code: func prepareCommandLine(cmd string, args ...string) (string, error) {
  	if isBlank(cmd) {
  		return "", errors.AssertionErrorf("command cannot be empty")
  	}
  
  	for i, arg := range args {
  		if strings.ContainsRune(arg, '"') {
  			return "", errors.Errorf(errors.ParseError, "arg at index %d contains an invalid double quote: %s", i, arg)
  		}
  		if containsWhitespace(arg) {
  			args[i] = fmt.Sprintf("\"%s\"", arg)
  		}
  	}
  
  	// Prepend the command to the args array.
  	if len(args) > 0 {
  		cmd = fmt.Sprintf("%s %s", cmd, strings.Join(args, " "))
  	}
  
  	return cmd, nil
  }
docs: // prepareCommandLine validates the command and argument strings, quotes
  // arguments if required, and joins them into a valid adb command string.