Following the advice on this mailing list, I rebuilt (from scratch) Centos 7.8. I added the following line into /etc/yum.conf before the first update:
exclude=grub2* shim* mokutil
The system worked fine. I was able to do general updates yum -y update and all was well.
HOWEVER, following Johnny Hughes' directions:
a) remove the 'exclude' line from yum.conf b) perform update: yum -y update c) reinstall kernel. Here the instructions were vague, so I typed in... yum reinstall kernel-3.10.0 deriving the "3.10.0" label by the output of rpm -qa | grep kernel
Yum got up to the point:
Running transaction Installing : kernel-3.10.0-1127.el7.x86_64 1/1
at which point the process appeared to hang. No further output happened for five minutes. I opened a different terminal and entered "shutdown -r now". The result is an unbootable system.
What did I do wrong? I must admit that there are multiple copies of advice on the mailing list, so perhaps I followed the wrong one?
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So now, I tried the 'repair' mechanism, but couldn't make that work either. So, it was back to initial from-scratch install.
David