Boot Process and Recovery¶

The boot sequence¶

1. Firmware (UEFI or BIOS) finds a boot device.
2. Bootloader (GRUB2) loads the kernel and the initramfs.
3. Kernel starts and mounts the root filesystem.
4. systemd starts services up to the default target.

Find out why a boot failed¶

journalctl -b -1 -p err         # previous boot errors
systemctl --failed

Recovery targets¶

sudo systemctl rescue           # single-user, with basic services
sudo systemctl emergency        # minimal shell with the root filesystem

A common cause of a failed boot is a bad /etc/fstab entry. Fix it from the emergency shell and test with mount -a.

Reset a lost root password (RHEL 8 and 9)¶

Only do this on systems you are authorised to administer, with console access.

1. At the GRUB menu press e, add rd.break to the end of the linux line, then press Ctrl+x.
2. In the emergency shell:

mount -o remount,rw /sysroot
chroot /sysroot
passwd root
touch /.autorelabel          # fixes SELinux labels on the next boot
exit
exit