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child_process.md
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# Child process
<!--introduced_in=v0.10.0-->
> Stability: 2 - Stable
<!-- source_link=lib/child_process.js -->
The `node: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:
```js
const { spawn } = require('node:child_process');
const ls = spawn('ls', ['-lh', '/usr']);
ls.stdout.on('data', (data) => {
console.log(`stdout: ${data}`);
});
ls.stderr.on('data', (data) => {
console.error(`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 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 `env` is in the `options` object. Otherwise, `process.env.PATH` is
used. If `options.env` is set without `PATH`, lookup on Unix is performed
on a default search path search of `/usr/bin:/bin` (see your operating system's
manual for execvpe/execvp), on Windows the current processes environment
variable `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 `node: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.
```js
// On Windows Only...
const { spawn } = require('node:child_process');
const bat = spawn('cmd.exe', ['/c', 'my.bat']);
bat.stdout.on('data', (data) => {
console.log(data.toString());
});
bat.stderr.on('data', (data) => {
console.error(data.toString());
});
bat.on('exit', (code) => {
console.log(`Child exited with code ${code}`);
});
```
```js
// OR...
const { exec, spawn } = require('node:child_process');
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])`
<!-- YAML
added: v0.1.90
changes:
- version:
- v16.4.0
- v14.18.0
pr-url: https://github.com/nodejs/node/pull/38862
description: The `cwd` option can be a WHATWG `URL` object using
`file:` protocol.
- version: v15.4.0
pr-url: https://github.com/nodejs/node/pull/36308
description: AbortSignal support was added.
- version: v8.8.0
pr-url: https://github.com/nodejs/node/pull/15380
description: The `windowsHide` option is supported now.
-->
* `command` {string} The command to run, with space-separated arguments.
* `options` {Object}
* `cwd` {string|URL} Current working directory of the child process.
**Default:** `process.cwd()`.
* `env` {Object} Environment key-value pairs. **Default:** `process.env`.
* `encoding` {string} **Default:** `'utf8'`
* `shell` {string} Shell to execute the command with. See
[Shell requirements][] and [Default Windows shell][]. **Default:**
`'/bin/sh'` on Unix, `process.env.ComSpec` on Windows.
* `signal` {AbortSignal} allows aborting the child process using an
AbortSignal.
* `timeout` {number} **Default:** `0`
* `maxBuffer` {number} 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` {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](https://en.wikipedia.org/wiki/List_of_command-line_interpreters))
need to be dealt with accordingly:
```js
const { exec } = require('node:child_process');
exec('"/path/to/test file/test.sh" arg1 arg2');
// Double quotes are used so that the space in the path is not interpreted as
// a delimiter of multiple arguments.
exec('echo "The \\$HOME variable is $HOME"');
// The $HOME variable is escaped in the first instance, but not in the second.
```
**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.
```js
const { exec } = require('node:child_process');
exec('cat *.js missing_file | wc -l', (error, stdout, stderr) => {
if (error) {
console.error(`exec error: ${error}`);
return;
}
console.log(`stdout: ${stdout}`);
console.error(`stderr: ${stderr}`);
});
```
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`.
```js
const util = require('node:util');
const exec = util.promisify(require('node:child_process').exec);
async function lsExample() {
const { stdout, stderr } = await exec('ls');
console.log('stdout:', stdout);
console.error('stderr:', stderr);
}
lsExample();
```
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`:
```js
const { exec } = require('node:child_process');
const controller = new AbortController();
const { signal } = controller;
const child = exec('grep ssh', { signal }, (error) => {
console.log(error); // an AbortError
});
controller.abort();
```
### `child_process.execFile(file[, args][, options][, callback])`
<!-- YAML
added: v0.1.91
changes:
- version:
- v16.4.0
- v14.18.0
pr-url: https://github.com/nodejs/node/pull/38862
description: The `cwd` option can be a WHATWG `URL` object using
`file:` protocol.
- version:
- v15.4.0
- v14.17.0
pr-url: https://github.com/nodejs/node/pull/36308
description: AbortSignal support was added.
