You don't have write permissions

执行 gem install fus 时，得到如下提示：
、、、
ERROR: While executing gem ... (Gem::FilePermissionError)
You don't have write permissions for the /Library/Ruby/Gems/2.3.0 directory.
、、、

字面意思：没有安装权限，执行 sudo gem install fus 就可以了。输入密码后可正常安装。