Ubuntu换源

$ sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak
$ sudo sed -i 's/security.ubuntu/mirrors.aliyun/g' /etc/apt/sources.list
$ sudo sed -i 's/archive.ubuntu/mirrors.aliyun/g' /etc/apt/sources.list
$ sudo apt update
$ sudo apt-get upgrade	//更新已安装的包到最新，这个是可选的