update
update 是同步 /etc/apt/sources.list 和 /etc/apt/sources.list.d 中列出的源的索引，這樣才能獲取到最新的軟件包。
upgrade
upgrade 是升級已安裝的所有軟件包，升級之後的版本就是本地索引裏的，因此，在執行 upgrade 之前一定要執行 update, 這樣才能是最新的。
An update should always be performed before an upgrade or dist-upgrade.
upgrade is used to install the newest versions of all packages currently installed on the system from the sources enumerated in /etc/apt/sources.list. Packages currently installed with new versions available are retrieved and upgraded.
