code: private function getProcessOutput(): ProcessOutputInterface
      {
          if (is_null($this->output)) {
              $this->output = new ProcessOutput($this->exitCode, $this->fullStdOut, $this->fullStdErr);
              $this->observer->processExited($this->pid, $this->output);
          }
  
          return $this->output;
      }
docs: Returns the output of the process (exit code, stdout & stderr).