在 VMware 上安装了一个 Ubuntu20.04 的虚拟机， lsb_release -a 显示如下信息：

    1. $ lsb_release -a
    2. No LSB modules are available.
    3. Distributor ID: Ubuntu
    4. Description:    Ubuntu 20.04 LTS
    5. Release:        20.04
    6. Codename:       focal

    该系统里自带了国内阿里的 apt 镜像源：

    1. $ cat /etc/apt/sources.list
    2. deb http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse
    4. deb-src http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse
    8. deb http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
    10. deb-src http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
    14. deb http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
    16. deb-src http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
    18. deb http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse
    20. deb-src http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse
    22. deb http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse
    24. deb-src http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse

    虽然上面的地址对应的是 Codename 为 bionic（18.04.5 LTS）的 Ubuntu 版本，而 Ubuntu 20.04 LTS 的 Codename 为 focal，然而能正常工作。
    反而网上 focal 版本的阿里镜像源不能工作~~