After a kernel update on RHEL, Rocky Linux, AlmaLinux, CentOS Stream, Oracle Linux, or Fedora, the newest build is commonly selected for the next boot. However, the running kernel and configured default may still differ. When a new kernel misbehaves, you need a safe way to point GRUB at an older build you already trust.
This guide uses grubby to inspect the running kernel, list boot entries, set a persistent default by kernel path, and verify the choice before you reboot. Current Fedora releases also use BLS-style kernel entries and provide grubby for inspecting and changing kernel defaults, although package versions and boot-menu behaviour can differ from enterprise Linux.
Tested on: Rocky Linux 10.2 (Red Quartz); kernel 6.12.0-211.16.1.el10_2.0.1.x86_64; grubby 8.40-83.el10.
Check the Running and Default Kernel
The kernel in memory and the kernel GRUB will load on the next reboot are two different questions. Changing the default with grubby does not replace the running kernel—you need a reboot for that. After an update you have not rebooted into yet, uname -r and grubby --default-kernel often disagree, and that is normal.
| Term | Meaning |
|---|---|
| Running kernel | Kernel currently loaded in memory |
| Default kernel | Kernel configured for the next normal boot |
| Fallback kernel | Older tested kernel retained for recovery |
| One-time selection | Kernel chosen only for the next boot via grub2-reboot or the GRUB menu |
Start with the kernel your session is using right now:
uname -rSample output:
6.12.0-211.16.1.el10_2.0.1.x86_64That is the build loaded in memory. Next, ask GRUB which kernel it will boot on a normal restart:
grubby --default-kernelSample output:
/boot/vmlinuz-6.12.0-211.28.1.el10_2.x86_64On this host the running kernel is older than the configured default—a typical post-update state where the new package is queued for the next boot but this session has not rebooted yet.
The default index is the position of that entry in the boot menu list. The newest kernel commonly appears at index 0, but do not assume that ordering without checking grubby --info=ALL:
grubby --default-indexSample output:
0Index 0 here matches the newer vmlinuz path returned by --default-kernel. If you set a fallback kernel, expect this number to change.
For the human-readable menu title GRUB shows:
grubby --default-titleSample output:
Rocky Linux (6.12.0-211.28.1.el10_2.x86_64) 10.2 (Red Quartz)The version string in parentheses should match the kernel release you intend to boot.
List Installed Kernels and Boot Entries
Before you change the default, confirm which kernel packages are on disk and which paths grubby knows about. A package can be installed while its boot entry is missing—or the other way around if cleanup was interrupted.
List installed kernel packages
On current RHEL-family releases, a kernel installation consists of several related packages, commonly including kernel, kernel-core, kernel-modules, and sometimes kernel-modules-core or kernel-modules-extra. Fedora package composition can differ by release, so treat the exact set as distribution-specific.
Query kernel and kernel-core to list the standard distribution kernel versions installed on current RHEL-family systems:
rpm -q kernel kernel-coreSample output:
kernel-6.12.0-211.16.1.el10_2.0.1.x86_64
kernel-6.12.0-211.28.1.el10_2.x86_64
kernel-core-6.12.0-211.16.1.el10_2.0.1.x86_64
kernel-core-6.12.0-211.28.1.el10_2.x86_64On current RHEL-family releases, kernel-core contains the bootable kernel image while kernel is the associated install-only package. Fedora and alternative kernels may use different package names. Real-time, debug, vendor, or third-party kernels may require separate queries.
The version strings here should match the vmlinuz files under /boot. For a wider inventory that also lists tools and development packages, run:
rpm -qa 'kernel*' | sort -VSample output:
kernel-6.12.0-211.16.1.el10_2.0.1.x86_64
kernel-6.12.0-211.28.1.el10_2.x86_64
kernel-core-6.12.0-211.16.1.el10_2.0.1.x86_64
kernel-core-6.12.0-211.28.1.el10_2.x86_64
kernel-modules-6.12.0-211.16.1.el10_2.0.1.x86_64
kernel-modules-6.12.0-211.28.1.el10_2.x86_64
kernel-modules-core-6.12.0-211.16.1.el10_2.0.1.x86_64
kernel-modules-core-6.12.0-211.28.1.el10_2.x86_64
kernel-modules-extra-6.12.0-211.16.1.el10_2.0.1.x86_64
kernel-tools-6.12.0-211.16.1.el10_2.0.1.x86_64
kernel-tools-libs-6.12.0-211.16.1.el10_2.0.1.x86_64That output is useful for inventory, not as a list of bootable kernels—entries such as kernel-tools or kernel-devel are not boot menu targets. For broader RPM query patterns, see the linked guide when you need filters beyond kernel*.
