update
update 是同步 /etc/apt/sources.list 和 /etc/apt/sources.list.d 中列出的源的索引，这样才能获取到最新的软件包。
upgrade 是升级已安装的所有软件包，升级之后的版本就是本地索引里的，因此，在执行 upgrade 之前一定要执行 update, 这样才能是最新的。
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.
