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Process execution for humans

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

^{Special thanks to:}

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.

Features

• Simple syntax: promises and template strings, like zx.
• Script interface.
• No escaping nor quoting needed. No risk of shell injection.
• Execute locally installed binaries without npx.
• Improved Windows support: shebangs, PATHEXT, graceful termination, and more.
• Detailed errors, verbose mode and custom logging, for debugging.
• Pipe multiple subprocesses better than in shells: retrieve intermediate results, use multiple sources/destinations, unpipe.
• Split the output into text lines, or iterate progressively over them.
• Strip unnecessary newlines.
• Pass any input to the subprocess: files, strings, Uint8Arrays, iterables, objects and almost any other type.
• Return almost any type from the subprocess, or redirect it to files.
• Get interleaved output from stdout and stderr similar to what is printed on the terminal.
• Retrieve the output programmatically and print it on the console at the same time.
• Transform or filter the input and output with simple functions.
• Pass Node.js streams or web streams to subprocesses, or convert subprocesses to a stream.
• Exchange messages with the subprocess.
• Ensure subprocesses exit even when they intercept termination signals, or when the current process ends abruptly.

Install

npm install execa

Documentation

Execution:

• ▶️ Basic execution
• 💬 Escaping/quoting
• 💻 Shell
• 📜 Scripts
• 🐢 Node.js files
• 🌐 Environment
• ❌ Errors
• 🏁 Termination

Input/output:

• 🎹 Input
• 📢 Output
• 📃 Text lines
• 🤖 Binary data
• 🧙 Transforms

Advanced usage:

• 🔀 Piping multiple subprocesses
• ⏳️ Streams
• 📞 Inter-process communication
• 🐛 Debugging
• 📎 Windows
• 🔍 Difference with Bash and zx
• 🐭 Small packages
• 🤓 TypeScript
• 📔 API reference

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`;

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

• nano-spawn - Like Execa but smaller
• gulp-execa - Gulp plugin for Execa
• nvexeca - Run Execa using any Node.js version

Maintainers

• Sindre Sorhus
• @ehmicky