在ubuntu下用apt-get install安裝軟件時，發現package list中沒有所需的軟件，
估計可能是package list太舊了，於是需要apt-get update & apt-get upgrade。
但又怕原始的源慢，故修改為阿里雲鏡像。步驟如下：
1、備份原始源的配置文件：
sudo cp /etc/apt/sources.list /etc/apt/sources.list.bakcup
2、添加阿里雲源到sources.list文件：
sudo vim /etc/apt/sources.list
deb http://mirrors.aliyun.com/ubuntu/ xenial main restricted universe multiverse deb http://mirrors.aliyun.com/ubuntu/ xenial-security main restricted universe multiverse deb http://mirrors.aliyun.com/ubuntu/ xenial-updates main restricted universe multiverse deb http://mirrors.aliyun.com/ubuntu/ xenial-proposed main restricted universe multiverse deb http://mirrors.aliyun.com/ubuntu/ xenial-backports main restricted universe multiverse deb-src http://mirrors.aliyun.com/ubuntu/ xenial main restricted universe multiverse deb-src http://mirrors.aliyun.com/ubuntu/ xenial-security main restricted universe multiverse deb-src http://mirrors.aliyun.com/ubuntu/ xenial-updates main restricted universe multiverse deb-src http://mirrors.aliyun.com/ubuntu/ xenial-proposed main restricted universe multiverse deb-src http://mirrors.aliyun.com/ubuntu/ xenial-backports main restricted universe multiverse
3、update & upgrade
sudo apt-get update sudo apt-get upgrade
這里，再說一下update 和upgrade的區別：
update命令，會訪問源列表里的每個網址，並讀取軟件列表，然后保存在本地電腦。
upgrade命令，會把本地已安裝的軟件，與剛下載的軟件列表里對應軟件進行對比，如果發現已安裝的軟件版本太低，就會提示你更新。
總而言之，update是更新軟件列表，upgrade是更新軟件。