查看版本：
lsb_release -c
返回：Codename: cosmic
替换中科大 18.04源：
##中科大源
deb https://mirrors.ustc.edu.cn/ubuntu/ cosmic main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ cosmic main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ cosmic-updates main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ cosmic-updates main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ cosmic-backports main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ cosmic-backports main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ cosmic-security main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ cosmic-security main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ cosmic-proposed main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ cosmic-proposed main restricted universe multiverse
写入 /etc/apt/sources.list
