1.  ubuntu 配置静态 ip ;   2. 修改为 动态ip  ;  3. ubuntu20 阿里云 apt 源 ;  4. ubuntu20 清华 apt 源。 apt update 与 apt upgrade 区别：只查看更新，和 更新软件包   

 #    修改为静态指定ip

ip a                           # 通过ip a 查看网卡名字，然后再下面配置中将 eth0 改为正确的网卡名

nano /etc/netplan/01-network-manager-all.yaml

network:
  ethernets:
    enp1s0:
        addresses: [192.168.122.80/24]
        dhcp4: no
        optional: true
        gateway4: 192.168.122.1
        nameservers: 
          addresses: [192.168.122.1,223.5.5.5]
  version: 2    
  renderer: NetworkManager

  # renderer 可不加,部分服务器加上这个参数网卡无法启动

  # 223.5.5.5  网络速度慢只有1M/s 速度，建议dns 修改为网关一个ip

sudo netplan apply                       # 生效配置

 #    修改动态获取ip

vi /etc/netplan/01-network-manager-all.yaml 

network:

  ethernets:

    eth0:

      dhcp4: true

  version: 2

sudo netplan apply                       # 生效配置

 #    ubuntu20 阿里云 apt 源

阿里云源 选择系统版本， 镜像仓库：https://developer.aliyun.com/mirror/

vi /etc/apt/sources.list

deb http://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse

deb-src http://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse

deb http://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse

deb-src http://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse

deb http://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse

deb-src http://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse

deb http://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse

deb-src http://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse

deb http://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse

deb-src http://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse

 #    ubuntu20 清华 apt 源

清华apt 源选择版本：https://mirrors.tuna.tsinghua.edu.cn/help/ubuntu/

vi /etc/apt/sources.list

# 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

apt update        # 命令更新软件源(只检查不更新)

apt upgrade      # 更新已安装的软件包