在用Ubuntu18.04根據官方文檔安裝ROS（Melodic Morenia）時出現了問題（Ubuntu16.04+Kinetic Kame 並未出現這種問題）
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.
已有的解決方案：
https://zhuanlan.zhihu.com/p/77483614
我嘗試了上述辦法，發現均不成功。即使自己建立/ros/rosdep/sources.list.d/20-default.list並寫入文件依然不行。
對比錯誤原因：
已有解決方案：
rosdep update 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]: <urlopen error [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:590)> (https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/osx-homebrew.yaml) ERROR: unable to process source [https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/base.yaml]: <urlopen error [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:590)> (https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/base.yaml) ERROR: unable to process source [https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/python.yaml]: <urlopen error [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:590)> (https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/python.yaml) ERROR: unable to process source [https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/ruby.yaml]: <urlopen error [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:590)> (https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/ruby.yaml) ERROR: unable to process source [https://raw.githubusercontent.com/ros/rosdistro/master/releases/fuerte.yaml]: Failed to download target platform data for gbpdistro: <urlopen error [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:590)> Query rosdistro index https://raw.githubusercontent.com/ros/rosdistro/master/index-v4.yaml ERROR: error loading sources list: <urlopen error <urlopen error [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:590)> (https://raw.githubusercontent.com/ros/rosdistro/master/index-v4.yaml)>
自己嘗試結果：
rosdep update reading in sources list data from /etc/ros/rosdep/sources.list.d Warning: running 'rosdep update' as root is not recommended. You should run 'sudo rosdep fix-permissions' and invoke 'rosdep update' again without sudo. ERROR: unable to process source [https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/osx-homebrew.yaml]: <urlopen error [Errno 111] Connection refused> (https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/osx-homebrew.yaml) ERROR: unable to process source [https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/base.yaml]: <urlopen error [Errno 111] Connection refused> (https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/base.yaml) ERROR: unable to process source [https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/python.yaml]: <urlopen error [Errno 111] Connection refused> (https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/python.yaml) ERROR: unable to process source [https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/ruby.yaml]: <urlopen error [Errno 111] Connection refused> (https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/ruby.yaml) ERROR: unable to process source [https://raw.githubusercontent.com/ros/rosdistro/master/releases/fuerte.yaml]: Failed to download target platform data for gbpdistro: <urlopen error [Errno 111] Connection refused> Query rosdistro index https://raw.githubusercontent.com/ros/rosdistro/master/index-v4.yaml ERROR: error loading sources list: <urlopen error <urlopen error [Errno 111] Connection refused> (https://raw.githubusercontent.com/ros/rosdistro/master/index-v4.yaml)>
判斷原因還是無法連接https://raw.githubusercontent.com。
最終采取方案為：
https://blog.csdn.net/u013468614/article/details/102917569
#打開hosts文件 sudo gedit /etc/hosts #在文件末尾添加 151.101.84.133 raw.githubusercontent.com #保存后退出再嘗試
完成上述步驟后重新 rosdep init和 rosdep update均正常
得到下面的結果：
rosdep update reading in sources list data from /etc/ros/rosdep/sources.list.d Warning: running 'rosdep update' as root is not recommended. You should run 'sudo rosdep fix-permissions' and invoke 'rosdep update' again without sudo. #這里我是采用root用戶導致的警告，可以無視 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 Skip end-of-life distro "ardent" Skip end-of-life distro "bouncy" Skip end-of-life distro "crystal" Add distro "dashing" Add distro "eloquent" Add distro "foxy" Skip end-of-life distro "groovy" Skip end-of-life distro "hydro" Skip end-of-life distro "indigo" Skip end-of-life distro "jade" Add distro "kinetic" Skip end-of-life distro "lunar" Add distro "melodic" Add distro "noetic" updated cache in /root/.ros/rosdep/sources.cache
其他參考博客：