3. Ubuntu 20.04 换源

换源步骤如下：

1. 备份原有的源

cd /etc/apt/
sudo cp sources.list sources.list.bak

2. 使用 vim 编辑源文件

sudo vim /etc/apt/sources.list

在 vim 中删除原来所有的内容，粘帖如下文本内容(下列源任选其一)。

阿里云的源：

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

清华的源：

deb 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 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

华为云镜像源:

deb https://mirrors.huaweicloud.com/ubuntu/ focal main restricted universe multiverse
deb https://mirrors.huaweicloud.com/ubuntu/ focal-security main restricted universe multiverse
deb https://mirrors.huaweicloud.com/ubuntu/ focal-updates main restricted universe multiverse
deb https://mirrors.huaweicloud.com/ubuntu/ focal-backports main restricted universe multiverse

中科大镜像源

deb https://mirrors.ustc.edu.cn/ubuntu/ focal main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ focal-security main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ focal-updates main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ focal-backports main restricted universe multiverse

换源后执行如下命令，来更新 Ubuntu 软件包的索引（元数据）

sudo apt update

准备就绪，可以使用 apt 命令来安装软件了！