修改 Ubuntu的源為阿里源

安裝Ubuntu 18.04后，使用國外源太慢了，修改為國內源會快很多。

修改阿里源為Ubuntu 18.04默認的源

備份/etc/apt/sources.list
#備份
cp /etc/apt/sources.list /etc/apt/sources.list.bak

在/etc/apt/sources.list文件前面添加如下條目
#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

最后執行如下命令更新源
##更新
sudo apt-get update
sudo apt-get upgrade

apt-get三個重要命令

apt-get update 更新軟件源中的所有軟件列表。 
apt-get upgrade 更新軟件。 
apt-get dist-upgrade 更新系統版本。如果你對新版本軟件的需求不是那么迫切，可以不執行