修改阿里源为Ubuntu 18.04默认的源
//备份/etc/apt/sources.list
//备份
#]cp /etc/apt/sources.list /etc/apt/sources.list.bak
//在/etc/apt/sources.list文件前面添加如下条目(原有源url可注释掉)
//添加阿里源
deb http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
deb 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 main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse
//最后执行如下命令更新源
//更新
#]sudo apt-get update
#]sudo apt-get upgrade
//apt-get三个重要命令
#]apt-get update //更新软件源中的所有软件列表。
#]apt-get upgrade //更新软件。
#]apt-get dist-upgrade //更新系统版本。如果你对新版本软件的需求不是那么迫切，可以不执行