according with man yum
update If run without any packages, update will update every currently installed package. If one or more packages are specified, Yum will only update the listed packages. While
updating packages, yum will ensure that all dependencies are satisfied. If no package matches the given package name(s), they are assumed to be a shell glob and any matches
are then installed.
If the --obsoletes flag is present yum will include package obsoletes in its calculations - this makes it better for distro-version changes, for example: upgrading from
somelinux 8.0 to somelinux 9.
Is the same as the update command with the --obsoletes flag set. See update for more details.
Itamar Reis Peixoto
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