ROS安装方法：ros安装后，初始化时rosdep update出错解决办法(2021-06-30)
初始化时rosdep update出错解决办法
2021年以前，通过科学上网、手机开热点等方式，还是可以安装上ROS的，但从2021年开始，突然发觉ubuntu上安装ROS愈发困难，科学上网、手机开热点等方式基本也不能成功安装ROS，不是timeout就是website refuse。因此在分析ROS安装方式后，尝试不通过访问github.com的方式安装ROS。经过亲测，发现同样可行。但前提是，你已经获得了ROS的相关安装文件。下载路径：https://github.com/ros/rosdistro。
下面开始对安装ROS过程中最后两步出现的问题，说明解决方法。
1.执行 sudo rosdep init 失败
原理分析：
sudo rosdep init 操作最主要的工作就是从https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/sources.list.d/20-default.list，下载文件20-default.list，并把它放到/etc/ros/rosdep/sources.list.d/这个目录下。
20-default.list 文件的內容是：
# 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
2. 执行rosdep update失败
原理分析：
rosdep update 操作最主要的工作就是根据从20-default.list 文件的內容下载相应文件，但是因为网络问题，从raw.githubusercontent.com/下载文件通常失败，采用科学上网可以成功，但也可能要尝试很多次。
基于上述分析，既然无法直接从网络上下载文件，那么可以把相关文件下载到本地，然后直接从本地获取。
修改20-default.list脚本文件，需要超级权限。可以跟前面的脚本内容做下对比。脚本内容如下：
# os-specific listings first
#yaml https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/osx-homebrew.yaml osx
yaml file:///etc/ros/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/rosdistro/master/rosdep/base.yaml
yaml file:///etc/ros/rosdistro/master/rosdep/python.yaml
yaml file:///etc/ros/rosdistro/master/rosdep/ruby.yaml
yaml file:///etc/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
退出保存后。
sudo 打开/usr/lib/python2.7/dist-packages/rosdistro/init.py这个文件，按照下面的代码进行修改。
# index information
#DEFAULT_INDEX_URL = 'https://raw.githubusercontent.com/ros/rosdistro/master/index-v4.yaml'
DEFAULT_INDEX_URL = 'file:///etc/ros/rosdistro/master/index-v4.yaml'
保存后，执行rosdep update。
如果一次不成功，再执行个一两次，应该就可以成功，执行結果如下：
reading in sources list data from /etc/ros/rosdep/sources.list.d
Hit file:///etc/ros/rosdistro/master/rosdep/osx-homebrew.yaml
Hit file:///etc/ros/rosdistro/master/rosdep/base.yaml
Hit file:///etc/ros/rosdistro/master/rosdep/python.yaml
Hit file:///etc/ros/rosdistro/master/rosdep/ruby.yaml
Hit file:///etc/ros/rosdistro/master/releases/fuerte.yaml
Query rosdistro index file:///etc/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/xxxx/.ros/rosdep/sources.cache
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