Process execution for humans

---

^{Sindre's open source work is supported by the community}

^{Special thanks to:}

---

Why

This package improves child_process methods with:

• Promise interface.
• Scripts interface, like zx.
• Improved Windows support, including shebang binaries.
• Executes locally installed binaries without npx.
• Cleans up child processes when the parent process ends.
• Graceful termination.
• Get interleaved output from stdout and stderr similar to what is printed on the terminal.
• Strips the final newline from the output so you don't have to do stdout.trim().
• Convenience methods to pipe processes' input and output.
• Can specify file and arguments as a single string without a shell.
• Verbose mode for debugging.
• More descriptive errors.
• Higher max buffer: 100 MB instead of 1 MB.

Install

npm install execa

Usage

Promise interface

import {execa} from 'execa';

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

Scripts interface

For more information about Execa scripts, please see this page.

Basic

import {$} from 'execa';

const branch = await $`git branch --show-current`;
await $`dep deploy --branch=${branch}`;

Multiple arguments

import {$} from 'execa';

const args = ['unicorns', '&', 'rainbows!'];
const {stdout} = await $`echo ${args}`;
console.log(stdout);
//=> 'unicorns & rainbows!'

With options

import {$} from 'execa';

await $({stdio: 'inherit'})`echo unicorns`;
//=> 'unicorns'

Shared options

import {$} from 'execa';

const $$ = $({stdio: 'inherit'});

await $$`echo unicorns`;
//=> 'unicorns'

await $$`echo rainbows`;
//=> 'rainbows'

Verbose mode

> node file.js
unicorns
rainbows

> NODE_DEBUG=execa node file.js
[16:50:03.305] echo unicorns
unicorns
[16:50:03.308] echo rainbows
rainbows

Input/output

Redirect output to a file

import {execa} from 'execa';

// Similar to `echo unicorns > stdout.txt` in Bash
await execa('echo', ['unicorns']).pipeStdout('stdout.txt');

// Similar to `echo unicorns 2> stdout.txt` in Bash
await execa('echo', ['unicorns']).pipeStderr('stderr.txt');

// Similar to `echo unicorns &> stdout.txt` in Bash
await execa('echo', ['unicorns'], {all: true}).pipeAll('all.txt');

Redirect input from a file

import {execa} from 'execa';

// Similar to `cat < stdin.txt` in Bash
const {stdout} = await execa('cat', {inputFile: 'stdin.txt'});
console.log(stdout);
//=> 'unicorns'

Save and pipe output from a child process

import {execa} from 'execa';

const {stdout} = await execa('echo', ['unicorns']).pipeStdout(process.stdout);
// Prints `unicorns`
console.log(stdout);
// Also returns 'unicorns'

Pipe multiple processes

import {execa} from 'execa';

// Similar to `echo unicorns | cat` in Bash
const {stdout} = await execa('echo', ['unicorns']).pipeStdout(execa('cat'));
console.log(stdout);
//=> 'unicorns'

Handling Errors

import {execa} from 'execa';

// 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',
		escapedCommand: 'unknown command',
		stdout: '',
		stderr: '',
		failed: true,
		timedOut: false,
		isCanceled: false,
		killed: false
	}
	*/
}

Graceful termination

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

const subprocess = execa('node');

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

API

Methods

execa(file, arguments?, options?)

Executes a command using file ...arguments. arguments are specified as an array of strings. Returns a childProcess.

Arguments are automatically escaped. They can contain any character, including spaces.

This is the preferred method when executing single commands.

execaNode(scriptPath, arguments?, options?)

Executes a Node.js file using node scriptPath ...arguments. arguments are specified as an array of strings. Returns a childProcess.

Arguments are automatically escaped. They can contain any character, including spaces.

This is the preferred method when executing Node.js files.

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

$`command`

Executes a command. The command string includes both the file and its arguments. Returns a childProcess.

Arguments are automatically escaped. They can contain any character, but spaces must use ${} like $`echo ${'has space'}`.

This is the preferred method when executing multiple commands in a script file.

