicitly set the value of argv[0] sent to the child process. This will be set to command if not specified.
• stdio | Child’s stdio configuration (see options.stdio ).
• detached Prepare child to run independently of its parent process. Specific behavior depends on the platform, see options.detached ).
• uid Sets the user identity of the process (see setuid(2) ).
• gid Sets the group identity of the process (see setgid(2) ).
• serialization Specify the kind of serialization used for sending messages between processes. Possible values are ‘json’ and ‘advanced’ . See Advanced serialization for more details. Default: ‘json’ .
• shell | 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 No quoting or escaping of arguments is done on Windows. Ignored on Unix. This is set to true automatically when shell is specified and is CMD. Default: false .
• windowsHide Hide the subprocess console window that would normally be created on Windows systems. Default: false .
• signal allows aborting the child process using an AbortSignal.
• timeout In milliseconds the maximum amount of time the process is allowed to run. Default: undefined .
• killSignal | The signal value to be used when the spawned process will be killed by timeout or abort signal. Default: ‘SIGTERM’ .

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.

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:

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. If given, but the path does not exist, the child process emits an ENOENT error and exits immediately. ENOENT is also emitted when the command does not exist.

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:

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

Example of checking for failed spawn :

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

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.

If the signal option is enabled, calling .abort() on the corresponding AbortController is similar to calling .kill() on the child process except the error passed to the callback will be an AbortError :

options.detached #

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

On non-Windows platforms, if options.detached is set to true , the child process will be made the leader of a new process group and session. 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 to exit, 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 the 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:

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

options.stdio #

History

Added the overlapped stdio flag.

The value 0 is now accepted as a file descriptor.

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)
• ‘overlapped’ : equivalent to [‘overlapped’, ‘overlapped’, ‘overlapped’]
• ‘ignore’ : equivalent to [‘ignore’, ‘ignore’, ‘ignore’]
• ‘inherit’ : equivalent to [‘inherit’, ‘inherit’, ‘inherit’] 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:

‘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, 1, and 2 are also available as subprocess.stdin , subprocess.stdout and subprocess.stderr , respectively.

‘overlapped’ : Same as ‘pipe’ except that the FILE_FLAG_OVERLAPPED flag is set on the handle. This is necessary for overlapped I/O on the child process’s stdio handles. See the docs for more details. This is exactly the same as ‘pipe’ on non-Windows systems.

‘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() and process.disconnect() methods, as well as ‘disconnect’ and ‘message’ events within the child.

Accessing the IPC channel fd in any way other than process.send() or using the IPC channel with a child process that is not a Node.js instance is not supported.

‘ignore’ : Instructs Node.js to ignore the fd in the child. While Node.js will always open fds 0, 1, and 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.

‘inherit’ : Pass through the corresponding stdio stream to/from the parent process. In the first three positions, this is equivalent to process.stdin , process.stdout , and process.stderr , respectively. In any other position, equivalent to ‘ignore’ .

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. The stream must have an underlying descriptor (file streams do not until the ‘open’ event has occurred).

Positive integer: The integer value is interpreted as a file descriptor that is currently open in the parent process. It is shared with the child process, similar to how objects can be shared. Passing sockets is not supported on Windows.

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’ .

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 ‘disconnect’ event or the ‘message’ event. This allows the child to exit normally without the process being held open by the open IPC channel.

On Unix-like operating systems, the child_process.spawn() method performs memory operations synchronously before decoupling the event loop from the child. Applications with a large memory footprint may find frequent child_process.spawn() calls to be a bottleneck. For more information, see V8 issue 7381.

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

The input option can now be any TypedArray or a DataView .

The windowsHide option is supported now.

The input option can now be a Uint8Array .

The encoding option can now explicitly be set to buffer .

Added in: v0.11.12

• file The name or path of the executable file to run.
• args List of string arguments.
• options
  • cwd Current working directory of the child process.
  • input | | | The value which will be passed as stdin to the spawned process. Supplying this value will override stdio[0] .
  • stdio | Child’s stdio configuration. stderr by default will be output to the parent process’ stderr unless stdio is specified. Default: ‘pipe’ .
  • env Environment key-value pairs. Default: process.env .
  • uid Sets the user identity of the process (see setuid(2) ).
  • gid Sets the group identity of the process (see setgid(2) ).
  • timeout In milliseconds the maximum amount of time the process is allowed to run. Default: undefined .
  • killSignal | The signal value to be used when the spawned process will be killed. Default: ‘SIGTERM’ .
  • maxBuffer Largest amount of data in bytes allowed on stdout or stderr. If exceeded, the child process is terminated. See caveat at maxBuffer and Unicode. Default: 1024 * 1024 .
  • encoding The encoding used for all stdio inputs and outputs. Default: ‘buffer’ .
  • windowsHide Hide the subprocess console window that would normally be created on Windows systems. Default: false .
  • shell | 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).
• Returns: | 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.

