execa

What is execa?

The execa npm package is a process execution tool that simplifies working with child processes in Node.js. It provides a better user experience than the default child_process module by offering a promise-based API, improved Windows support, and additional convenience options.

What are execa's main functionalities?

Executing a shell command

This feature allows you to execute a shell command and obtain the result. The example shows how to execute the 'echo' command and print 'unicorns' to the console.

const execa = require('execa');

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

Running a command synchronously

This feature is used to execute a command synchronously, blocking the event loop until the process has finished. The example synchronously executes the 'echo' command and logs the result.

const execa = require('execa');

const { stdout } = execa.sync('echo', ['unicorns']);
console.log(stdout);

Handling errors

This feature demonstrates error handling when a command fails to execute. The example attempts to run a non-existent command and catches the error.

const execa = require('execa');

(async () => {
  try {
    const { stdout } = await execa('wrong-command');
    console.log(stdout);
  } catch (error) {
    console.error('Error occurred:', error);
  }
})();

Streaming output

This feature allows you to stream the output of a command directly to the console or another stream. The example streams the output of the 'echo' command to the process's stdout.

const execa = require('execa');

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

Other packages similar to execa

shelljs

ShellJS is a portable Unix shell commands implementation for Node.js. It offers a higher-level API for executing commands but does not support returning promises natively.

cross-spawn

Cross-spawn is a cross-platform solution for spawning child processes. It aims to solve compatibility issues on Windows but does not provide a promise-based API.

README

execa

A better child_process

Why

• Promise interface.
• Strips EOF from the output so you don't have to stdout.trim().
• Supports shebang binaries cross-platform.
• Improved Windows support.
• Higher max buffer. 10 MB instead of 200 KB.
• Executes locally installed binaries by name.
• Cleans up spawned processes when the parent process dies.

Install

$ npm install execa

Usage

const execa = require('execa');

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

Additional examples:

const execa = require('execa');

(async () => {
	// Pipe the child process stdout to the current stdout
	execa('echo', ['unicorns']).stdout.pipe(process.stdout);


	// Run a shell command
	const {stdout} = await execa.shell('echo unicorns');
	//=> 'unicorns'


	// Catching an error
	try {
		await execa.shell('exit 3');
	} catch (error) {
		console.log(error);
		/*
		{
			message: 'Command failed: /bin/sh -c exit 3'
			killed: false,
			code: 3,
			signal: null,
			cmd: '/bin/sh -c exit 3',
			stdout: '',
			stderr: '',
			timedOut: false
		}
		*/
	}
})();

// Catching an error with a sync method
try {
	execa.shellSync('exit 3');
} catch (error) {
	console.log(error);
	/*
	{
		message: 'Command failed: /bin/sh -c exit 3'
		code: 3,
		signal: null,
		cmd: '/bin/sh -c exit 3',
		stdout: '',
		stderr: '',
		timedOut: false
	}
	*/
}

API

execa(file, [arguments], [options])

Execute a file.

Think of this as a mix of child_process.execFile and child_process.spawn.

Returns a child_process instance, which is enhanced to also be a Promise for a result Object with stdout and stderr properties.

execa.stdout(file, [arguments], [options])

Same as execa(), but returns only stdout.

execa.stderr(file, [arguments], [options])

Same as execa(), but returns only stderr.

execa.shell(command, [options])

Execute a command through the system shell. Prefer execa() whenever possible, as it's both faster and safer.

Returns a child_process instance.

The child_process instance is enhanced to also be promise for a result object with stdout and stderr properties.

execa.sync(file, [arguments], [options])

Execute a file synchronously.

Returns the same result object as child_process.spawnSync.

This method throws an Error if the command fails.

execa.shellSync(file, [options])

Execute a command synchronously through the system shell.

Returns the same result object as child_process.spawnSync.

options

Type: Object

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.

extendEnv

Type: boolean
Default: true

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

argv0

Type: string

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

stdio

Type: string[] string
Default: pipe

Child's stdio configuration.

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

stripEof

Type: boolean
Default: true

Strip EOF (last newline) from the output.

preferLocal

Type: boolean
Default: true

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).

input

Type: string Buffer stream.Readable

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

reject

Type: boolean
Default: true

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

cleanup

Type: boolean
Default: true

Keep track of the spawned process and kill it when the parent process exits.

encoding

Type: string
Default: utf8

Specify the character encoding used to decode the stdout and stderr output.

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.

buffer

Type: boolean
Default: true

Buffer the output from the spawned process. When buffering is disabled you must consume the output of the stdout and stderr streams because the promise will not be resolved/rejected until they have completed.

maxBuffer

Type: number
Default: 10000000 (10MB)

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.

stdin

Type: string number Stream undefined null
Default: pipe

Same options as stdio.

stdout

Type: string number Stream undefined null
Default: pipe

Same options as stdio.

stderr

Type: string number Stream undefined null
Default: pipe

Same options as stdio.

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.

Tips

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 getStream = require('get-stream');

const stream = execa('echo', ['foo']).stdout;

stream.pipe(process.stdout);

getStream(stream).then(value => {
	console.log('child output:', value);
});

License

MIT © Sindre Sorhus