Control the boot and fix boot problems

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$ als gewone gebruiker# als root (of met sudo)

Vaardigheden in dit hoofdstuk

Het opstarttarget kiezen en opstartproblemen oplossen

Know the steps of the boot

The boot is a chain. When it breaks, the last step you see tells you where to look.

Firmware (UEFI or BIOS) tests the hardware and starts the boot loader.
GRUB 2 shows the menu and loads the kernel and the initramfs.
The kernel starts the hardware drivers and runs the initramfs.
The initramfs (made by dracut) finds the root file system (LVM, RAID, encryption) and mounts it on /sysroot.
systemd (PID 1) starts all units of the default target: file systems from fstab, network, services.
A login prompt (text or graphical) appears.

$systemd-analyze
Startup finished in 1.215s (kernel) + 2.810s (initrd) + 8.402s (userspace) = 12.428s
$systemd-analyze blame | head -5
$journalctl -b -o short-monotonic | head

12.2

Know the targets

A target is a system state: a group of units that run together.

Target	What runs
`graphical.target`	Everything + the desktop login
`multi-user.target`	Everything for a server, text login, network
`rescue.target`	A root shell with the base system, local file systems mounted, no network
`emergency.target`	A root shell with only the root file system, read-only
`poweroff.target`, `reboot.target`	Shut down / reboot

$systemctl get-default
multi-user.target
#systemctl set-default graphical.target
#systemctl isolate multi-user.target
$systemctl list-dependencies multi-user.target | head

set-default = for every boot. isolate = switch now, stop everything that is not part of the new target.

Examentip

"The system must boot into text mode by default" → systemctl set-default multi-user.target. Check with systemctl get-default.

12.3

Boot one time into another target

At the GRUB menu you can add a kernel option for this boot only: systemd.unit=TARGET.

Reboot. In the GRUB menu, select the entry and press e.
Go to the line that starts with linux and add at the end: systemd.unit=rescue.target (or emergency.target, or multi-user.target).
Press Ctrl+X to boot.

#systemctl reboot
#systemctl rescue

Notitie

rescue.target and emergency.target ask for the root password. If root has no password (locked), they can not be used — then use the method of Recover root access.

12.4

Change kernel options for good

grubby changes the kernel options of the boot entries directly. It is the easy and safe way on RHEL.

#grubby --default-kernel
/boot/vmlinuz-6.12.0-55.9.1.el10_0.x86_64
#grubby --info=DEFAULT
#grubby --update-kernel=ALL --args="quiet"
#grubby --update-kernel=ALL --remove-args="rhgb"
#grubby --info=ALL | grep -E '^(kernel|args)'
#grubby --set-default=/boot/vmlinuz-6.12.0-55.9.1.el10_0.x86_64

The other way: change GRUB_CMDLINE_LINUX in /etc/default/grub, then write the config again:

#vim /etc/default/grub
#grub2-mkconfig -o /boot/grub2/grub.cfg --update-bls-cmdline

File	Meaning
`/etc/default/grub`	Main settings: timeout, default options
`/boot/loader/entries/*.conf`	One file per kernel (Boot Loader Specification)
`/boot/grub2/grub.cfg`	The generated config — do not edit by hand

Onthoud

Without --update-bls-cmdline, grub2-mkconfig does not change the options of the existing kernel entries. grubby always changes them.

12.5

Repair a wrong fstab line

A wrong line in /etc/fstab (wrong UUID, missing disk, typo) stops the boot in emergency mode. Make root writable, fix the file, go on.

You are in emergency mode. After logging in, type "journalctl -xb" to view
system logs, "systemctl reboot" to reboot, "systemctl default" or "exit"
to boot into default mode.
Give root password for maintenance

#journalctl -xb | grep -i -A3 'failed'
#mount -o remount,rw /
#vim /etc/fstab
#systemctl daemon-reload
#mount -a
#systemctl default

Op het werk

Prevent it: after every fstab change run mount -a and findmnt --verify before you reboot. For extra disks, add nofail.

12.6

Use the debug shell for hard cases

When the boot hangs, the debug shell gives a root shell on TTY9 early in the boot — no password. Use it only for repair, then turn it off.

#systemctl enable debug-shell.service
#grubby --update-kernel=ALL --args="systemd.debug-shell=1"
$systemctl list-jobs
#systemctl disable debug-shell.service
#grubby --update-kernel=ALL --remove-args="systemd.debug-shell=1"

Press Ctrl+Alt+F9 during the boot to get the shell. systemctl list-jobs shows which unit the boot waits for.

Let op

The debug shell is a root shell without password. Never leave it on.

12.7

Repair the boot loader and the initramfs

When GRUB or the initramfs is broken, boot from the RHEL installation ISO in rescue mode, enter the installed system, and rebuild them.

#chroot /mnt/sysroot
#dracut -f --regenerate-all
#grub2-install /dev/vda
#grub2-mkconfig -o /boot/grub2/grub.cfg
#exit

grub2-install is for BIOS machines. On UEFI machines, reinstall the packages instead: dnf reinstall grub2-efi-x64 shim-x64.

More: Fix a system that does not boot.