GRUB2, kernel arguments and the default kernel

What GRUB2 does for you

GRUB2 (GRand Unified Bootloader, version 2) lists the installed kernels, waits a few seconds (GRUB_TIMEOUT), and boots the default one. Each menu entry knows which kernel and initramfs to load and which kernel arguments to pass. Kernel arguments are settings for the kernel and for early boot, such as root=, ro, quiet, or systemd.unit=rescue.target.

RHEL 9 uses the Boot Loader Specification (BLS): each kernel has its own small entry file in /boot/loader/entries/. The main GRUB config (/boot/grub2/grub.cfg) reads those files. On UEFI systems, /boot/efi/EFI/redhat/grub.cfg is only a small stub that points to /boot/grub2/grub.cfg.

One-time changes from the menu

1. Reboot. When the menu appears, press an arrow key to stop the countdown.
2. Select the entry and press e.
3. Find the line that starts with "linux" and move to its end (Ctrl+E).
4. Add or remove arguments, for example: systemd.unit=rescue.target
5. Press Ctrl+X to boot. The change is NOT saved.

Permanent changes with grubby

# grubby --default-kernel
/boot/vmlinuz-5.14.0-427.13.1.el9_4.x86_64
# grubby --info=ALL | grep -E '^(index|kernel|args)'
index=0
kernel="/boot/vmlinuz-5.14.0-427.13.1.el9_4.x86_64"
args="ro crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet"
index=1
kernel="/boot/vmlinuz-5.14.0-362.8.1.el9_3.x86_64"
...
# grubby --update-kernel=ALL --args="console=ttyS0,115200"    # add to every kernel
# grubby --update-kernel=ALL --remove-args="rhgb quiet"       # remove
# grubby --set-default /boot/vmlinuz-5.14.0-362.8.1.el9_3.x86_64   # boot the older kernel by default

The other way: /etc/default/grub

# grep -E 'TIMEOUT|CMDLINE' /etc/default/grub
GRUB_TIMEOUT=5
GRUB_CMDLINE_LINUX="crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet"
# vim /etc/default/grub            # e.g. GRUB_TIMEOUT=10
# grub2-mkconfig -o /boot/grub2/grub.cfg
Generating grub configuration file ...
done

Booting an older kernel

After a kernel update, if the server misbehaves (a driver issue, a crash), reboot and pick the previous kernel from the menu. If it works, make it the default with grubby --set-default while you report the problem. dnf keeps 3 kernels by default for exactly this reason.

Protecting GRUB

Anyone at the console can press e and boot into a root shell (next lesson). On machines in shared spaces, set a GRUB password with grub2-setpassword; then editing entries needs the password. Physical security still matters: someone with the hardware can boot from a USB stick.

Common mistakes

• Editing arguments in the menu and expecting them to stay. Menu edits are for one boot only.
• Changing GRUB_CMDLINE_LINUX and forgetting grub2-mkconfig, so nothing changes.
• Removing the only working kernel. Always keep at least one known-good kernel.