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.