composer/src/Composer/Command/GlobalCommand.php

<?php

/*
 * This file is part of Composer.
 *
 * (c) Nils Adermann <naderman@naderman.de>
 *     Jordi Boggiano <j.boggiano@seld.be>
 *
 * For the full copyright and license information, please view the LICENSE
 * file that was distributed with this source code.
 */

namespace Composer\Command;

use Composer\Installer;
use Composer\Factory;
use Symfony\Component\Console\Input\InputInterface;
use Symfony\Component\Console\Input\InputArgument;
use Symfony\Component\Console\Input\StringInput;
use Symfony\Component\Console\Output\OutputInterface;

/**
 * @author Jordi Boggiano <j.boggiano@seld.be>
 */
class GlobalCommand extends Command
{
    protected function configure()
    {
        $this
            ->setName('global')
            ->setDescription('Allows running commands in the global composer dir ($COMPOSER_HOME).')
            ->setDefinition(array(
                new InputArgument('command-name', InputArgument::REQUIRED, ''),
                new InputArgument('args', InputArgument::IS_ARRAY | InputArgument::OPTIONAL, ''),
            ))
            ->setHelp(<<<EOT
Use this command as a wrapper to run other Composer commands
within the global context of COMPOSER_HOME.

You can use this to install CLI utilities globally, all you need
is to add the COMPOSER_HOME/vendor/bin dir to your PATH env var.

COMPOSER_HOME is c:\Users\<user>\AppData\Roaming\Composer on Windows
and /home/<user>/.composer on unix systems.

Note: This path may vary depending on customizations to bin-dir in 
composer.json or the environmental variable COMPOSER_BIN_DIR.

EOT
            )
        ;
    }

    public function run(InputInterface $input, OutputInterface $output)
    {
        // extract real command name
        $tokens = preg_split('{\s+}', $input->__toString());
        $args = array();
        foreach ($tokens as $token) {
            if ($token && $token[0] !== '-') {
                $args[] = $token;
                if (count($args) >= 2) {
                    break;
                }
            }
        }

        // show help for this command if no command was found
        if (count($args) < 2) {
            return parent::run($input, $output);
        }

        // change to global dir
        $config = Factory::createConfig();
        chdir($config->get('home'));

        // create new input without "global" command prefix
        $input = new StringInput(preg_replace('{\bg(?:l(?:o(?:b(?:a(?:l)?)?)?)?)?\b}', '', $input->__toString(), 1));

        return $this->getApplication()->get($args[1])->run($input, $output);
    }
}