protected function varnishCheckCommand()
    {
        $this->readProperties();

        $drushVersion = $this->taskDrushStack()
            ->drupalRootDirectory($this->getConfig()->get('digipolis.root.web'))
            ->getVersion();
        if (version_compare($drushVersion, '9.0', '<')) {
            return 'bash -c "[[ '
                . '\'$ENABLED_MODULES\' =~ \((varnish|purge)\) '
                . '&& \'$ENABLED_MODULES\' =~ \(page_cache\)'
                . ' ]]" && exit 1 || :';
        }
        return 'bash -c "[[ '
            . '\'$ENABLED_MODULES\' =~ (varnish|purge) '
            . '&& \'$ENABLED_MODULES\' =~ page_cache'
            . ' ]]" && exit 1 || :';
    }