Chapter 7. Making persistent changes to the GRUB boot loader
Chapter 7. Making persistent changes to the GRUB boot loader
https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/8/html/managing_monitoring_and_updating_the_kernel/assembly_making-persistent-changes-to-the-grub-boot-loader_managing-monitoring-and-updating-the-kernel
Use the grubby tool to make persistent changes in GRUB.
7.1. Prerequisites
Copy link You have successfully installed RHEL on your system. You have root permission. 7.2. Listing the default kernel
By listing the default kernel, you can find the file name and the index number of the default kernel to make permanent changes to the GRUB boot loader.
Procedure
To get the file name of the default kernel, enter:
grubby –default-kernel
/boot/vmlinuz-4.18.0-372.9.1.el8.x86_64
To get the index number of the default kernel, enter:
grubby –default-index
0
7.3. Viewing the GRUB menu entry for a kernel
You can list all the kernel menu entries or view the GRUB menu entry for a specific kernel.
Procedure
To list all kernel menu entries, enter:
grubby –info=ALL
index=0 kernel=”/boot/vmlinuz-4.18.0-372.9.1.el8.x86_64” args=”ro crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet $tuned_params zswap.enabled=1” root=”/dev/mapper/rhel-root” initrd=”/boot/initramfs-4.18.0-372.9.1.el8.x86_64.img $tuned_initrd” title=”Red Hat Enterprise Linux (4.18.0-372.9.1.el8.x86_64) 8.6 (Ootpa)” id=”67db13ba8cdb420794ef3ee0a8313205-4.18.0-372.9.1.el8.x86_64” index=1 kernel=”/boot/vmlinuz-0-rescue-67db13ba8cdb420794ef3ee0a8313205” args=”ro crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet” root=”/dev/mapper/rhel-root” initrd=”/boot/initramfs-0-rescue-67db13ba8cdb420794ef3ee0a8313205.img” title=”Red Hat Enterprise Linux (0-rescue-67db13ba8cdb420794ef3ee0a8313205) 8.6 (Ootpa)” id=”67db13ba8cdb420794ef3ee0a8313205-0-rescue” Show more
To view the GRUB menu entry for a specific kernel, enter:
grubby –info /boot/vmlinuz-4.18.0-372.9.1.el8.x86_64
grubby –info /boot/vmlinuz-4.18.0-372.9.1.el8.x86_64 index=0 kernel=”/boot/vmlinuz-4.18.0-372.9.1.el8.x86_64” args=”ro crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet $tuned_params zswap.enabled=1” root=”/dev/mapper/rhel-root” initrd=”/boot/initramfs-4.18.0-372.9.1.el8.x86_64.img $tuned_initrd” title=”Red Hat Enterprise Linux (4.18.0-372.9.1.el8.x86_64) 8.6 (Ootpa)” id=”67db13ba8cdb420794ef3ee0a8313205-4.18.0-372.9.1.el8.x86_64” Show more
Note Try tab completion to see available kernels within the /boot directory.
7.4. Editing a Kernel Argument
You can change a value in an existing kernel argument. For example, you can change the virtual console (screen) font and size.
Procedure
Change the virtual console font to latarcyrheb-sun with the size of 32:
grubby –args=vconsole.font=latarcyrheb-sun32 –update-kernel /boot/vmlinuz-4.18.0-372.9.1.el8.x86_64
7.5. Adding and removing arguments from a GRUB menu entry
You can add, remove, or simultaneously add and remove arguments from the GRUB Menu.
Procedure
To add arguments to a GRUB menu entry, use the –update-kernel option in combination with –args. For example, following command adds a serial console:
grubby –args=console=ttyS0,115200 –update-kernel /boot/vmlinuz-4.18.0-372.9.1.el8.x86_64
The console arguments are attached to the end of the line, the new console will take precedence over any other configured consoles.
To remove arguments from a GRUB menu entry, use the –update-kernel option in combination with –remove-args. For example:
grubby –remove-args=”rhgb quiet” –update-kernel /boot/vmlinuz-4.18.0-372.9.1.el8.x86_64
This command removes the Red Hat graphical boot argument and enables log messages, that is verbose mode.
