Process execution for humans

Why

This package improves child_process methods with:

• Promise interface.
• Strips the final newline from the output so you don't have to do stdout.trim().
• Supports shebang binaries cross-platform.
• Improved Windows support.
• Higher max buffer. 100 MB instead of 200 KB.
• Executes locally installed binaries by name.
• Cleans up spawned processes when the parent process dies.
• Get interleaved output from stdout and stderr similar to what is printed on the terminal. (Async only)
• Can specify file and arguments as a single string without a shell
• More descriptive errors.

Install

$ npm install execa

Usage

const execa = require('execa');

(async () => {
	const {stdout} = await execa('echo', ['unicorns']);
	console.log(stdout);
	//=> 'unicorns'
})();

Pipe the child process stdout to the parent

const execa = require('execa');

execa('echo', ['unicorns']).stdout.pipe(process.stdout);

Handling Errors

const execa = require('execa');

(async () => {
	// Catching an error
	try {
		await execa('unknown', ['command']);
	} catch (error) {
		console.log(error);
		/*
		{
			message: 'Command failed with ENOENT: unknown command spawn unknown ENOENT',
			errno: -2,
			code: 'ENOENT',
			syscall: 'spawn unknown',
			path: 'unknown',
			spawnargs: ['command'],
			originalMessage: 'spawn unknown ENOENT',
			shortMessage: 'Command failed with ENOENT: unknown command spawn unknown ENOENT',
			command: 'unknown command',
			stdout: '',
			stderr: '',
			all: '',
			failed: true,
			timedOut: false,
			isCanceled: false,
			killed: false
		}
		*/
	}

})();

Cancelling a spawned process

const execa = require('execa');

(async () => {
	const subprocess = execa('node');

	setTimeout(() => {
		subprocess.cancel();
	}, 1000);

	try {
		await subprocess;
	} catch (error) {
		console.log(subprocess.killed); // true
		console.log(error.isCanceled); // true
	}
})()

Catching an error with the sync method

try {
	execa.sync('unknown', ['command']);
} catch (error) {
	console.log(error);
	/*
	{
		message: 'Command failed with ENOENT: unknown command spawnSync unknown ENOENT',
		errno: -2,
		code: 'ENOENT',
		syscall: 'spawnSync unknown',
		path: 'unknown',
		spawnargs: ['command'],
		originalMessage: 'spawnSync unknown ENOENT',
		shortMessage: 'Command failed with ENOENT: unknown command spawnSync unknown ENOENT',
		command: 'unknown command',
		stdout: '',
		stderr: '',
		all: '',
		failed: true,
		timedOut: false,
		isCanceled: false,
		killed: false
	}
	*/
}

Kill a process

Using SIGTERM, and after 2 seconds, kill it with SIGKILL.

const subprocess = execa('node');

setTimeout(() => {
	subprocess.kill('SIGTERM', {
		forceKillAfterTimeout: 2000
	});
}, 1000);

API

execa(file, arguments, options?)

Execute a file. Think of this as a mix of child_process.execFile() and child_process.spawn().

No escaping/quoting is needed.

Unless the shell option is used, no shell interpreter (Bash, cmd.exe, etc.) is used, so shell features such as variables substitution (echo $PATH) are not allowed.

Returns a child_process instance which:

• is also a Promise resolving or rejecting with a childProcessResult.
• exposes the following additional methods and properties.

kill(signal?, options?)

Same as the original child_process#kill() except: if signal is SIGTERM (the default value) and the child process is not terminated after 5 seconds, force it by sending SIGKILL.

options.forceKillAfterTimeout

Type: number | false
Default: 5000

Milliseconds to wait for the child process to terminate before sending SIGKILL.

Can be disabled with false.

cancel()

Similar to childProcess.kill(). This is preferred when cancelling the child process execution as the error is more descriptive and childProcessResult.isCanceled is set to true.

all

Type: ReadableStream | undefined

Stream combining/interleaving stdout and stderr.

This is undefined if either:

• the all option is false (the default value)
• both stdout and stderr options are set to 'inherit', 'ipc', Stream or integer

execa.sync(file, arguments?, options?)

Execute a file synchronously.

Returns or throws a childProcessResult.

execa.command(command, options?)