The command string can inject any ${value} with the following types: string, number, childProcess or an array of those types. For example: $`echo one ${'two'} ${3} ${['four', 'five']}`. For ${childProcess}, the process's stdout is used.

For more information, please see this section and this page.

$(options)

Returns a new instance of $ but with different default options. Consecutive calls are merged to previous ones.

This can be used to either:

• Set options for a specific command: $(options)`command`
• Share options for multiple commands: const $$ = $(options); $$`command`; $$`otherCommand`;

execaCommand(command, options?)

Executes a command. The command string includes both the file and its arguments. Returns a childProcess.

Arguments are automatically escaped. They can contain any character, but spaces must be escaped with a backslash like execaCommand('echo has\\ space').

This is the preferred method when executing a user-supplied command string, such as in a REPL.

execaSync(file, arguments?, options?)

Same as execa() but synchronous.

Returns or throws a childProcessResult.

$.sync`command`

Same as $`command` but synchronous.

Returns or throws a childProcessResult.

execaCommandSync(command, options?)

Same as execaCommand() but synchronous.

Returns or throws a childProcessResult.

Shell syntax

For all the methods above, no shell interpreter (Bash, cmd.exe, etc.) is used unless the shell option is set. This means shell-specific characters and expressions ($variable, &&, ||, ;, |, etc.) have no special meaning and do not need to be escaped.

childProcess

The return value of all asynchronous methods is both:

• a Promise resolving or rejecting with a childProcessResult.
• a child_process instance with 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.

Note that this graceful termination does not work on Windows, because Windows doesn't support signals (SIGKILL and SIGTERM has the same effect of force-killing the process immediately.) If you want to achieve graceful termination on Windows, you have to use other means, such as taskkill.

options.forceKillAfterTimeout

Type: number | false
Default: 5000

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

Can be disabled with false.

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

pipeStdout(target)

Pipe the child process's stdout to target, which can be:

• Another execa() return value
• A writable stream
• A file path string

If the target is another execa() return value, it is returned. Otherwise, the original execa() return value is returned. This allows chaining pipeStdout() then awaiting the final result.

The stdout option must be kept as pipe, its default value.

pipeStderr(target)

Like pipeStdout() but piping the child process's stderr instead.

The stderr option must be kept as pipe, its default value.

pipeAll(target)

Combines both pipeStdout() and pipeStderr().

Either the stdout option or the stderr option must be kept as pipe, their default value. Also, the all option must be set to true.

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, for logging purposes.

This is not escaped and should not be executed directly as a process, including using execa() or execaCommand().

escapedCommand

Type: string

Same as command but escaped.

This is meant to be copy and pasted into a shell, for debugging purposes. Since the escaping is fairly basic, this should not be executed directly as a process, including using execa() or execaCommand().

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)
• execaSync() 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.

You can cancel the spawned process using the signal option.

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.

cwd

Type: string

The cwd of the command if provided in the command options. Otherwise it is process.cwd().

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: true with $, false otherwise

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

localDir

Type: string | URL
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 | Uint8Array | stream.Readable

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

If the input is a file, use the inputFile option instead.

inputFile

Type: string

Use a file as input to the the stdin of your binary.

If the input is not a file, use the input option instead.

stdin

Type: string | number | Stream | undefined
Default: inherit with $, pipe otherwise

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 | URL
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 execaNode(): - json: Uses JSON.stringify() and JSON.parse(). - advanced: Uses v8.serialize()

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 'buffer' or 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.

signal

Type: AbortSignal

You can abort the spawned process using AbortController.

When AbortController.abort() is called, .isCanceled becomes true.

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.

verbose

Type: boolean
Default: false

Print each command on stderr before executing it.

This can also be enabled by setting the NODE_DEBUG=execa environment variable in the current process.

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:

import pRetry from 'p-retry';

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

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

Cancelling a spawned process

import {execa} from 'execa';

const abortController = new AbortController();
const subprocess = execa('node', [], {signal: abortController.signal});

setTimeout(() => {
	abortController.abort();
}, 1000);

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

Execute the current package's binary

import {getBinPath} from 'get-bin-path';

const binPath = await getBinPath();
await 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