关于ubuntu18.04.04安装ROS sudo rosdep init 报错ERROR: cannot download default sources list from:

ubuntu18.04安装ros到此步骤出现问题。

sudo rosdep init

问题：
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.
该问题主要是访问不了上述链接，因此在这一步无法下载20-default.list这个文件。
解决方法1：
将上述网站ip添加进hosts

sudo gedit /etc/hosts

添加内容：

# GitHub Start
199.232.28.133 raw.githubusercontent.com

接着步骤正常运行

sudo rosdep init

如下显示即导入正常。

Wrote /etc/ros/rosdep/sources.list.d/20-default.list
Recommended: please run

	rosdep update

接着：

rosdep update

错误1：

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]:
	 (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
Add distro "ardent"
ERROR: error loading sources list:
	('The read operation timed out',)

错误2：

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
Add distro "ardent"
Add distro "bouncy"
Add distro "crystal"
Add distro "dashing"
Add distro "eloquent"
Add distro "foxy"
Add distro "groovy"
Add distro "hydro"
ERROR: error loading sources list:
	('The read operation timed out',)

等等错误，反正各式各样的错误，但是有成功的就说明步骤上没有问题。

打开手机热点，连接手机热点进行运行rosdep update

我可能进行了大约十次左右才完全成功。

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
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 /home/zyh/.ros/rosdep/sources.cache

亲测可用，另外网上还有一个手动更改这个链接的方法，没有成功，可能出现此错误的原因多种多样，解决办法不一样。

zyh@zyh-dp:~$ roscore
... logging to /home/zyh/.ros/log/a564b614-8456-11ea-bcc9-84ef1810a437/roslaunch-zyh-dp-19269.log
Checking log directory for disk usage. This may take a while.
Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.

started roslaunch server http://zyh-dp:34049/
ros_comm version 1.14.5


SUMMARY
========

PARAMETERS
 * /rosdistro: melodic
 * /rosversion: 1.14.5

NODES

auto-starting new master
process[master]: started with pid [19280]
ROS_MASTER_URI=http://zyh-dp:11311/

setting /run_id to a564b614-8456-11ea-bcc9-84ef1810a437
process[rosout-1]: started with pid [19291]
started core service [/rosout]
^C[rosout-1] killing on exit
[master] killing on exit
shutting down processing monitor...
... shutting down processing monitor complete
done