macOS10.14.2 gem 更新问题

macOS10.14.2，最近cocoapods不能正常使用了。
终端输入
sudo gem update –system
显示如下错误
ERROR: While executing gem … (Gem::FilePermissionError)
You don’t have write permissions for the /usr/bin directory.

重新输入下面
sudo gem install -n /usr/local/bin sass
sudo gem install -n /usr/local/bin cocoapods