I was practicing, trying to reinstall GRUB from a failed boot. I was able to fix it and boot it up just fine after regenerating the /boot/grub2/grub.cfg.
However, what has been bothering me is that I couldn't reinstall the original /etc/default/grub file for some reason after deleting all the configuration files related to grub2-tools package on CentOS. All the related config files are outputted from rpm -qc grub2-tools command. I've tried downloading the RPM package and extracted it with rpm2cpio command to find the relevant file. I still got nothing.
So far two sources for Ubuntu and Scientific Linux distros confirmed the same issue I had, and the only workaround is to copy the original file from another system. IMHO, this workaround is pretty pathetic as I thought a simple reinstall should at least replace the original configuration files.
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