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 an Error that will include the full result of the underlying child_process.spawnSync() .

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.

child_process.execSync(command[, options]) #

History

The input option can now be any TypedArray or a DataView .

The windowsHide option is supported now.

The input option can now be a Uint8Array .

Added in: v0.11.12

• command The command to run.
• options
  • cwd Current working directory of the child process.
  • input | | | The value which will be passed as stdin to the spawned process. Supplying this value will override stdio[0] .
  • stdio | Child’s stdio configuration. stderr by default will be output to the parent process’ stderr unless stdio is specified. Default: ‘pipe’ .
  • env Environment key-value pairs. Default: process.env .
  • shell Shell to execute the command with. See Shell requirements and Default Windows shell. Default: ‘/bin/sh’ on Unix, process.env.ComSpec on Windows.
  • uid Sets the user identity of the process. (See setuid(2) ).
  • gid Sets the group identity of the process. (See setgid(2) ).
  • timeout In milliseconds the maximum amount of time the process is allowed to run. Default: undefined .
  • killSignal | The signal value to be used when the spawned process will be killed. Default: ‘SIGTERM’ .
  • maxBuffer Largest amount of data in bytes allowed on stdout or stderr. If exceeded, the child process is terminated and any output is truncated. See caveat at maxBuffer and Unicode. Default: 1024 * 1024 .
  • encoding The encoding used for all stdio inputs and outputs. Default: ‘buffer’ .
  • windowsHide Hide the subprocess console window that would normally be created on Windows systems. Default: false .
• Returns: | 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. 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() .

Never pass unsanitized 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

The input option can now be any TypedArray or a DataView .

The windowsHide option is supported now.

The input option can now be a Uint8Array .

The shell option is supported now.

The encoding option can now explicitly be set to buffer .

Added in: v0.11.12

• command The command to run.
• args List of string arguments.
• options
  • cwd Current working directory of the child process.
  • input | | | The value which will be passed as stdin to the spawned process. Supplying this value will override stdio[0] .
  • argv0 Explicitly set the value of argv[0] sent to the child process. This will be set to command if not specified.
  • stdio | Child’s stdio configuration.
  • env Environment key-value pairs. Default: process.env .
  • uid Sets the user identity of the process (see setuid(2) ).
  • gid Sets the group identity of the process (see setgid(2) ).
  • timeout In milliseconds the maximum amount of time the process is allowed to run. Default: undefined .
  • killSignal | The signal value to be used when the spawned process will be killed. Default: ‘SIGTERM’ .
  • maxBuffer Largest amount of data in bytes allowed on stdout or stderr. If exceeded, the child process is terminated and any output is truncated. See caveat at maxBuffer and Unicode. Default: 1024 * 1024 .
  • encoding The encoding used for all stdio inputs and outputs. Default: ‘buffer’ .
  • shell | 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 No quoting or escaping of arguments is done on Windows. Ignored on Unix. This is set to true automatically when shell is specified and is CMD. Default: false .
  • windowsHide Hide the subprocess console window that would normally be created on Windows systems. Default: false .
• Returns:
  • pid Pid of the child process.
  • output Array of results from stdio output.
  • stdout | The contents of output[1] .
  • stderr | The contents of output[2] .
  • status | The exit code of the subprocess, or null if the subprocess terminated due to a signal.
  • signal | The signal used to kill the subprocess, or null if the subprocess did not terminate due to a signal.
  • 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. If the process intercepts and handles the SIGTERM signal and doesn’t exit, the parent process will wait until the child process has exited.

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.

Class: ChildProcess #

Instances of the ChildProcess 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 The exit code if the child exited on its own.
• signal 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’ #

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.

The ‘exit’ event may or may not fire after an error has occurred. When listening to both the ‘exit’ and ‘error’ events, guard against accidentally invoking handler functions multiple times.

Event: ‘exit’ #

• code The exit code if the child exited on its own.
• signal 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 .

When the ‘exit’ event is triggered, child process stdio streams might still be open.

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.

