1.ubuntu18.04安装ROS
一开始我是直接找安装说明的博客，后来发现还是有不少问题的，就比如秘钥的问题，很有可能不一样。但是官网的一定是准确的，所以以后安装还是参考官网比较好。
http://wiki.ros.org/melodic/Installation/Ubuntu
2.初始化 rosdep init 报错
感谢知乎文章https://zhuanlan.zhihu.com/p/77483614
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.
这个问题真的把我坑惨了，也是因为这个问题，让我下定决心写博客记录自己踩过的坑.....
我搜了好多方法来解决这个问题，现在大致总结如下：
(1) ca-certificates证书的问题，需要重新安装
sudo apt-get install ca-certificates
(2) 还是ssl certs问题
sudo c_rehash /etc/ssl/certs
sudo -E rosdep init
(3) 发现还是有问题
然后我连上VPN，直接在chrome浏览器输入这个链接：https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/sources.list.d/20-default.list
发现是一个文本文件，既然下载不了，那就自己sudo建一下
全程sudo，建立一个文件夹/etc/ros/rosdep/sources.list.d
然后写入文件
sudo gedit 20-default.list
输入网址里面的内容，保存
# os-specific listings first
yaml https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/osx-homebrew.yaml osx
# generic
yaml https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/base.yaml
yaml https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/python.yaml
yaml https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/ruby.yaml
gbpdistro https://raw.githubusercontent.com/ros/rosdistro/master/releases/fuerte.yaml fuerte
# newer distributions (Groovy, Hydro, ...) must not be listed anymore, they are being fetched from the rosdistro index.yaml instead
完事再进行下一步,
rosdep update
3.rosdep update报错
reading in sources list data from /etc/ros/rosdep/sources.list.d
ERROR: unable to process source [[https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/osx-homebrew.yaml](http://jump.bdimg.com/safecheck/index?url=rN3wPs8te/pX/RXZrrwmgz1HhLKFfuYdBR6elVnGSRMPTZwDs+V1Iw2DN2GIDg0c0lXFaWYCi6F5eugIuTik7Xf24HznH1heOw2gAEfI/e4pyVgDjNm3SaoaxT39lV2h1C6ZRqu3Ohsi0ZhQy1cgW5DaxsU0VVq099hJLALtyPraHkhVQYEleySdzQDlWLfuyu4YXs62n7WWp87t6F1nMhvv0JflI+P2dj2oeHoEzTI=)]:
<urlopen error _ssl.c:495: The handshake operation timed out> ([https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/osx-homebrew.yaml](http://jump.bdimg.com/safecheck/index?url=rN3wPs8te/pX/RXZrrwmgz1HhLKFfuYdBR6elVnGSRMPTZwDs+V1Iw2DN2GIDg0c0lXFaWYCi6F5eugIuTik7Xf24HznH1heOw2gAEfI/e4pyVgDjNm3SaoaxT39lV2h1C6ZRqu3Ohsi0ZhQy1cgW5DaxsU0VVq099hJLALtyPraHkhVQYEleySdzQDlWLfuyu4YXs62n7WWp87t6F1nMhvv0JflI+P2dj2oeHoEzTI=))
Hit [https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/base.yaml](http://jump.bdimg.com/safecheck/index?url=rN3wPs8te/pX/RXZrrwmgz1HhLKFfuYdBR6elVnGSRMPTZwDs+V1Iw2DN2GIDg0c0lXFaWYCi6F5eugIuTik7Xf24HznH1heOw2gAEfI/e64EpZhV/nSSfaMCRlMqNMjqsw5JFtciolkdT5HNLb22Z8xZQwNWZn8DhrNz7QOmul3vNSiiiuylvEgZ+MKQ61lxfjiJ1xO6k1T0sjgIIm+zjA8Zu4mdgY0)
ERROR: error loading sources list:
The read operation timed out
解决办法如下
sudo apt-get update
sudo apt-get install python-rosdep
