- Ubuntu20.04版本安装ros系统时碰到了与博主 "菜鸟不在家" 相同的问题，也在这里总结和记录一下。
参考链接：Ubuntu20.04系统ros-neotic版本的安装与安装过程中遇到的一些问题（纯小白教程）https://blog.csdn.net/weixin_43433726/article/details/106899359
问题：按照ROS官网的安装步骤，输入 sudo rosdep init 后，提示找不到命令。
- 系统版本：Ubuntu20.04
- ROS版本：Noetic Ninjemys
- 要注意的是Ubuntu的版本与要安装的ros版本一定要对应（如Ubuntu20.04对应安装Noetic Ninjemys；Ubuntu18.04对应安装Melodic Morenia），具体可以百度搜索。
解决办法：
a) 首先输入：
sudo apt install python3-rosdep
注意不是 python2 ，而是 python3 。
b) 再输入：
sudo rosdep init
出现报错：
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.
（根据博主 "菜鸟不在家" 的百度，有答主回答说是因为国内防火墙的原因， raw.githubusercontent.com 这个网站进不去，要下载的内容下载不了。）
解决办法是修改 hosts 文件。
c) 修改 hosts 文件：
- 打开 terminal，输入：
sudo gedit /etc/hosts
- 将以下内容写入hosts中：
192.30.253.118 gist.github.com
185.199.110.153 github.io
151.101.113.194 github.global.ssl.fastly.net
52.216.227.168 github-cloud.s3.amazonaws.com
52.74.223.119 github.com
199.232.28.133 avatars1.githubusercontent.com
199.232.28.133 avatars2.githubusercontent.com
199.232.28.133 avatars0.githubusercontent.com
199.232.28.133 avatars3.githubusercontent.com
199.232.28.133 raw.githubusercontent.com
199.232.28.133 user-images.githubusercontent.com
199.232.28.133 avatars.githubusercontent.com
199.232.28.133 github.map.fastly.net
199.232.28.133 avatars7.githubusercontent.com
#Amazon AWS Start
54.239.31.69 aws.amazon.com
54.239.30.25 console.aws.amazon.com
54.239.96.90 ap-northeast-1.console.aws.amazon.com
54.240.226.81 ap-southeast-1.console.aws.amazon.com
54.240.193.125 ap-southeast-2.console.aws.amazon.com
54.239.54.102 eu-central-1.console.aws.amazon.com
177.72.244.194 sa-east-1.console.aws.amazon.com
176.32.114.59 eu-west-1.console.aws.amazon.com
54.239.31.128 us-west-1.console.aws.amazon.com
54.240.254.230 us-west-2.console.aws.amazon.com
54.239.38.102 s3-console-us-standard.console.aws.amazon.com
54.231.49.3 s3.amazonaws.com
52.219.0.4 s3-ap-northeast-1.amazonaws.com
54.231.242.170 s3-ap-southeast-1.amazonaws.com
54.231.251.21 s3-ap-southeast-2.amazonaws.com
54.231.193.37 s3-eu-central-1.amazonaws.com
52.218.16.140 s3-eu-west-1.amazonaws.com
52.92.72.2 s3-sa-east-1.amazonaws.com
54.231.236.6 s3-us-west-1.amazonaws.com
54.231.168.160 s3-us-west-2.amazonaws.com
52.216.80.48 github-cloud.s3.amazonaws.com
54.231.40.3 github-com.s3.amazonaws.com
52.216.20.171 github-production-release-asset-2e65be.s3.amazonaws.com
52.216.228.168 github-production-user-asset-6210df.s3.amazonaws.com
d) 再输入：
sudo rosdep init
到这里问题得以解决，接着ROS官网（http://wiki.ros.org/noetic/Installation/Ubuntu）的安装步骤进行安装，顺利完成。
