update is used to resynchronize the packageindex files from their sources. The indexes of available packages are fetchedfrom the location(s) specified in /etc/apt/sources.list. For example, whenusing a Debian archive, this command retrieves and scans the Packages.gz files,so that information about new and updated packages is available. An updateshould always be performed before an upgrade or dist-upgrade. Please be awarethat the overall progress meter will be incorrect as the size of the packagefiles cannot be known in advance.
upgrade is used to install the newestversions 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 circumstancesare currently installed packages removed, or packages not already installed retrieved and installed. Newversions of currently installed packages that cannot be upgraded without changing the install statusof another package will be left at their current version. An update must be performed first so that apt-getknows that new versions of packages are available.
dist-upgrade in addition to performing the function of upgrade, alsointelligently 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 lessimportant ones if necessary. The /etc/apt/sources.list file contains a list of locations from which toretrieve desired package files. See also apt_preferences(5) for a mechanism for overriding the generalsettings for individual packages.
update：当执行apt-get update时，update重点更新的是来自软件源的软件包的索引记录（即index files）。
从上述可以看出，在执行apt-get upgrade和apt-get dist-upgrade之前应该先执行apt-get update。
apt-get upgrade 和 apt-get dist-upgrade 本质上是没有什么不同的。
例如软件包 a 原先依赖 b c d，但是在源里面可能已经升级了，现在是 a 依赖 b c
e。这种情况下，dist-upgrade 会删除 d 安装 e，并把 a 软件包升级，而 upgrade 会认为依赖关系改变而拒绝升级 a