To add and remove arguments simultaneously, enter:
grubby –remove-args=”rhgb quiet” –args=console=ttyS0,115200 –update-kernel /boot/vmlinuz-4.18.0-372.9.1.el8.x86_64
Verification
To review the permanent changes you have made, enter:
grubby –info /boot/vmlinuz-4.18.0-372.9.1.el8.x86_64
index=0 kernel=”/boot/vmlinuz-4.18.0-372.9.1.el8.x86_64” args=”ro crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap $tuned_params zswap.enabled=1 console=ttyS0,115200” root=”/dev/mapper/rhel-root” initrd=”/boot/initramfs-4.18.0-372.9.1.el8.x86_64.img $tuned_initrd” title=”Red Hat Enterprise Linux (4.18.0-372.9.1.el8.x86_64) 8.6 (Ootpa)” id=”67db13ba8cdb420794ef3ee0a8313205-4.18.0-372.9.1.el8.x86_64” Show more
7.6. Adding a new boot entry
You can add a new boot entry to the boot loader menu entries.
Procedure
Copy all the kernel arguments from your default kernel to this new kernel entry:
grubby –add-kernel=new_kernel –title=”entry_title” –initrd=”new_initrd” –copy-default
Get the list of available boot entries:
ls -l /boot/loader/entries/*
-rw-r–r–. 1 root root 408 May 27 06:18 /boot/loader/entries/67db13ba8cdb420794ef3ee0a8313205-0-rescue.conf -rw-r–r–. 1 root root 536 Jun 30 07:53 /boot/loader/entries/67db13ba8cdb420794ef3ee0a8313205-4.18.0-372.9.1.el8.x86_64.conf -rw-r–r– 1 root root 336 Aug 15 15:12 /boot/loader/entries/d88fa2c7ff574ae782ec8c4288de4e85-4.18.0-193.el8.x86_64.conf Show more
Create a new boot entry. For example, for the 4.18.0-193.el8.x86_64 kernel, issue the command as follows:
grubby –grub2 –add-kernel=/boot/vmlinuz-4.18.0-193.el8.x86_64 –title=”Red Hat Enterprise 8 Test” –initrd=/boot/initramfs-4.18.0-193.el8.x86_64.img –copy-default
Verification
Verify that the newly added boot entry is listed among the available boot entries:
ls -l /boot/loader/entries/*
-rw-r–r–. 1 root root 408 May 27 06:18 /boot/loader/entries/67db13ba8cdb420794ef3ee0a8313205-0-rescue.conf -rw-r–r–. 1 root root 536 Jun 30 07:53 /boot/loader/entries/67db13ba8cdb420794ef3ee0a8313205-4.18.0-372.9.1.el8.x86_64.conf -rw-r–r– 1 root root 287 Aug 16 15:17 /boot/loader/entries/d88fa2c7ff574ae782ec8c4288de4e85-4.18.0-193.el8.x86_64.0~custom.conf -rw-r–r– 1 root root 287 Aug 16 15:29 /boot/loader/entries/d88fa2c7ff574ae782ec8c4288de4e85-4.18.0-193.el8.x86_64.conf Show more
7.7. Changing the default boot entry with grubby
With the grubby tool, you can change the default boot entry.
Procedure
To make a persistent change in the kernel designated as the default kernel, enter:
grubby –set-default /boot/vmlinuz-4.18.0-372.9.1.el8.x86_64
The default is /boot/loader/entries/67db13ba8cdb420794ef3ee0a8313205-4.18.0-372.9.1.el8.x86_64.conf with index 0 and kernel /boot/vmlinuz-4.18.0-372.9.1.el8.x86_64
7.8. Updating all kernel menus with the same arguments
You can add the same kernel boot arguments to all the kernel menu entries.
Procedure
To add the same kernel boot arguments to all the kernel menu entries, attach the –update-kernel=ALL parameter. For example, this command adds a serial console to all kernels:
grubby –update-kernel=ALL –args=console=ttyS0,115200
Note The –update-kernel parameter also accepts DEFAULT or a comma-separated list of kernel index numbers.
7.9. Changing default kernel options for current and future kernels
By using the kernelopts variable, you can change the default kernel options for both current and future kernels.
Procedure
List the kernel parameters from the kernelopts variable:
grub2-editenv - list | grep kernelopts
kernelopts=root=/dev/mapper/rhel-root ro crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet
Make the changes to the kernel command-line parameters. You can add, remove or modify a parameter. For example, to add the debug parameter, enter:
grub2-editenv - set “$(grub2-editenv - list | grep kernelopts) "
Optional: Verify the parameter newly added to kernelopts:
grub2-editenv - list | grep kernelopts
kernelopts=root=/dev/mapper/rhel-root ro crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet debug
Reboot the system for the changes to take effect. Note As an alternative, you can use the grubby command to pass the arguments to current and future kernels:
grubby –update-kernel ALL –args=”"
7.10. Additional resources
The /usr/share/doc/grub2-common directory. The info grub2 command.