GRUB2

Created: July 09, 2019

Configuration

/etc/default/grub contains configuration options.
rhgb and quiet parameters are enabled by default and cause all output to be hidden when booting. rhgb is specific to the RHEL ecosystem.
man 7 bootparam gives an overview of kernel boot parameters.
Changes are not applied automatically; the grub2-mkconfig command needs to be used to update a file in the boot, or EFI partitions.
For BIOS systems the file is: /boot/grub2/grub.cfg or /boot/grub/grub.cfg.
For UEFI systems the file is: /boot/efi/EFI/redhat/grub.cfg or /boot/efi/EFI/centos/grub.cfg
For example: grub2-mkconfig > /boot/grub2/grub.cfg or grub2-mkconfig -o /boot/efi/EFI/centos/grub.cfg
/etc/grub.d contains complex scripts to tell GRUB what order to load things in.

Press c to enter command mode.
In command mode, you can:

List disks and partitions with: ls To make GRUB LVM aware: insmod lvm Search for a GRUB config file: search.file /grub2/grub.cfg Cat a file: cat

Press e to change boot parameters.
The key line begins with the word linux; three recovery modes can be accessed by adding options to this line.
rd.break: breaks to an interactive shell while still in initrd. Doesn’t mount the system disk. The system disk appears as /sysroot. rd.break is specific to the RHEL ecoysystem.
emergency: like rd.break but mounts the system disk.
recovery: more services than emergency mode and the system disk mounted.
After adding an option, press ^x to continue.

In an rd.break environment:

mount -o rw,remount /sysroot
passwd root
touch /.autorelabel
exit
mount -o ro,remount ./sysroot
reboot

The system will reboot twice and take some time to boot due to SELinux relabelling the filesystem.
/.autorelabel is necessary to relabel files with SELinux contexts.