List boot entries with grubby
grubby --info=ALL prints the boot entries known to the active bootloader configuration. You do not need every field on day one—focus on these:
index— menu position; can change when kernels are added or removedkernel— path to thevmlinuzimage; use this with--set-defaultinitrd— paired initramfs; must exist or boot failstitle— label shown in the GRUB menuid— BLS entry identifier under/boot/loader/entries/
grubby --info=ALLSample output:
index=0
kernel="/boot/vmlinuz-6.12.0-211.28.1.el10_2.x86_64"
args="ro resume=UUID=26765c90-fea0-4216-9293-9936cabf9135 rd.lvm.lv=rlm/root"
root="/dev/mapper/rlm-root"
initrd="/boot/initramfs-6.12.0-211.28.1.el10_2.x86_64.img"
title="Rocky Linux (6.12.0-211.28.1.el10_2.x86_64) 10.2 (Red Quartz)"
id="28a80846932844369fe693bfcdfb81bb-6.12.0-211.28.1.el10_2.x86_64"
index=1
kernel="/boot/vmlinuz-6.12.0-211.16.1.el10_2.0.1.x86_64"
...
index=2
kernel="/boot/vmlinuz-0-rescue-28a80846932844369fe693bfcdfb81bb"
...Each block is one boot menu entry. Copy the kernel= path for --set-default or the id= value for grub2-reboot.
For a shorter list of kernel paths only:
grubby --info=ALL | grep '^kernel='Sample output:
kernel="/boot/vmlinuz-6.12.0-211.28.1.el10_2.x86_64"
kernel="/boot/vmlinuz-6.12.0-211.16.1.el10_2.0.1.x86_64"
kernel="/boot/vmlinuz-0-rescue-28a80846932844369fe693bfcdfb81bb"The rescue entry is a recovery boot image created by the installation or kernel-install workflow. It is separate from the normal versioned kernel entries and may not match your newest installed kernel. Keep it unless you have verified another recovery method.
Inspect one entry in detail with --info and the full vmlinuz path:
grubby --info="/boot/vmlinuz-6.12.0-211.16.1.el10_2.0.1.x86_64"Sample output:
index=1
kernel="/boot/vmlinuz-6.12.0-211.16.1.el10_2.0.1.x86_64"
args="ro resume=UUID=26765c90-fea0-4216-9293-9936cabf9135 rd.lvm.lv=rlm/root"
root="/dev/mapper/rlm-root"
initrd="/boot/initramfs-6.12.0-211.16.1.el10_2.0.1.x86_64.img"
title="Rocky Linux (6.12.0-211.16.1.el10_2.0.1.x86_64) 10.2 (Red Quartz)"
id="28a80846932844369fe693bfcdfb81bb-6.12.0-211.16.1.el10_2.0.1.x86_64"Confirm the initrd= path exists on disk before you set this entry as default.
Change the Default Kernel with grubby
Use the full /boot/vmlinuz-* path as the primary method. grubby updates the bootloader's saved default to reference the selected kernel entry. You do not need to edit /boot/grub2/grub.cfg or the BLS files under /boot/loader/entries/ manually.
Set an older kernel as default
Suppose the newest kernel misbehaves and you want the previous build to boot every time until you finish testing. Point grubby at the older vmlinuz path:
sudo grubby --set-default "/boot/vmlinuz-6.12.0-211.16.1.el10_2.0.1.x86_64"Sample output:
The default is /boot/loader/entries/28a80846932844369fe693bfcdfb81bb-6.12.0-211.16.1.el10_2.0.1.x86_64.conf with index 1 and kernel /boot/vmlinuz-6.12.0-211.16.1.el10_2.0.1.x86_64The confirmation line names the BLS config file and the new index.
Switch back to the latest kernel
When you are ready to default to the newest build again, set the path for that vmlinuz:
sudo grubby --set-default "/boot/vmlinuz-6.12.0-211.28.1.el10_2.x86_64"Sample output:
The default is /boot/loader/entries/28a80846932844369fe693bfcdfb81bb-6.12.0-211.28.1.el10_2.x86_64.conf with index 0 and kernel /boot/vmlinuz-6.12.0-211.28.1.el10_2.x86_64Index-based selection is an alternative when you already verified the menu order:
sudo grubby --set-default-index=1Prefer kernel paths over indexes. Installing or removing a kernel can reorder entries, so an index that pointed at your fallback yesterday may point somewhere else tomorrow. More grubby examples cover title-based lookups and kernel argument updates.
Verify the Kernel Before and After Reboot
Always confirm the default before you reboot—especially on production systems where console access may be limited.
After setting the default, read it back:
grubby --default-kernelSample output:
/boot/vmlinuz-6.12.0-211.16.1.el10_2.0.1.x86_64The path should match the vmlinuz you passed to --set-default. Check the index as well:
grubby --default-indexSample output:
1Cross-check the boot entry details:
grubby --info="/boot/vmlinuz-6.12.0-211.16.1.el10_2.0.1.x86_64"Sample output:
index=1
kernel="/boot/vmlinuz-6.12.0-211.16.1.el10_2.0.1.x86_64"
...
title="Rocky Linux (6.12.0-211.16.1.el10_2.0.1.x86_64) 10.2 (Red Quartz)"Reboot when you are satisfied with the configured default and have console or out-of-band access if something goes wrong:
sudo rebootAfter the system comes back, confirm the active kernel:
uname -rSample output:
6.12.0-211.16.1.el10_2.0.1.x86_64That release string should match the kernel path you selected.
| Mistake | Fix |
|---|---|
| Assuming the running kernel is the default | Check uname -r and grubby --default-kernel separately |
| Selecting by index only | Prefer the full kernel path |
| Removing the previous kernel immediately | Keep a tested fallback until the new default is verified |
| Expecting the kernel to change without reboot | Reboot and verify with uname -r |
| Editing generated GRUB files directly | Use grubby |
| Using Ubuntu commands on RHEL-family systems | Keep the workflows in separate articles |
Test an Older Kernel Before Making It Default
Before changing the persistent default, you can select an older kernel for the next boot only.
