Install Extension (via CLI) for Joomla 3.9

vdmInstallExtension.php

<?php
/**
 * @package    VDM Install Extension (via CLI)
 *
 * @author     Llewellyn van der Merwe <https://www.vdm.io>
 * @gitHub     https://gist.github.com/Llewellynvdm/fb2a8bcd24c42a955b67ec3154aa7b54
 * @copyright  Copyright (C) 2020 Vast Development Method. All rights reserved.
 * @license    GNU General Public License version 2 or later
 */

// Set flag that this is a parent file.
const _JEXEC = 1;
// turn debug off/on
const JDEBUG = 1;

error_reporting(E_ALL | E_NOTICE);
ini_set('display_errors', 1);

// Load system defines
if (file_exists(dirname(__DIR__) . '/defines.php'))
{
	require_once dirname(__DIR__) . '/defines.php';
}

if (!defined('_JDEFINES'))
{
	define('JPATH_BASE', dirname(__DIR__));
	require_once JPATH_BASE . '/includes/defines.php';
}

require_once JPATH_LIBRARIES . '/import.legacy.php';
require_once JPATH_LIBRARIES . '/cms.php';

// Load the configuration
require_once JPATH_CONFIGURATION . '/configuration.php';

/**
 * CLI Install Extension
 */
class vdmInstallExtension extends JApplicationCli
{

	/**
	 * Joomla! Site Application
	 *
	 * @var    JApplicationSite
	 */
	protected $app = null;

	public function doExecute()
	{
		$_SERVER['HTTP_HOST'] = 'localhost';
		$this->app            = JFactory::getApplication('site');
		// stout an empty line
		$this->out('');
		// Get Extension Folder Path
		if (($path = $this->input->get('p', false, 'PATH')) !== false)
		{
			// Start the Installation
			$this->out('Starting Install....... ', false);
			// Get the Installer Instance
			$installer = JInstaller::getInstance();
			// Install
			if ($installer->install($path))
			{
				// Give Completion Notice
				$this->out(
					'Successfully Completed Installation of ' . JText::_(
						$installer->manifest->name
					) . '!'
				);
				// check if we have a message
				if (isset($installer->message) && strlen($installer->message))
				{
					// setup a little component message with frame (nice to have)
					$message = '  | ' . implode(
							PHP_EOL . '  | ', array_filter(
							array_map(
								"trim", explode(
									PHP_EOL, strip_tags($installer->message)
								)
							), function ($line) {
							return strlen($line);
						}));
					$this->out('');
					$this->out(' _|______________________________', false);
					$this->out('_______________________ vdm.io _________');
					$this->out('  |');
					$this->out($message);
					$this->out(' _|______________________________', false);
					$this->out('________________________________________');
					$this->out('  |');
				}
			}
			else
			{
				// Give Error Notice
				if (isset($installer->manifest->name))
				{
					$this->out(
						'Installation of ' . JText::_(
							$installer->manifest->name
						) . ' FAILED!'
					);
				}
				else
				{
					$this->out(
						'Installation FAILED! (try removing the trail slash from your path)'
					);
				}
			}
		}
		else
		{
			$this->out('');
			$this->out(' _|______________________________', false);
			$this->out('_______________________ vdm.io _________');
			$this->out('  |');
			$this->out('  | THE PATH IS MISSING!');
			$this->out(
				'  | The path to your extension folder must be added via the -p tag'
			);
			$this->out('  |');
			$this->out('  | Example: php ./vdmInstallExtension.php -p "/path/to/extension_folder"'
			);
			$this->out(' _|______________________________', false);
			$this->out('________________________________________');
			$this->out('  |');
		}
		// stout an empty line
		$this->out('');
	}
}

JApplicationCli::getInstance('vdmInstallExtension')->execute();