换源原因

还用说吗，官方服务器在海外，速度太慢了。

更换源步骤

1.更改源文件的权限

如果是以普通用户状态进入系统的，推荐先更改源文件的独写权限，这样子方便后续维护操作。

sudo chmod 777 /etc/apt/sources.list 

2.更改源文件内容

可以利用vim等编辑器进入sources.list之后禁用默认的源，直接在默认源之前加入#即可。
然后在sources.list里面添加以下内容（阿里云）

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

更新与升级

sudo apt-get update 

sudo apt-get upgrade 

上面这两个命令的顺序尽量不要错，调换顺序的影响暂时不清楚，也不想尝试，麻烦。。