code: func usage(errorMessage string) {
  	appName := filepath.Base(os.Args[0])
  	appName = strings.TrimSuffix(appName, filepath.Ext(appName))
  	fmt.Fprintln(os.Stderr, errorMessage)
  	fmt.Fprintln(os.Stderr, "Usage:")
  	fmt.Fprintf(os.Stderr, "  %s [OPTIONS] <command> <args...>\n\n",
  		appName)
  	fmt.Fprintln(os.Stderr, showHelpMessage)
  	fmt.Fprintln(os.Stderr, listCmdMessage)
  }
docs: // usage displays the general usage when the help flag is not displayed and
  // and an invalid command was specified.  The commandUsage function is used
  // instead when a valid command was specified.