APT package handling utility - - command-line interface

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.

apt-get [-sqdyfmubV] [-o= config_string ] [-c= config_file ] [-t= target_release]
        [-a= default_architecture] {update | upgrade | dselect-upgrade | dist-upgrade |
        install pkg [ { =pkg_version_number | /target_release } ] ...  | remove pkg...  | purge pkg...  |
        source pkg [ { =pkg_version_number | /target_release } ] ...  | build-dep pkg...  | check | clean
        | autoclean | autoremove | {-v | --version} | {-h | --help}}

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.