ubantu国内镜像

Ubuntu 国内镜像有很多，比如阿里云、清华大学、中科大等等。以下是阿里云的 Ubuntu 镜像源：

1. 首先备份原有的源列表文件：`sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak`
2. 然后打开 `/etc/apt/sources.list` 文件：`sudo vim /etc/apt/sources.list`
3. 将以下内容复制到文件中并保存：

   # 阿里云 Ubuntu 镜像源
   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-backports main restricted universe multiverse
   deb-src http://mirrors.aliyun.com/ubuntu/ focal-backports 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

4. 最后更新一下源列表：`sudo apt-get update`