Configure the GRUB2 Bootloader on RHEL

On RHEL 10, GRUB2 reads BLS snippets, loads a kernel and initramfs, and passes kernel command-line arguments to the running system. This walkthrough on vm1.lab.example uses grubby for everyday changes and /etc/default/grub plus grub2-mkconfig when you need distribution-wide defaults synchronized into every entry.

How GRUB2 Booting Works on RHEL

Firmware
  ↓
GRUB2
  ↓
Boot Loader Specification entry
  ↓
Kernel + initramfs + kernel arguments

Package updates install new kernels and BLS snippets automatically. Your job is usually to pick the default entry and adjust options, not to hand-edit grub.cfg for every kernel change.

RHEL GRUB2 Quick Reference

For additional grubby flags and title-based lookups, see grubby command examples.

Identify BIOS vs UEFI Boot

Firmware mode determines where the platform firmware looks first, but RHEL 10 regenerates the same main configuration path for routine administration.

Check firmware mode:

[ -d /sys/firmware/efi ] && echo UEFI || echo BIOS

BIOS

On this lab host firmware is legacy BIOS. On UEFI systems the same check prints UEFI.

Locate the active GRUB configuration:

ls -l /boot/grub2/grub.cfg

-rw-------. 1 root root 7352 Aug  3 11:45 /boot/grub2/grub.cfg

On RHEL 10, write regenerated configuration to /boot/grub2/grub.cfg for both BIOS and UEFI. The UEFI stub under /boot/efi/EFI/redhat/ loads that file; do not replace the stub grub.cfg with grub2-mkconfig output.

Understand RHEL Boot Loader Specification Entries

BLS snippets live in /boot/loader/entries/:

ls /boot/loader/entries/

23b7a5ba4a464d768c37c2b2990e7d06-0-rescue.conf
23b7a5ba4a464d768c37c2b2990e7d06-6.12.0-211.42.1.el10_2.x86_64.conf
23b7a5ba4a464d768c37c2b2990e7d06-6.12.0-211.42.1.el10_2.x86_64+debug.conf

A typical entry contains:

Inspect one file:

head -8 /boot/loader/entries/23b7a5ba4a464d768c37c2b2990e7d06-6.12.0-211.42.1.el10_2.x86_64.conf

title Red Hat Enterprise Linux (6.12.0-211.42.1.el10_2.x86_64) 10.2 (Coughlan)
version 6.12.0-211.42.1.el10_2.x86_64
linux /vmlinuz-6.12.0-211.42.1.el10_2.x86_64
initrd /initramfs-6.12.0-211.42.1.el10_2.x86_64.img $tuned_initrd
options root=/dev/mapper/rhel-root ro resume=UUID=78531da6-a389-46e8-9bad-267572940826 rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet 
grub_users $grub_users
grub_arg --unrestricted
grub_class rhel

Treat these files as output of package management and grubby, not the primary edit surface. For file layout background, see what GRUB files do.

List Installed Kernels and Boot Entries

grubby --info=ALL summarizes every BLS-backed entry:

grubby --info=ALL

index=0
kernel="/boot/vmlinuz-6.12.0-211.42.1.el10_2.x86_64+debug"
args="ro resume=UUID=78531da6-a389-46e8-9bad-267572940826 rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet"
root="/dev/mapper/rhel-root"
initrd="/boot/initramfs-6.12.0-211.42.1.el10_2.x86_64+debug.img $tuned_initrd"
title="Red Hat Enterprise Linux (6.12.0-211.42.1.el10_2.x86_64+debug) 10.2 (Coughlan) with debugging"
id="23b7a5ba4a464d768c37c2b2990e7d06-6.12.0-211.42.1.el10_2.x86_64+debug"
index=1
kernel="/boot/vmlinuz-6.12.0-211.42.1.el10_2.x86_64"
args="ro resume=UUID=78531da6-a389-46e8-9bad-267572940826 rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet"
root="/dev/mapper/rhel-root"
initrd="/boot/initramfs-6.12.0-211.42.1.el10_2.x86_64.img $tuned_initrd"
title="Red Hat Enterprise Linux (6.12.0-211.42.1.el10_2.x86_64) 10.2 (Coughlan)"
id="23b7a5ba4a464d768c37c2b2990e7d06-6.12.0-211.42.1.el10_2.x86_64"
index=2
kernel="/boot/vmlinuz-0-rescue-23b7a5ba4a464d768c37c2b2990e7d06"
args="ro resume=UUID=78531da6-a389-46e8-9bad-267572940826 rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet"
root="/dev/mapper/rhel-root"
initrd="/boot/initramfs-0-rescue-23b7a5ba4a464d768c37c2b2990e7d06.img"
title="Red Hat Enterprise Linux (0-rescue-23b7a5ba4a464d768c37c2b2990e7d06) 10.2 (Coughlan)"
id="23b7a5ba4a464d768c37c2b2990e7d06-0-rescue"

Read index for menu ordering, kernel for the path you pass to grubby --set-default, and args for stored kernel parameters. The 0-rescue entry is the rescue kernel created at installation.