- version: v8.8.0
pr-url: https://github.com/nodejs/node/pull/15380
description: The `windowsHide` option is supported now.
-->
* `file` {string} The name or path of the executable file to run.
* `args` {string\[]} List of string arguments.
* `options` {Object}
* `cwd` {string|URL} Current working directory of the child process.
* `env` {Object} Environment key-value pairs. **Default:** `process.env`.
* `encoding` {string} **Default:** `'utf8'`
* `timeout` {number} **Default:** `0`
* `maxBuffer` {number} 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` {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`.
* `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).
* `signal` {AbortSignal} allows aborting the child process using an
AbortSignal.
* `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 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.
```js
const { execFile } = require('node:child_process');
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.
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`.
```js
const util = require('node:util');
const execFile = util.promisify(require('node:child_process').execFile);
async function getVersion() {
const { stdout } = await execFile('node', ['--version']);
console.log(stdout);
}
getVersion();
```
**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`:
```js
const { execFile } = require('node:child_process');
const controller = new AbortController();
const { signal } = controller;
const child = execFile('node', ['--version'], { signal }, (error) => {
console.log(error); // an AbortError
});
controller.abort();
```
### `child_process.fork(modulePath[, args][, options])`
<!-- YAML
added: v0.5.0
changes:
- version:
- v17.4.0
- v16.14.0
pr-url: https://github.com/nodejs/node/pull/41225
description: The `modulePath` parameter can be a WHATWG `URL` object using
`file:` protocol.
- version:
- v16.4.0
- v14.18.0
pr-url: https://github.com/nodejs/node/pull/38862
description: The `cwd` option can be a WHATWG `URL` object using
`file:` protocol.
- version:
- v15.13.0
- v14.18.0
pr-url: https://github.com/nodejs/node/pull/37256
description: timeout was added.
- version:
- v15.11.0
- v14.18.0
pr-url: https://github.com/nodejs/node/pull/37325
description: killSignal for AbortSignal was added.
- version:
- v15.6.0
- v14.17.0
pr-url: https://github.com/nodejs/node/pull/36603
description: AbortSignal support was added.
- version:
- v13.2.0
- v12.16.0
pr-url: https://github.com/nodejs/node/pull/30162
description: The `serialization` option is supported now.
- version: v8.0.0
pr-url: https://github.com/nodejs/node/pull/10866
description: The `stdio` option can now be a string.
- version: v6.4.0
pr-url: https://github.com/nodejs/node/pull/7811
description: The `stdio` option is supported now.
-->
* `modulePath` {string|URL} The module to run in the child.
* `args` {string\[]} List of string arguments.
* `options` {Object}
* `cwd` {string|URL} Current working directory of the child process.
* `detached` {boolean} Prepare child to run independently of its parent
process. Specific behavior depends on the platform, see
[`options.detached`][]).
* `env` {Object} Environment key-value pairs. **Default:** `process.env`.
* `execPath` {string} Executable used to create the child process.
* `execArgv` {string\[]} List of string arguments passed to the executable.
**Default:** `process.execArgv`.
* `gid` {number} Sets the group identity of the process (see setgid(2)).
* `serialization` {string} 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` {AbortSignal} Allows closing the child process using an
AbortSignal.
* `killSignal` {string|integer} The signal value to be used when the spawned
process will be killed by timeout or abort signal. **Default:** `'SIGTERM'`.
* `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']`.
* `uid` {number} Sets the user identity of the process (see setuid(2)).
* `windowsVerbatimArguments` {boolean} No quoting or escaping of arguments is
done on Windows. Ignored on Unix. **Default:** `false`.
* `timeout` {number} In milliseconds the maximum amount of time the process
is allowed to run. **Default:** `undefined`.
* 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.