List the available entries and note the id, title, or index for the kernel you want to test:
grubby --info=ALLSample output:
index=0
kernel="/boot/vmlinuz-6.12.0-211.28.1.el10_2.x86_64"
...
title="Rocky Linux (6.12.0-211.28.1.el10_2.x86_64) 10.2 (Red Quartz)"
id="28a80846932844369fe693bfcdfb81bb-6.12.0-211.28.1.el10_2.x86_64"
index=1
kernel="/boot/vmlinuz-6.12.0-211.16.1.el10_2.0.1.x86_64"
...
title="Rocky Linux (6.12.0-211.16.1.el10_2.0.1.x86_64) 10.2 (Red Quartz)"
id="28a80846932844369fe693bfcdfb81bb-6.12.0-211.16.1.el10_2.0.1.x86_64"Pass the entry id, exact title, or a verified index to grub2-reboot. Prefer the id when you have it:
sudo grub2-reboot "28a80846932844369fe693bfcdfb81bb-6.12.0-211.16.1.el10_2.0.1.x86_64"grub2-reboot exits silently on success. Reboot to use the one-time selection:
sudo rebootAfter login, verify the running kernel:
uname -rThe one-time selection does not change the persistent default:
grubby --default-kernelSample output:
/boot/vmlinuz-6.12.0-211.28.1.el10_2.x86_64The path still points at the persistent default, not the kernel you booted once with grub2-reboot.
Prefer an entry id or exact title over a numeric index—kernel installation or removal can change the index order. You can also select the required kernel manually from the GRUB menu during startup.
Common Problems and Fixes
| Symptom | Likely cause | Fix |
|---|---|---|
grubby: command not found |
grubby package not installed |
Run dnf install grubby (or yum install grubby on older hosts) |
Kernel missing from grubby --info=ALL |
BLS entry not generated or broken install | Reinstall the matching kernel package and verify the transaction recreates the BLS entry, vmlinuz, and initramfs. Use kernel-install manually only when following instructions for your distribution and release |
| Default changes but another kernel boots | Manual edits to GRUB files, or firmware loading a different GRUB installation | Re-run sudo grubby --set-default with the path; on RHEL 9/10-family systems regenerate /boot/grub2/grub.cfg for both BIOS and UEFI—do not overwrite the UEFI stub under /boot/efi/EFI/ |
| Default index changes after update | New kernel inserted at index 0 |
Re-check grubby --info=ALL; set default by path, not index |
| Boot fails with missing initramfs | initrd path in BLS entry does not match disk |
Confirm /boot/initramfs-<version>.img exists; reinstall the matching kernel-core package for that NEVRA |
| Removed kernel still configured as default | Default pointed at deleted vmlinuz |
Set default to an installed path with sudo grubby --set-default; see kernel-core update notes before removing packages |
| GRUB menu never appears | GRUB_TIMEOUT=0 or hidden timeout style |
Adjust /etc/default/grub timeout settings; on RHEL 9/10-family systems run sudo grub2-mkconfig -o /boot/grub2/grub.cfg for both BIOS and UEFI—older releases and some Fedora versions may differ, so verify the documentation for the installed release first |
grubby changes are ignored |
Firmware is loading another GRUB installation, another ESP, or a nonstandard bootloader | Check the active UEFI boot entry and mounted EFI System Partition before modifying files |
Best Practices
- Keep at least one known-working fallback kernel installed until you have rebooted into and exercised the new default.
- Test an older kernel with
grub2-rebootor the GRUB menu once before you make it the persistent default. - Prefer
sudo grubby --set-defaultwith the full/boot/vmlinuz-*path over--set-default-index. - Run
grubby --default-kernelandgrubby --default-indeximmediately beforesudo rebooton remote systems. - Do not remove the running kernel or the kernel currently set as default—DNF will block the running build, but you can still break the next boot if you delete the configured default.
- Maintain console, IPMI, or snapshot access when changing boot defaults on production hosts.
Summary
On RHEL-family systems and Fedora, grubby is the supported way to read and set the default boot kernel. Check the running kernel with uname -r, check the next boot target with grubby --default-kernel, list entries with grubby --info=ALL, and set the persistent default with sudo grubby --set-default and the full vmlinuz path. Use sudo grub2-reboot when you need a one-time boot only. Reboot and confirm with uname -r before you remove older kernels or declare the change complete.
Official references: grubby(8), GRUB2 on Red Hat Enterprise Linux, and Fedora kernel package management.
