安装ros-melodic命令行：
$ sudo rosdep init
$ rosdep update报错：
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.
解决方案：
$cd /etc/ros/rosdep/sources.list.d
#创建文件
$sudo gedit 20-default.list 写入下面文字：
# os-specific listings first
yaml https://raw.github.com/ros/rosdistro/master/rosdep/osx-homebrew.yaml osx# generic
yaml https://raw.github.com/ros/rosdistro/master/rosdep/base.yaml
yaml https://raw.github.com/ros/rosdistro/master/rosdep/python.yaml
yaml https://raw.github.com/ros/rosdistro/master/rosdep/ruby.yaml
gbpdistro https://raw.github.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
保存文件
运行：
$rosdep update
完成通过！！！
参考
1.
https://blog.csdn.net/u013468614/article/details/102917569