Event: ‘message’ #

• message A parsed JSON object or primitive value.
• sendHandle A net.Socket or net.Server object, or undefined.

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

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

If the serialization option was set to ‘advanced’ used when spawning the child process, the message argument can contain data that JSON is not able to represent. See Advanced serialization for more details.

Event: ‘spawn’ #

The ‘spawn’ event is emitted once the child process has spawned successfully. If the child process does not spawn successfully, the ‘spawn’ event is not emitted and the ‘error’ event is emitted instead.

If emitted, the ‘spawn’ event comes before all other events and before any data is received via stdout or stderr .

The ‘spawn’ event will fire regardless of whether an error occurs within the spawned process. For example, if bash some-command spawns successfully, the ‘spawn’ event will fire, though bash may fail to spawn some-command . This caveat also applies when using < shell: true >.

subprocess.channel #

History

The object no longer accidentally exposes native C++ bindings.

• 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.channel.ref() #

This method makes the IPC channel keep the event loop of the parent process running if .unref() has been called before.

subprocess.channel.unref() #

This method makes the IPC channel not keep the event loop of the parent process running, and lets it finish even while the channel is open.

subprocess.connected #

• 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() .

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.exitCode #

The subprocess.exitCode property indicates the exit code of the child process. If the child process is still running, the field will be null .

subprocess.kill([signal]) #

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. This function returns true if kill(2) succeeds, and false otherwise.

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.

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

See kill(2) for reference.

On Windows, where POSIX signals do not exist, the signal argument will be ignored, and the process will be killed forcefully and abruptly (similar to ‘SIGKILL’ ). See Signal Events for more details.

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 the use of the shell option of ChildProcess :

subprocess.killed #

• 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 #

Returns the process identifier (PID) of the child process. If the child process fails to spawn due to errors, then the value is undefined and error is emitted.

subprocess.ref() #

Calling subprocess.ref() after making a call to subprocess.unref() will restore the removed reference count for the child process, forcing the parent to wait for the child to exit before exiting itself.

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

History

The options parameter, and the keepOpen option in particular, is supported now.

This method returns a boolean for flow control now.

The callback parameter is supported now.

• message
• sendHandle
• options 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 value that can be used when passing instances of net.Socket . When true , the socket is kept open in the sending process. Default: false .
• callback
• Returns:

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 ‘message’ event.

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:

And then the child script, ‘sub.js’ might look like this:

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 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 ‘message’ event. Rather, such messages are emitted using the ‘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 ‘message’ event. Any data that is received and buffered in the socket will not be sent to the child.

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.

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:

The child would then receive the server object as:

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:

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

Do not use .maxConnections on a socket that has been passed to a subprocess. The parent cannot track when the socket is destroyed.

Any ‘message’ handlers in the subprocess should verify that socket exists, as the connection may have been closed during the time it takes to send the connection to the child.

subprocess.signalCode #

The subprocess.signalCode property indicates the signal received by the child process if any, else null .

subprocess.spawnargs #

The subprocess.spawnargs property represents the full list of command-line arguments the child process was launched with.

subprocess.spawnfile #

The subprocess.spawnfile property indicates the executable file name of the child process that is launched.

For child_process.fork() , its value will be equal to process.execPath . For child_process.spawn() , its value will be the name of the executable file. For child_process.exec() , its value will be the name of the shell in which the child process is launched.

subprocess.stderr #

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

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

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

The subprocess.stderr property can be null if the child process could not be successfully spawned.

subprocess.stdin #

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

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.

The subprocess.stdin property can be undefined if the child process could not be successfully spawned.

subprocess.stdio #

A sparse array of pipes to the child process, corresponding with positions in the stdio option passed to child_process.spawn() that have been set to the value ‘pipe’ . 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 .

The subprocess.stdio property can be undefined if the child process could not be successfully spawned.

subprocess.stdout #

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

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

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

The subprocess.stdout property can be null if the child process could not be successfully spawned.

subprocess.unref() #

By default, the parent will wait for the detached child to exit. To prevent the parent from waiting for a given subprocess to exit, 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 the parent.

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. If the shell is ‘cmd.exe’ , it should understand the /d /s /c switches and command-line parsing should be compatible.

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.

Advanced serialization #

Child processes support a serialization mechanism for IPC that is based on the serialization API of the v8 module, based on the HTML structured clone algorithm. This is generally more powerful and supports more built-in JavaScript object types, such as BigInt , Map and Set , ArrayBuffer and TypedArray , Buffer , Error , RegExp etc.

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