code: public function getVersion()
      {
          if (empty($this->drushVersion)) {
              $isPrinted = $this->isPrinted;
              $this->isPrinted = false;
              $result = $this->executeCommand($this->executable . ' version');
              $output = $result->getMessage();
              $this->drushVersion = 'unknown';
              if (preg_match('#[0-9.]+#', $output, $matches)) {
                  $this->drushVersion = $matches[0];
              }
              $this->isPrinted = $isPrinted;
          }
  
          return $this->drushVersion;
      }
docs: Returns the drush version.
  
  @return string