Which some might consider a security hole, but you know the old Unix dictum: whoever has physical access to the machine owns it. In fact, it gives you complete access to all filesystems on the local machine regardless of permissions or other protections. You can use it to discover boot images, kernels, and root filesystems. The GRUB 2 command shell is just as powerful as the shell in legacy GRUB. So you can look for your boot files at the GRUB prompt, set their locations, and then boot your system and fix your GRUB configuration. In these scenarios your boot files are still there, but GRUB can’t find them. How does this happen? The kernel might have changed drive assignments or you moved your hard drives, you changed some partitions, or installed a new operating system and moved things around. If you see grub rescue> that means it couldn’t find normal.mod, so it probably couldn’t find any of your boot files. That means GRUB 2 started normally and loaded the normal.mod module (and other modules which are located in /boot/grub//), but it didn’t find your grub.cfg file. When you boot up your system and it stops at the grub> prompt, that is the full GRUB 2 command shell. We’re going to learn how to fix two of the more common failures. The good news is that the update-grub script is reliable for finding kernels, boot files, and adding all operating systems to your GRUB boot menu, so you don’t have to do it manually. boot/grub/grub.cfg is built from /etc/default/grub and /etc/grub.d/* when you run the update-grub command, which you must run every time you make changes. These are the scripts that boot your operating systems, control external applications such as memtest and os_prober, and theming. We may also edit the scripts in /etc/grub.d/.
We lowly humans may edit /etc/default/grub, which controls mainly the appearance of the GRUB menu. Which you don’t edit directly, oh no, for this is not for mere humans to touch, but only other scripts. It’s more complicated to configure with all kinds of scripts to wade through, and instead of having a nice fairly simple /boot/grub/menu.lst file with all configurations in one place, the default is /boot/grub/grub.cfg. It boots removable media, and can be configured with an option to enter your system BIOS.
GRUB 2 is a major rewrite with several significant differences. Legacy GRUB had many virtues, but it became old and its developers did yearn for more functionality, and thus did GRUB 2 come into the world. Once upon a time we had legacy GRUB, the Grand Unified Linux Bootloader version 0.97.
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