protected function buildCommandString($source)
    {
        $commandPattern = '%s %s %s %s %s';
        $command = sprintf(
        // pattern
            $commandPattern,
            // compass binary path
            $this->getBin(),
            // compass command
            'compile',
            // sources
            $source->getRealPath(),
            // destination file
            '--css-dir='.$this->getCacheDir(),
            // sass directory
            '--sass-dir='.$source->getPath()
        );

        return $command;
    }