預設下載源很慢，改成阿里的下載速度超快
sudo vim /etc/apt/sources.list

將檔案內容替換成

deb http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse

deb-src http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse

deb http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse

deb-src http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse

deb http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse

deb-src http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse

deb http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse

deb-src http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse

deb http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse

deb-src http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse

更新
sudo apt-get update
sudo apt-get upgrade

sudo apt-get update
這個命令，會訪問源列表裡的每個網址，並讀取軟體列表，然後儲存在本地電腦。我們在新立得軟體包管理器裡看到的軟體列表，都是通過update命令更新的。

sudo apt-get upgrade
這個命令，會把本地已安裝的軟體，與剛下載的軟體列表裡對應軟體進行對比，如果發現已安裝的軟體版本太低，就會提示你更新。如果你的軟體都是最新版本，會提示：

升級了 0 個軟體包，新安裝了 0 個軟體包，要解除安裝 0 個軟體包，有 0 個軟體包未被升級。

總而言之，update是更新軟體列表，upgrade是更新軟體。