修改 Ubuntu的源为阿里源

安装Ubuntu 18.04后，使用国外源太慢了，修改为国内源会快很多。

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
#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