Ubuntu 18.04.4 LTS 更換國內系統源
1.1) 好習慣先做備份在干活：
mv /etc/apt/sources.list /etc/apt/sources.list.bak
1.2) 查看ubuntu版本:
lsb_release -c | grep -o "\s.*"
bionic
1.3) 阿里雲的源(將下列內容復制/etc/apt/sources.list)
vim /etc/apt/sources.list
*** 注意將$SYS_VERSION替換為上一步中獲取的版本信息。***這點非常重要，否則會失敗。(我這里的版本是：bionic) ***
## 阿里雲的源(將下列內容復制/etc/apt/sources.list)：
deb http://mirrors.aliyun.com/ubuntu/ $SYS_VERSION main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ $SYS_VERSION main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ $SYS_VERSION-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ $SYS_VERSION-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ $SYS_VERSION-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ $SYS_VERSION-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ $SYS_VERSION-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ $SYS_VERSION-backports main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ $SYS_VERSION-proposed main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ $SYS_VERSION-proposed main restricted universe multiverse
1.4) 執行更新命令：
apt-get update