Check and Change the Default Kernel

Show which kernel boots by default:

grubby --default-kernel

/boot/vmlinuz-6.12.0-211.42.1.el10_2.x86_64

Set the default to the installed production kernel path (adjust the version to match your host):

sudo grubby --set-default /boot/vmlinuz-6.12.0-211.42.1.el10_2.x86_64

The default is /boot/loader/entries/23b7a5ba4a464d768c37c2b2990e7d06-6.12.0-211.42.1.el10_2.x86_64.conf with index 1 and kernel /boot/vmlinuz-6.12.0-211.42.1.el10_2.x86_64

Confirm the saved default:

grubby --default-kernel

/boot/vmlinuz-6.12.0-211.42.1.el10_2.x86_64

Reboot to run the newly selected default, then verify the running kernel:

uname -r

6.12.0-211.42.1.el10_2.x86_64

Use kernel paths rather than menu indexes when you script changes. Installing or removing kernels can reorder indexes. For booting a previous build after an upgrade, see boot with old kernel.

Select a Different Kernel for One Boot

Persistent default: grubby --set-default updates saved_entry in /boot/grub2/grubenv for every future boot until you change it again.

One boot only: pick another entry at the GRUB menu, or set next_entry with the BLS entry id (most stable), title, or index:

sudo grub2-reboot 23b7a5ba4a464d768c37c2b2990e7d06-0-rescue

Indexes also work (grubby --info=ALL shows each index=), but installing or removing kernels can reorder them. Red Hat recommends id or title when you need a repeatable one-time boot target.

Inspect the environment block:

sudo grub2-editenv /boot/grub2/grubenv list

saved_entry=23b7a5ba4a464d768c37c2b2990e7d06-6.12.0-211.42.1.el10_2.x86_64
menu_auto_hide=1
boot_success=0
boot_indeterminate=0
next_entry=23b7a5ba4a464d768c37c2b2990e7d06-0-rescue

saved_entry remains the normal default. next_entry applies only to the next boot, then GRUB clears it.

Clear a pending one-time boot without rebooting:

sudo grub2-editenv - unset next_entry

Add Kernel Command-Line Parameters

Use grubby for the everyday workflow. printk.time=1 adds timestamps to kernel log lines and is safe for lab testing:

sudo grubby --update-kernel=ALL --args="printk.time=1"

Confirm the argument landed on every entry:

grubby --info=/boot/vmlinuz-6.12.0-211.42.1.el10_2.x86_64 | grep ^args=

args="ro resume=UUID=78531da6-a389-46e8-9bad-267572940826 rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet printk.time=1"

The BLS options line matches:

grep ^options /boot/loader/entries/23b7a5ba4a464d768c37c2b2990e7d06-6.12.0-211.42.1.el10_2.x86_64.conf

options root=/dev/mapper/rhel-root ro resume=UUID=78531da6-a389-46e8-9bad-267572940826 rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet printk.time=1

Reboot before the running kernel picks up new arguments:

cat /proc/cmdline

BOOT_IMAGE=(hd0,gpt2)/vmlinuz-6.12.0-211.42.1.el10_2.x86_64 root=/dev/mapper/rhel-root ro resume=UUID=78531da6-a389-46e8-9bad-267572940826 rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet

Until reboot, /proc/cmdline still reflects the currently running kernel, not the updated BLS entry.

Remove Kernel Parameters

Remove the test parameter from all kernels:

sudo grubby --update-kernel=ALL --remove-args="printk.time=1"

Verify it disappeared from the stored entry:

grubby --info=/boot/vmlinuz-6.12.0-211.42.1.el10_2.x86_64 | grep ^args=

args="ro resume=UUID=78531da6-a389-46e8-9bad-267572940826 rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet"

Reboot and check /proc/cmdline again when you need to confirm the running kernel no longer has the parameter.

Configure Parameters Through /etc/default/grub

/etc/default/grub holds distribution-wide settings. Important keys on this host:

grep -E '^GRUB_(TIMEOUT|DEFAULT|CMDLINE_LINUX)' /etc/default/grub

GRUB_TIMEOUT=5
GRUB_DEFAULT=saved
GRUB_CMDLINE_LINUX="resume=UUID=78531da6-a389-46e8-9bad-267572940826 rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet"

grubby edits existing BLS options directly. /etc/default/grub holds distribution-wide defaults that grub2-mkconfig --update-bls-cmdline can overwrite into every BLS options line.

