最近在Ubuntu18.04安装ROS melodic时踩下的坑
问题1.rosdep init
ERROR: cannot download default sources list from:
这是因为网络无法访问raw.githubusercontent.com网站
（1）首先在https://site.ip138.com/raw.Githubusercontent.com/网站中查询raw.githubusercontent.com网址的IP，然后找一个可以用ping+ip查询可以网络延迟较小的IP并记录
（2）修改hosts文件
sudo vi /etc/hosts在hosts中添加如下内容
# GitHub Start
52.74.223.119 github.com
192.30.253.119 gist.github.com
54.169.195.247 api.github.com
185.199.111.153 assets-cdn.github.com
151.101.76.133 raw.githubusercontent.com
151.101.108.133 user-images.githubusercontent.com
151.101.76.133 gist.githubusercontent.com
151.101.76.133 cloud.githubusercontent.com
151.101.76.133 camo.githubusercontent.com
151.101.76.133 avatars0.githubusercontent.com
151.101.76.133 avatars1.githubusercontent.com
151.101.76.133 avatars2.githubusercontent.com
151.101.76.133 avatars3.githubusercontent.com
151.101.76.133 avatars4.githubusercontent.com
151.101.76.133 avatars5.githubusercontent.com
151.101.76.133 avatars6.githubusercontent.com
151.101.76.133 avatars7.githubusercontent.com
151.101.76.133 avatars8.githubusercontent.com
# GitHub End
问题2：在rosdep update时出现如下错误
reading in sources list data from /etc/ros/rosdep/sources.list.d
Hit https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/osx-homebrew.yaml
Hit https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/base.yaml
Hit https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/python.yaml
Hit https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/ruby.yaml
Hit https://raw.githubusercontent.com/ros/rosdistro/master/releases/fuerte.yaml
Query rosdistro index https://raw.githubusercontent.com/ros/rosdistro/master/index-v4.yaml
ERROR: error loading sources list:
('The read operation timed out',)
这是网络问题。可以科学上网一下或者更换手机热点多试几次。