apt-get update 和 upgrade 的区别

update

update 命令用于同步 /etc/apt/sources.list 和 /etc/apt/sources.list.d 中列出的源的索引，从而获取最新的软件包列表。

upgrade

upgrade 命令用于升级系统中已安装的所有软件包。升级后的版本取决于本地索引中的信息，因此执行 upgrade 之前必须先执行 update。

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.