code: func (c *LogCmd) Output() ([]byte, error) {
  	st := time.Now()
  	data, err := c.Cmd.Output()
  	if err != nil {
  		ee, ok := err.(*exec.ExitError)
  		if ok {
  			c.log(st, err, ee.Stderr)
  			return data, err
  		}
  	}
  	c.log(st, err, nil)
  	return data, err
  }
docs: // Output overrides exec.Cmd.Output to record the result.
  // If Cmd.Stderr is nil, Output logs outputs to stderr as well.