Same as execa() except both file and arguments are specified in a single command string. For example, execa('echo', ['unicorns']) is the same as execa.command('echo unicorns').

If the file or an argument contains spaces, they must be escaped with backslashes. This matters especially if command is not a constant but a variable, for example with __dirname or process.cwd(). Except for spaces, no escaping/quoting is needed.

The shell option must be used if the command uses shell-specific features, as opposed to being a simple file followed by its arguments.

execa.commandSync(command, options?)

Same as execa.command() but synchronous.

Returns or throws a childProcessResult.

execa.node(scriptPath, arguments?, options?)

Execute a Node.js script as a child process.

Same as execa('node', [scriptPath, ...arguments], options) except (like child_process#fork()):

• the current Node version and options are used. This can be overridden using the nodePath and nodeOptions options.
• the shell option cannot be used
• an extra channel ipc is passed to stdio

childProcessResult

Type: object

Result of a child process execution. On success this is a plain object. On failure this is also an Error instance.

The child process fails when:

• its exit code is not 0
• it was killed with a signal
• timing out
• being canceled
• there's not enough memory or there are already too many child processes

command

Type: string

The file and arguments that were run.

exitCode

Type: number

The numeric exit code of the process that was run.

stdout

Type: string | Buffer

The output of the process on stdout.

stderr

Type: string | Buffer

The output of the process on stderr.

all

Type: string | Buffer | undefined

The output of the process with stdout and stderr interleaved.

This is undefined if either:

• the all option is false (the default value)
• execa.sync() was used

failed

Type: boolean

Whether the process failed to run.

timedOut

Type: boolean

Whether the process timed out.

isCanceled

Type: boolean

Whether the process was canceled.

killed

Type: boolean

Whether the process was killed.

signal

Type: string | undefined

The name of the signal that was used to terminate the process. For example, SIGFPE.

If a signal terminated the process, this property is defined and included in the error message. Otherwise it is undefined.

signalDescription

Type: string | undefined

A human-friendly description of the signal that was used to terminate the process. For example, Floating point arithmetic error.

If a signal terminated the process, this property is defined and included in the error message. Otherwise it is undefined. It is also undefined when the signal is very uncommon which should seldomly happen.

message

Type: string

Error message when the child process failed to run. In addition to the underlying error message, it also contains some information related to why the child process errored.

The child process stderr then stdout are appended to the end, separated with newlines and not interleaved.

shortMessage

Type: string

This is the same as the message property except it does not include the child process stdout/stderr.

originalMessage

Type: string | undefined

Original error message. This is the same as the message property except it includes neither the child process stdout/stderr nor some additional information added by Execa.

This is undefined unless the child process exited due to an error event or a timeout.

options

Type: object

cleanup

Type: boolean
Default: true

Kill the spawned process when the parent process exits unless either: - the spawned process is detached - the parent process is terminated abruptly, for example, with SIGKILL as opposed to SIGTERM or a normal exit

preferLocal

Type: boolean
Default: false

Prefer locally installed binaries when looking for a binary to execute.
If you $ npm install foo, you can then execa('foo').

localDir

Type: string
Default: process.cwd()

Preferred path to find locally installed binaries in (use with preferLocal).

execPath

Type: string
Default: process.execPath (Current Node.js executable)

Path to the Node.js executable to use in child processes.

This can be either an absolute path or a path relative to the cwd option.

Requires preferLocal to be true.

For example, this can be used together with get-node to run a specific Node.js version in a child process.

buffer

Type: boolean
Default: true

Buffer the output from the spawned process. When set to false, you must read the output of stdout and stderr (or all if the all option is true). Otherwise the returned promise will not be resolved/rejected.

If the spawned process fails, error.stdout, error.stderr, and error.all will contain the buffered data.

input

Type: string | Buffer | stream.Readable

Write some input to the stdin of your binary.
Streams are not allowed when using the synchronous methods.

stdin

Type: string | number | Stream | undefined
Default: pipe

Same options as stdio.

stdout

Type: string | number | Stream | undefined
Default: pipe

Same options as stdio.

stderr

Type: string | number | Stream | undefined
Default: pipe

Same options as stdio.

all

Type: boolean
Default: false

Add an .all property on the promise and the resolved value. The property contains the output of the process with stdout and stderr interleaved.

reject

Type: boolean
Default: true

Setting this to false resolves the promise with the error instead of rejecting it.

stripFinalNewline

Type: boolean
Default: true

Strip the final newline character from the output.

extendEnv

Type: boolean
Default: true

Set to false if you don't want to extend the environment variables when providing the env property.