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`:
```js
if (process.argv[2] === 'child') {
setTimeout(() => {
console.log(`Hello from ${process.argv[2]}!`);
}, 1_000);
} else {
const { fork } = require('node:child_process');
const controller = new AbortController();
const { signal } = controller;
const child = fork(__filename, ['child'], { signal });
child.on('error', (err) => {
// This will be called with err being an AbortError if the controller aborts
});
controller.abort(); // Stops the child process
}
```
### `child_process.spawn(command[, args][, options])`
<!-- YAML
added: v0.1.90
changes:
- version:
- v16.4.0
- v14.18.0
pr-url: https://github.com/nodejs/node/pull/38862
description: The `cwd` option can be a WHATWG `URL` object using
`file:` protocol.
- version:
- v15.13.0
- v14.18.0
pr-url: https://github.com/nodejs/node/pull/37256
description: timeout was added.
- version:
- v15.11.0
- v14.18.0
pr-url: https://github.com/nodejs/node/pull/37325
description: killSignal for AbortSignal was added.
- version:
- v15.5.0
- v14.17.0
pr-url: https://github.com/nodejs/node/pull/36432
description: AbortSignal support was added.
- version:
- v13.2.0
- v12.16.0
pr-url: https://github.com/nodejs/node/pull/30162
description: The `serialization` option is supported now.
- version: v8.8.0
pr-url: https://github.com/nodejs/node/pull/15380
description: The `windowsHide` option is supported now.
- version: v6.4.0
pr-url: https://github.com/nodejs/node/pull/7696
description: The `argv0` option is supported now.
- version: v5.7.0
pr-url: https://github.com/nodejs/node/pull/4598
description: The `shell` option is supported now.
-->
* `command` {string} The command to run.
* `args` {string\[]} List of string arguments.
* `options` {Object}
* `cwd` {string|URL} Current working directory of the child process.
* `env` {Object} Environment key-value pairs. **Default:** `process.env`.
* `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`][`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)).
* `serialization` {string} 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` {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 and is CMD. **Default:** `false`.
* `windowsHide` {boolean} Hide the subprocess console window that would
normally be created on Windows systems. **Default:** `false`.
* `signal` {AbortSignal} allows aborting the child process using an
AbortSignal.
* `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 by timeout or abort signal. **Default:** `'SIGTERM'`.
* 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.
**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:
```js
const 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. 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:
```js
const { spawn } = require('node:child_process');
const ls = spawn('ls', ['-lh', '/usr']);
ls.stdout.on('data', (data) => {
console.log(`stdout: ${data}`);
});
ls.stderr.on('data', (data) => {
console.error(`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`
```js
const { spawn } = require('node:child_process');
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.error(`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.error(`grep stderr: ${data}`);
});
grep.on('close', (code) => {
if (code !== 0) {
console.log(`grep process exited with code ${code}`);
}
});
```
Example of checking for failed `spawn`:
```js
const { spawn } = require('node:child_process');
const subprocess = spawn('bad_command');
subprocess.on('error', (err) => {
console.error('Failed to start subprocess.');
});
```
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`:
```js
const { spawn } = require('node:child_process');
const controller = new AbortController();
const { signal } = controller;
const grep = spawn('grep', ['ssh'], { signal });
grep.on('error', (err) => {
// This will be called with err being an AbortError if the controller aborts
});
controller.abort(); // Stops the child process
```
#### `options.detached`
<!-- YAML
added: 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. 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:
```js
const { spawn } = require('node:child_process');
const subprocess = spawn(process.argv[0], ['child_program.js'], {
detached: true,
stdio: 'ignore'
});
subprocess.unref();
```
Alternatively one can redirect the child process' output into files:
```js
const fs = require('node:fs');
const { spawn } = require('node:child_process');
const out = fs.openSync('./out.log', 'a');
const err = fs.openSync('./out.log', 'a');
const subprocess = spawn('prg', [], {
detached: true,
stdio: [ 'ignore', out, err ]
});
subprocess.unref();
```
#### `options.stdio`
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added: v0.7.10
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- version:
- v15.6.0
- v14.18.0
pr-url: https://github.com/nodejs/node/pull/29412
description: Added the `overlapped` stdio flag.
