index.js

import process from 'node:process';
import fs, {promises as fsPromises} from 'node:fs';
import execa from 'execa';

function extractDarwin(line) {
	const columns = line.split(':');

	// Darwin passwd(5)
	// 0 name      User's login name.
	// 1 password  User's encrypted password.
	// 2 uid       User's id.
	// 3 gid       User's login group id.
	// 4 class     User's general classification (unused).
	// 5 change    Password change time.
	// 6 expire    Account expiration time.
	// 7 gecos     User's full name.
	// 8 home_dir  User's home directory.
	// 9 shell     User's login shell.

	return {
		username: columns[0],
		password: columns[1],
		userIdentifier: Number(columns[2]),
		groupIdentifier: Number(columns[3]),
		fullName: columns[7],
		homeDirectory: columns[8],
		shell: columns[9],
	};
}

function extractLinux(line) {
	const columns = line.split(':');

	// Linux passwd(5):
	// 0 login name
	// 1 optional encrypted password
	// 2 numerical user ID
	// 3 numerical group ID
	// 4 user name or comment field
	// 5 user home directory
	// 6 optional user command interpreter

	return {
		username: columns[0],
		password: columns[1],
		userIdentifier: Number(columns[2]),
		groupIdentifier: Number(columns[3]),
		fullName: columns[4] && columns[4].split(',')[0],
		homeDirectory: columns[5],
		shell: columns[6],
	};
}

const extract = process.platform === 'linux' ? extractLinux : extractDarwin;

function getUser(passwd, username) {
	const lines = passwd.split('\n');
	const linesCount = lines.length;
	let index = 0;

	while (index < linesCount) {
		const user = extract(lines[index++]);

		if (user.username === username || user.userIdentifier === Number(username)) {
			return user;
		}
	}
}

export async function passwdUser(username) {
	if (username === undefined) {
		if (typeof process.getuid !== 'function') {
			// eslint-disable-next-line unicorn/prefer-type-error
			throw new Error('Platform not supported');
		}

		username = process.getuid();
	}

	if (process.platform === 'linux') {
		return getUser(await fsPromises.readFile('/etc/passwd', 'utf8'), username);
	}

	if (process.platform === 'darwin') {
		const {stdout} = await execa('/usr/bin/id', ['-P', username]);
		return getUser(stdout, username);
	}

	throw new Error('Platform not supported');
}

export function passwdUserSync(username) {
	if (username === undefined) {
		if (typeof process.getuid !== 'function') {
			// eslint-disable-next-line unicorn/prefer-type-error
			throw new Error('Platform not supported');
		}

		username = process.getuid();
	}

	if (process.platform === 'linux') {
		return getUser(fs.readFileSync('/etc/passwd', 'utf8'), username);
	}

	if (process.platform === 'darwin') {
		return getUser(execa.sync('/usr/bin/id', ['-P', username]).stdout, username);
	}

	throw new Error('Platform not supported');
}