1. 更换清华源

输入下面的命令

sudo apt update# 将 sources.list 拷贝到桌面
cp /etc/apt/sources.list ~/Desktop # 打开 sources.list 进行编辑
sudo gedit /etc/apt/sources.list

打开文件后，将里面的所有内容替换为之前网页内文本框里的内容，例如

# 默认注释了源码镜像以提高 apt update 速度，如有需要可自行取消注释
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-backports main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-security main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-security main restricted universe multiverse# 预发布软件源，不建议启用
# deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-proposed main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ focal-proposed main restricted universe multiverse

2. 添加安装ROS的软件源

sudo sh -c '. /etc/lsb-release && echo "deb http://mirrors.ustc.edu.cn/ros/ubuntu/ $DISTRIB_CODENAME main" > /etc/apt/sources.list.d/ros-latest.list'

3.添加密钥

sudo apt-key adv --keyserver 'hkp://keyserver.ubuntu.com:80' --recv-key C1CF6E31E6BADE8868B172B4F42ED6FBAB17C654

4.更新软件源

sudo apt update

5. 安装ROS

sudo apt install ros-noetic-desktop-full

sudo apt install python3-pip

sudo pip3 install rosdepc

sudo rosdepc initrosdepc update

6. 设置环境变量

echo "source /opt/ros/noetic/setup.bash" >> ~/.bashrcsource ~/.bashrc

7. 安装rosinstall

sudo apt install python3-rosinstall python3-rosinstall-generator python3-wstool

8. 测试roscore

roscore

出现下面这样的就是正确的

打开第二个终端窗口，输入：

rosrun turtlesim turtlesim_node

小海龟测试成功

参考如下：
【ROS】在 Ubuntu 20.04 安装 ROS 的详细教程
ROS1 基础与常用命令