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update 更新我們的套件清單 /etc/apt/sources.list，這樣在我們更新時才能比對最新的套件清單，決定是否更新
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upgrade 根據前述的更新清單 /etc/apt/sources.list，比對是否更新套件。若有相依性問題，則放棄更新。
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dist-upgrade 根據前述的更新清單 /etc/apt/sources.list，比對是否更新套件。若有相依性問題，會嘗試做安裝/移除，將相依性問題解決並更新。 PS. 通常視為較危險的更新
參考資料：
man page
update update is used to resynchronize the package index files from their sources. The indexes of available packages are fetched from the location(s) specified in /etc/apt/sources.list. For example, when using a Debian archive, this command retrieves and scans the Packages.gz files, so that information about new and updated packages is available. An update should always be performed before an upgrade or dist-upgrade. Please be aware that the overall progress meter will be incorrect as the size of the package files cannot be known in advance.
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; under no circumstances are currently installed packages removed, or packages not already installed retrieved and installed. New versions of currently installed packages that cannot be upgraded without changing the install status of another package will be left at their current version. An update must be performed first so that apt-get knows that new versions of packages are available.
dist-upgrade dist-upgrade in addition to performing the function of upgrade, also intelligently handles changing dependencies with new versions of packages; apt-get has a “smart” conflict resolution system, and it will attempt to upgrade the most important packages at the expense of less important ones if necessary. The dist-upgrade command may therefore remove some packages. The /etc/apt/sources.list file contains a list of locations from which to retrieve desired package files. See also apt_preferences(5) for a mechanism for overriding the general settings for individual packages.