sudo apt-get upgrade: 不会对系统产生重大的影响，可以在任何时候运行。
sudo apt-get dist-upgrade: 涉及核心的升级，通常会对系统功能产生实际的影响，可能在需要的情况下卸载一些已经安装的软件也可能安装一些额外的软件，通常应该谨慎操作，特别是有未完成的工作，但是在允许的情况下应该尽力升级，应为可能涉及一些重大的核心升级，比如安全。
dist-upgrade will perform updates to software that makes actual changes to the system and can have an impact on the functionality of your system. Using ubuntu, a dist-upgrade often involves kernel upgrades. Kernel upgrades can really change things, especially if you have manually installed proprietary drivers. If you are currently working on a project for school or work, it is best to wait until the project is finished before performing a dist-upgrade. You can safely perform an upgrade at any time and it will not have a large impact on system functionality. You should, however, perform a dist-upgrade whenever possible as there are often security updates for the kernel. Also, a dist-upgrade may uninstall existing packages or install extra packages where an upgrade will not.
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. So, dist-upgrade
command may 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.ref: https://www.quora.com/What-does-command-sudo-apt-get-dist-upgrade
ref: https://askubuntu.com/questions/81585/what-is-dist-upgrade-and-why-does-it-upgrade-more-than-upgrade
