sudo rosdep init->Website may be down.

sudo rosdep init 
ERROR: cannot download default sources list from:
https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/sources.list.d/20-default.list
Website may be down.

先在浏览器里复制粘贴报错信息里面的地址，如果访问不了则为网络问题

sudo gedit /etc/hosts

添加下面内容到hosts文件

199.232.28.133 raw.githubusercontent.com

保存退出,重新在浏览器中访问报错信息里面的地址.