重要！重要！重要！
解决melodic过程中遇到的问题。
执行命令：
rosdep update
出现错误如下：（
注意：命令中不需要sudo
）
意思就是网速太慢了，下载不了20-default.list这个文件。
自己在路径下（/etc/ros/rosdep/sources.list.d）创建这个文件。
mkdir -r /etc/ros/rosdep/sources.list.d可以复制添加文件内容
# os-specific listings first
# yaml https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/osx-homebrew.yaml osx
yaml file:/etc/ros/rosdep/sources.list.d/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
在/etc/resolv.conf将原先的nameserver屏蔽，添加两个nameserver
打开etc/hosts文件添加
还有的教程是修改时间值，我改了没有用
解决掉20-default.list这个文件。
执行rosdep update更新不成功的问题。
太多方法都没解决。
解决方法：
1、下载rosdistro-master.zip
链接：https://pan.baidu.com/s/1WSZRVfL0cld0WIzhrYKkBw
提取码：zxcv
2、修改20-default.list文件内容如下
# os-specific listings first
# yaml https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/osx-homebrew.yaml osx
yaml file:/etc/ros/rosdep/sources.list.d/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
yaml file:/etc/ros/rosdep/sources.list.d/rosdistro-master/rosdep/base.yaml
yaml file:/etc/ros/rosdep/sources.list.d/rosdistro-master/rosdep/python.yaml
yaml file:/etc/ros/rosdep/sources.list.d/rosdistro-master/rosdep/ruby.yaml
gbpdistro file:/etc/ros/rosdep/sources.list.d/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
3、将下载好的rosdistro-master.zip解压后复制到路径（/etc/ros/rosdep/sources.list.d）下，
执行命令：
该路径中文件情况
【关键步骤】下面执行3个命令修改其中的网址信息
DEFAULT_SOURCES_LIST_URL行的信息
执行命令，
REP3_TARGETS_URL行
执行命令，
DEFAULT_INDEX_URL行
3条命令修改完成后，执行
rosdep update出现下述界面说明安装成功。
接下来测试安装环境：
我擦，这么情况，继续解决
继续执行命令：
sudo apt-get install ros-melodic-desktop
待安装完成后
再行执行roscore
打开3个终端，测试小乌龟
ROS环境搭建完成。