Add a shared argument to /etc/default/grub:

sudo sed -i 's/rhgb quiet"/rhgb quiet audit=1"/' /etc/default/grub

Push GRUB_CMDLINE_LINUX into every BLS entry:

sudo grub2-mkconfig -o /boot/grub2/grub.cfg --update-bls-cmdline

Generating grub configuration file ...
Adding boot menu entry for UEFI Firmware Settings ...
done

Check one entry:

grep ^options /boot/loader/entries/23b7a5ba4a464d768c37c2b2990e7d06-6.12.0-211.42.1.el10_2.x86_64.conf

options root=/dev/mapper/rhel-root ro resume=UUID=78531da6-a389-46e8-9bad-267572940826 rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet audit=1

Revert the test change on a lab host when finished:

sudo sed -i 's/ audit=1//' /etc/default/grub

Regenerate so BLS entries drop the test argument:

sudo grub2-mkconfig -o /boot/grub2/grub.cfg --update-bls-cmdline

Use grubby for routine kernel-argument changes directly in BLS entries. RHEL 10 normally carries those arguments forward when a newer kernel is installed. Use /etc/default/grub plus --update-bls-cmdline when you intentionally want the BLS command lines realigned with GRUB_CMDLINE_LINUX. That step overwrites BLS snippets; it is not merely appending missing arguments, so grubby-only parameters absent from GRUB_CMDLINE_LINUX can be removed.

Change the GRUB Menu Timeout

Edit GRUB_TIMEOUT in /etc/default/grub, then regenerate:

sudo grub2-mkconfig -o /boot/grub2/grub.cfg

Generating grub configuration file ...
Adding boot menu entry for UEFI Firmware Settings ...
done

Confirm at the next boot that the menu waits the configured number of seconds. GRUB_TIMEOUT=0 hides the menu immediately unless you interrupt boot.

Inspect the Active Kernel Command Line

The running kernel exposes the command line it received at boot:

cat /proc/cmdline

BOOT_IMAGE=(hd0,gpt2)/vmlinuz-6.12.0-211.42.1.el10_2.x86_64 root=/dev/mapper/rhel-root ro resume=UUID=78531da6-a389-46e8-9bad-267572940826 rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet

Compare this line to grubby --info or the BLS options field. If they differ, you likely changed stored entries but have not rebooted yet, or a one-time menu edit overrode the saved entry.

Rebuild GRUB Configuration

You do not need grub2-mkconfig after every grubby change. grubby updates BLS snippets and grubenv directly.

Regenerate when you change /etc/default/grub, after some manual recovery steps, or when generated grub.cfg is clearly stale:

sudo grub2-mkconfig -o /boot/grub2/grub.cfg

Generating grub configuration file ...
Adding boot menu entry for UEFI Firmware Settings ...
done

On RHEL 10 the correct target is always:

sudo grub2-mkconfig -o /boot/grub2/grub.cfg

for BIOS and UEFI. Add --update-bls-cmdline when GRUB_CMDLINE_LINUX or other defaults must flow into /boot/loader/entries/.

Recover from a Bad Kernel Parameter

If a new kernel argument prevents boot:

1. Interrupt the GRUB menu at startup.
2. Highlight the entry and press e to edit temporarily.
3. Remove or fix the bad argument on the linux line.
4. Press Ctrl+x to boot with the temporary change.
5. After the system is up, remove the argument permanently with grubby:

sudo grubby --update-kernel=ALL --remove-args="bad.parameter"

Reboot again to confirm clean startup.

This workflow fixes kernel command-line mistakes. Locked root accounts or forgotten passwords use a different procedure in reset root password on RHEL.

Troubleshoot GRUB and Kernel Entries

Practical GRUB Configuration Examples

Safe patterns on a lab host:

1. grubby --info=ALL — inventory kernels before you change anything
2. grubby --set-default /boot/vmlinuz-VERSION — pick the production kernel
3. grubby --update-kernel=ALL --args="printk.time=1" — test parameter add
4. Reboot and cat /proc/cmdline — confirm the running kernel
5. grubby --update-kernel=ALL --remove-args="printk.time=1" — revert
6. Edit GRUB_TIMEOUT in /etc/default/grub, then grub2-mkconfig -o /boot/grub2/grub.cfg
7. grub2-reboot 23b7a5ba4a464d768c37c2b2990e7d06-0-rescue — one-time rescue boot by BLS id; clear with grub2-editenv - unset next_entry

Avoid parameters that disable storage, networking, or SELinux unless you have snapshot recovery. Do not use this article as a root password reset guide.

References

• grubby(8) — Linux manual page
• kernel-command-line(7) — Linux manual page
• Red Hat — Configuring kernel command-line parameters
• Red Hat — Managing, monitoring, and updating the kernel

Summary

You configured GRUB2 on RHEL by working with BLS snippets under /boot/loader/entries/ rather than treating grub.cfg as the primary edit target. grubby lists entries, sets the default kernel, and adds or removes kernel arguments on one or all installed kernels. grubby --default-kernel and uname -r confirm default selection after reboot; cat /proc/cmdline shows what the running kernel actually received.

The distinction that saves time on RHEL 10 is tooling choice: routine per-kernel work stays in grubby, while /etc/default/grub plus grub2-mkconfig -o /boot/grub2/grub.cfg --update-bls-cmdline synchronizes distribution-wide GRUB_CMDLINE_LINUX values into every BLS entry. Regenerate /boot/grub2/grub.cfg on both BIOS and UEFI; do not overwrite the UEFI stub configuration directly.

If a bad parameter blocks boot, edit once at the GRUB menu, boot, then make the fix permanent with grubby. For exhaustive command coverage, continue with grubby command examples; for booting a previous kernel build, see boot with old kernel. Root password recovery stays in reset root password on RHEL.