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execa logo

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Process execution for humans





Execa runs commands in your script, application or library. Unlike shells, it is optimized for programmatic usage. Built on top of the child_process core module.


One of the maintainers @ehmicky is looking for a remote full-time position. Specialized in Node.js back-ends and CLIs, he led Netlify Build, Plugins and Configuration for 2.5 years. Feel free to contact him on his website or on LinkedIn!


Features

Install

npm install execa

Documentation

Execution:

Input/output:

Advanced usage:

Examples

Execution

Simple syntax

import {execa} from 'execa';

const {stdout} = await execa`npm run build`;
// Print command's output
console.log(stdout);

Script

import {$} from 'execa';

const {stdout: name} = await $`cat package.json`.pipe`grep name`;
console.log(name);

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

await Promise.all([
	$`sleep 1`,
	$`sleep 2`,
	$`sleep 3`,
]);

const directoryName = 'foo bar';
await $`mkdir /tmp/${directoryName}`;

Local binaries

$ npm install -D eslint
await execa({preferLocal: true})`eslint`;

Pipe multiple subprocesses

const {stdout, pipedFrom} = await execa`npm run build`
	.pipe`sort`
	.pipe`head -n 2`;

// Output of `npm run build | sort | head -n 2`
console.log(stdout);
// Output of `npm run build | sort`
console.log(pipedFrom[0].stdout);
// Output of `npm run build`
console.log(pipedFrom[0].pipedFrom[0].stdout);

Input/output

Interleaved output

const {all} = await execa({all: true})`npm run build`;
// stdout + stderr, interleaved
console.log(all);

Programmatic + terminal output

const {stdout} = await execa({stdout: ['pipe', 'inherit']})`npm run build`;
// stdout is also printed to the terminal
console.log(stdout);

Simple input

const getInputString = () => { /* ... */ };
const {stdout} = await execa({input: getInputString()})`sort`;
console.log(stdout);

File input

// Similar to: npm run build < input.txt
await execa({stdin: {file: 'input.txt'}})`npm run build`;

File output

// Similar to: npm run build > output.txt
await execa({stdout: {file: 'output.txt'}})`npm run build`;

Split into text lines

const {stdout} = await execa({lines: true})`npm run build`;
// Print first 10 lines
console.log(stdout.slice(0, 10).join('\n'));

Streaming

Iterate over text lines

for await (const line of execa`npm run build`) {
	if (line.includes('WARN')) {
		console.warn(line);
	}
}

Transform/filter output

let count = 0;

// Filter out secret lines, then prepend the line number
const transform = function * (line) {
	if (!line.includes('secret')) {
		yield `[${count++}] ${line}`;
	}
};

await execa({stdout: transform})`npm run build`;

Web streams

const response = await fetch('https://example.com');
await execa({stdin: response.body})`sort`;

Convert to Duplex stream

import {execa} from 'execa';
import {pipeline} from 'node:stream/promises';
import {createReadStream, createWriteStream} from 'node:fs';

await pipeline(
	createReadStream('./input.txt'),
	execa`node ./transform.js`.duplex(),
	createWriteStream('./output.txt'),
);

IPC

Exchange messages

// parent.js
import {execaNode} from 'execa';

const subprocess = execaNode`child.js`;
await subprocess.sendMessage('Hello from parent');
const message = await subprocess.getOneMessage();
console.log(message); // 'Hello from child'
// child.js
import {getOneMessage, sendMessage} from 'execa';

const message = await getOneMessage(); // 'Hello from parent'
const newMessage = message.replace('parent', 'child'); // 'Hello from child'
await sendMessage(newMessage);

Any input type

// main.js
import {execaNode} from 'execa';

const ipcInput = [
	{task: 'lint', ignore: /test\.js/},
	{task: 'copy', files: new Set(['main.js', 'index.js']),
}];
await execaNode({ipcInput})`build.js`;
// build.js
import {getOneMessage} from 'execa';

const ipcInput = await getOneMessage();

Any output type

// main.js
import {execaNode} from 'execa';

const {ipcOutput} = await execaNode`build.js`;
console.log(ipcOutput[0]); // {kind: 'start', timestamp: date}
console.log(ipcOutput[1]); // {kind: 'stop', timestamp: date}
// build.js
import {sendMessage} from 'execa';

const runBuild = () => { /* ... */ };

await sendMessage({kind: 'start', timestamp: new Date()});
await runBuild();
await sendMessage({kind: 'stop', timestamp: new Date()});

Graceful termination

// main.js
import {execaNode} from 'execa';

const controller = new AbortController();
setTimeout(() => {
	controller.abort();
}, 5000);

await execaNode({
	cancelSignal: controller.signal,
	gracefulCancel: true,
})`build.js`;
// build.js
import {getCancelSignal} from 'execa';

const cancelSignal = await getCancelSignal();
const url = 'https://example.com/build/info';
const response = await fetch(url, {signal: cancelSignal});

Debugging

Detailed error

import {execa, ExecaError} from 'execa';

try {
	await execa`unknown command`;
} catch (error) {
	if (error instanceof ExecaError) {
		console.log(error);
	}
	/*
	ExecaError: Command failed with ENOENT: unknown command
	spawn unknown ENOENT
			at ...
			at ... {
		shortMessage: 'Command failed with ENOENT: unknown command\nspawn unknown ENOENT',
		originalMessage: 'spawn unknown ENOENT',
		command: 'unknown command',
		escapedCommand: 'unknown command',
		cwd: '/path/to/cwd',
		durationMs: 28.217566,
		failed: true,
		timedOut: false,
		isCanceled: false,
		isTerminated: false,
		isMaxBuffer: false,
		code: 'ENOENT',
		stdout: '',
		stderr: '',
		stdio: [undefined, '', ''],
		pipedFrom: []
		[cause]: Error: spawn unknown ENOENT
				at ...
				at ... {
			errno: -2,
			code: 'ENOENT',
			syscall: 'spawn unknown',
			path: 'unknown',
			spawnargs: [ 'command' ]
		}
	}
	*/
}

Verbose mode

await execa`npm run build`;
await execa`npm run test`;

execa verbose output

Custom logging

import {execa as execa_} from 'execa';
import {createLogger, transports} from 'winston';

// Log to a file using Winston
const transport = new transports.File({filename: 'logs.txt'});
const logger = createLogger({transports: [transport]});
const LOG_LEVELS = {
	command: 'info',
	output: 'verbose',
	ipc: 'verbose',
	error: 'error',
	duration: 'info',
};

const execa = execa_({
	verbose(verboseLine, {message, ...verboseObject}) {
		const level = LOG_LEVELS[verboseObject.type];
		logger[level](message, verboseObject);
	},
});

await execa`npm run build`;
await execa`npm run test`;

Related

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