- version: v3.3.1
pr-url: https://github.com/nodejs/node/pull/2727
description: 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:
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]`][`subprocess.stdio`]. Pipes
created for fds 0, 1, and 2 are also available as [`subprocess.stdin`][],
[`subprocess.stdout`][] and [`subprocess.stderr`][], respectively.
Currently, these are not actual Unix pipes and therefore the child process
can not use them by their descriptor files,
e.g. `/dev/fd/2` or `/dev/stdout`.
2. `'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](https://docs.microsoft.com/en-us/windows/win32/fileio/synchronous-and-asynchronous-i-o)
for more details. This is exactly the same as `'pipe'` on non-Windows
systems.
3. `'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.
4. `'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.
5. `'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'`.
6. {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. The stream must have an
underlying descriptor (file streams do not until the `'open'` event has
occurred).
7. 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. Passing sockets
is not supported on Windows.
8. `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'`.
```js
const { spawn } = require('node:child_process');
// 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 [`'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](https://bugs.chromium.org/p/v8/issues/detail?id=7381).
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])`
<!-- YAML
added: v0.11.12
changes:
- version:
- v16.4.0
- v14.18.0
pr-url: https://github.com/nodejs/node/pull/38862
description: The `cwd` option can be a WHATWG `URL` object using
`file:` protocol.
- version: v10.10.0
pr-url: https://github.com/nodejs/node/pull/22409
description: The `input` option can now be any `TypedArray` or a
`DataView`.
- version: v8.8.0
pr-url: https://github.com/nodejs/node/pull/15380
description: The `windowsHide` option is supported now.
- version: v8.0.0
pr-url: https://github.com/nodejs/node/pull/10653
description: The `input` option can now be a `Uint8Array`.
- version:
- v6.2.1
- v4.5.0
pr-url: https://github.com/nodejs/node/pull/6939
description: The `encoding` option can now explicitly be set to `buffer`.
-->
* `file` {string} The name or path of the executable file to run.
* `args` {string\[]} List of string arguments.
* `options` {Object}
* `cwd` {string|URL} Current working directory of the child process.
* `input` {string|Buffer|TypedArray|DataView} 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. `stderr` by default will
be output to the parent process' stderr unless `stdio` is specified.
**Default:** `'pipe'`.
* `env` {Object} Environment key-value pairs. **Default:** `process.env`.
* `uid` {number} Sets the user identity of the process (see setuid(2)).
* `gid` {number} Sets the group identity of the process (see setgid(2)).
* `timeout` {number} In milliseconds the maximum amount of time the process
is allowed to run. **Default:** `undefined`.
* `killSignal` {string|integer} The signal value to be used when the spawned
process will be killed. **Default:** `'SIGTERM'`.
* `maxBuffer` {number} Largest amount of data in bytes allowed on stdout or
stderr. If exceeded, the child process is terminated. See caveat at
[`maxBuffer` and Unicode][]. **Default:** `1024 * 1024`.
* `encoding` {string} The encoding used for all stdio inputs and outputs.
**Default:** `'buffer'`.
* `windowsHide` {boolean} Hide the subprocess console window that would
normally be created on Windows systems. **Default:** `false`.
* `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).
* 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.
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])`
<!-- YAML
added: v0.11.12
changes:
- version:
- v16.4.0
- v14.18.0
pr-url: https://github.com/nodejs/node/pull/38862
description: The `cwd` option can be a WHATWG `URL` object using
`file:` protocol.
- version: v10.10.0
pr-url: https://github.com/nodejs/node/pull/22409
description: The `input` option can now be any `TypedArray` or a
`DataView`.
- version: v8.8.0
pr-url: https://github.com/nodejs/node/pull/15380
description: The `windowsHide` option is supported now.
- version: v8.0.0
pr-url: https://github.com/nodejs/node/pull/10653
description: The `input` option can now be a `Uint8Array`.
-->
* `command` {string} The command to run.
* `options` {Object}
* `cwd` {string|URL} Current working directory of the child process.
* `input` {string|Buffer|TypedArray|DataView} 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. `stderr` by default will