Seems someone over at my favorite free software project mucked up yum again, since I just got the dreaded “Warning: group core does not exist” message on running “yum update”. Solution below.
This is actually on the list of known bugs in Fedora 19, and persists in Fedora 20.
My systems are all on Fedora 19, and experienced this with yum-3.4.3-122.fc19.
The bug appears to be related to a change in the way yum groups are handled (“groups as objects”) introduced in both Fedora 19 and 20 with yum-3.4.3-111.
The official workaround is to run the following command to disable yum’s treatment of “groups as objects”:
yum-config-manager --save --setopt=group_command=compat
What this will do is write the following line into /etc/yum.conf:
group_command = compat
Then run “yum update” to see if the problem persists. If it does, you can try downgrading yum to the previous version by running:
yum downgrade yum
In my case this downgraded me to yum-3.4.3-99.fc19.
If you do this be sure to add the following to /etc/yum.conf:
That will prevent an open-ended “yum upgrade” from updating yum back to the broken version.
In my case re-configuring to disable “groups as objects” treatment resulted in everything returning to normal, and so I was able to update back to the latest version of yum (after removing the exclude line from yum.conf).