APTupgrade和dist-upgrade的差?e

Debian/Ubuntu Linux都使用apt,升级时都是:

apt-get update
apt-get upgrade
apt-get dist-upgrade

但是常常有人会问, upgrade和dist-upgrade有何不同,仔细查查,似乎大家对upgrade和dist-upgrade的解释都有点不同,在此也纪录自己的看法.

我认为apt-get upgrade和dist-upgrade的差别:

upgrade:系统将现有的Package升级,如果有相依性的问题,而此相依性需要安装其它新的Package或影响到其它Package的相依性时,此Package就不会被升级,会保留下来.

dist-upgrade:可以聪明的解决相依性的问题,如果有相依性问题,需要安装/移除新的Package,就会试着去安装/移除它. (所以通常这个会被认为是有点风险的升级)

man apt-get的解释:

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