Execa also accepts the below options which are the same as the options for child_process#spawn()/child_process#exec()

cwd

Type: string
Default: process.cwd()

Current working directory of the child process.

env

Type: object
Default: process.env

Environment key-value pairs. Extends automatically from process.env. Set extendEnv to false if you don't want this.

argv0

Type: string

Explicitly set the value of argv[0] sent to the child process. This will be set to file if not specified.

stdio

Type: string | string[]
Default: pipe

Child's stdio configuration.

serialization

Type: string
Default: 'json'

Specify the kind of serialization used for sending messages between processes when using the stdio: 'ipc' option or execa.node(): - json: Uses JSON.stringify() and JSON.parse(). - advanced: Uses v8.serialize()

Requires Node.js 13.2.0 or later.

More info.

detached

Type: boolean

Prepare child to run independently of its parent process. Specific behavior depends on the platform.

uid

Type: number

Sets the user identity of the process.

gid

Type: number

Sets the group identity of the process.

shell

Type: boolean | string
Default: false

If true, runs file inside of a shell. Uses /bin/sh on UNIX and cmd.exe on Windows. A different shell can be specified as a string. The shell should understand the -c switch on UNIX or /d /s /c on Windows.

We recommend against using this option since it is:

• not cross-platform, encouraging shell-specific syntax.
• slower, because of the additional shell interpretation.
• unsafe, potentially allowing command injection.

encoding

Type: string | null
Default: utf8

Specify the character encoding used to decode the stdout and stderr output. If set to null, then stdout and stderr will be a Buffer instead of a string.

timeout

Type: number
Default: 0

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.

maxBuffer

Type: number
Default: 100_000_000 (100 MB)

Largest amount of data in bytes allowed on stdout or stderr.

killSignal

Type: string | number
Default: SIGTERM

Signal value to be used when the spawned process will be killed.

windowsVerbatimArguments

Type: boolean
Default: false

If true, no quoting or escaping of arguments is done on Windows. Ignored on other platforms. This is set to true automatically when the shell option is true.

windowsHide

Type: boolean
Default: true

On Windows, do not create a new console window. Please note this also prevents CTRL-C from working on Windows.

nodePath (For .node() only)

Type: string
Default: process.execPath

Node.js executable used to create the child process.

nodeOptions (For .node() only)

Type: string[]
Default: process.execArgv

List of CLI options passed to the Node.js executable.

Tips

Retry on error

Gracefully handle failures by using automatic retries and exponential backoff with the p-retry package:

const pRetry = require('p-retry');

const run = async () => {
	const results = await execa('curl', ['-sSL', 'https://sindresorhus.com/unicorn']);
	return results;
};

(async () => {
	console.log(await pRetry(run, {retries: 5}));
})();

Save and pipe output from a child process

Let's say you want to show the output of a child process in real-time while also saving it to a variable.

const execa = require('execa');

const subprocess = execa('echo', ['foo']);
subprocess.stdout.pipe(process.stdout);

(async () => {
	const {stdout} = await subprocess;
	console.log('child output:', stdout);
})();

Redirect output to a file

const execa = require('execa');

const subprocess = execa('echo', ['foo'])
subprocess.stdout.pipe(fs.createWriteStream('stdout.txt'))

Redirect input from a file

const execa = require('execa');

const subprocess = execa('cat')
fs.createReadStream('stdin.txt').pipe(subprocess.stdin)

Execute the current package's binary

const {getBinPathSync} = require('get-bin-path');

const binPath = getBinPathSync();
const subprocess = execa(binPath);

execa can be combined with get-bin-path to test the current package's binary. As opposed to hard-coding the path to the binary, this validates that the package.json bin field is correctly set up.

Related

• gulp-execa - Gulp plugin for execa
• nvexeca - Run execa using any Node.js version
• sudo-prompt - Run commands with elevated privileges.

Maintainers

• Sindre Sorhus
• @ehmicky