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= Introduction =
= What is GRUB 2 =


GRUB 2 is the latest version of GNU GRUB, the GRand Unified Bootloader. A bootloader is the first software program that runs when a computer starts. It is responsible for loading and transferring control to the operating system kernel, (Linux, in the case of Fedora). The kernel, in turn, initializes the rest of the operating system.
GRUB 2 is the latest version of GNU GRUB, the GRand Unified Bootloader. A bootloader is the first software program that runs when a computer starts. It is responsible for loading and transferring control to the operating system kernel - Linux in the case of Fedora. The kernel, in turn, initializes the rest of the operating system.


GRUB 2 has replaced what was formerly known as GRUB (i.e. version 0.9x), which has, in turn, become GRUB Legacy.
GRUB 2 has replaced GRUB version 0.9x, which has become GRUB Legacy. Upstream refers to GRUB 2 as just GRUB.


Starting with Fedora 16, GRUB 2 is the default bootloader on x86 BIOS systems. For upgrades of BIOS systems the default is also to install GRUB 2, but you can opt to skip bootloader configuration entirely.
GRUB 2 is the bootloader used on x86_64 systems, and on aarch64 if UEFI firmware is also used.


= Adding and removing kernel command-line parameters using grubby =
= Changing kernel command-line parameters with <code>grubby</code> =


Grubby is a utility that updates the bootloader-specific configuration files. The utility is a recommended way for making routine changes to the kernel boot parameters and setting a default kernel.  
The <code>grubby</code> utility updates the bootloader-specific configuration files. The utility is a recommended way for making routine changes to the kernel boot parameters and setting a default kernel.  


Following are some of the selected illustrations of <code>grubby</code> usage:
Following are some of the selected illustrations of <code>grubby</code> usage:
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<pre># grubby --args=<NEW_PARAMETER> --update-kernel=/boot/vmlinuz-5.11.14-300.fc34.x86_64</pre>
<pre># grubby --args=<NEW_PARAMETER> --update-kernel=/boot/vmlinuz-5.11.14-300.fc34.x86_64</pre>


* To add multiple kernel paramters to a single boot entry:
* To add multiple kernel parameters to a single boot entry:
<pre># grubby --args="<NEW_PARAMETER1> <NEW_PARAMETER2 <NEW_PARAMETER_n>" --update-kernel=/boot/vmlinuz-5.11.14-300.fc34.x86_64</pre>
<pre># grubby --args="<NEW_PARAMETER1> <NEW_PARAMETER2 <NEW_PARAMETER_n>" --update-kernel=/boot/vmlinuz-5.11.14-300.fc34.x86_64</pre>


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<pre># grubby --set-default=/boot/vmlinuz-5.11.12-300.fc34.x86_64</pre>
<pre># grubby --set-default=/boot/vmlinuz-5.11.12-300.fc34.x86_64</pre>


= Updating and repairing the GRUB 2 main configuration file =
= Updating the GRUB configuration file =


The <code>/boot/grub2/grub.cfg</code> is the main GRUB 2 configuration file. It is a static file that you rarely modify. Except in cases of disk replacement or installation of another Linux distribution.
The GRUB configuration file is located at <code>/boot/grub2/grub.cfg</code> and is intended to be a static file that does not need updating. In case of disk replacement, or installation of another Linux distribution, <code>/boot/grub2/grub.cfg</code> should be updated. Use the following commands:


== Discovering what firmware the system is running ==
<code>sudo grub2-mkconfig -o /etc/grub2.cfg</code>


To discover what firmware your machine is using, run the following command:
<code>sudo grub2-mkconfig -o /etc/grub2-efi.cfg</code>


* On UEFI systems:
<code>sudo grub2-mkconfig -o /boot/grub2/grub.cfg</code> - Legacy boot method for grub update.
<pre># ls -ld /sys/firmware/efi</pre>


* On BIOS systems:
These commands use information provided by the <code>os-prober</code> utility to add entries for other Linux distributions and Windows.
<pre># ls -lrt /etc/grub2.cfg</pre>


A directory listing of either of these commands indicate that you are running the corresponding firmware.
{{admon/note|Refrain from using `grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg` going forward. This is a valid location on Fedora 33 and earlier. However on Fedora 34 and later, it is a small stub file that merely forwards to `/boot/grub2/grub.cfg`. See the [https://fedoraproject.org/wiki/GRUB_2#Reinstalling_GRUB Reinstalling GRUB] section if you have accidentally overwritten this file.|}}


The <code>grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg</code> command adds entries for other detected operating systems. That will be done based on the output of the <code>os-prober</code> tool.
= I have a `grub>` prompt! Now what?  =


{{admon/note|The above command for updating the GRUB 2 configuration file is only applicable for UEFI systems with Fedora 33 and earlier. Everybody else should use <code>grub2-mkconfig -o /boot/grub2/grub.cfg</code>. |}}
If you are stuck at a `grub>` prompt, use a rescue mode to repair the already installed operating system. You can reach the rescue mode on any Fedora edition, spin of Network Installer, or DVD Installer.
 
== Repairing GRUB 2 ==
 
If your machine is not working because of the broken GRUB 2 bootloader, you can boot into the rescue mode to repair an already installed operating system.


For more details see [https://docs.fedoraproject.org/en-US/fedora/rawhide/install-guide/advanced/Boot_Options/#sect-boot-options-rescue Booting Your Computer in Rescue Mode].
For more details see [https://docs.fedoraproject.org/en-US/fedora/rawhide/install-guide/advanced/Boot_Options/#sect-boot-options-rescue Booting Your Computer in Rescue Mode].
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<pre># chroot /mnt/sysimage</pre>
<pre># chroot /mnt/sysimage</pre>


== Reinstalling GRUB 2 ==
Next, update the GRUB configuration file as described in the [https://fedoraproject.org/wiki/GRUB_2#Updating_the_GRUB_configuration_file Updating the GRUB configuration file] section. Afterwards, continue with the section below for firmware specific instructions on [https://fedoraproject.org/wiki/GRUB_2#Reinstalling_GRUB Reinstalling GRUB].
 
The GRUB 2 packages contain commands for installing the bootloader and for creating the <code>grub.cfg</code> configuration file.


The <code>grub2-install</code> command installs the GRUB 2 bootloader usually in the master boot record (MBR), in free and unpartioned space. The bootloader files are placed in the <code>/boot/</code> directory.
= Reinstalling GRUB =


To reinstall the GRUB 2 bootloader:
GRUB comes in two flavors, BIOS GRUB and UEFI GRUB. The instructions on reinstalling GRUB depend on the firmware type. Systems with UEFI firmware have their GRUB RPM packages updated, which in turn updates the bootloader installed on the EFI System volume.


* Learn what firmware your system is running. See [https://fedoraproject.org/wiki/GRUB_2#Discovering_what_firmware_the_system_is_running Discovering what firmware the system is running] section.
== Discovering the firmware type ==
 
* On UEFI systems run:
<pre># dnf reinstall shim-* grub-efi-*</pre>
 
* On BIOS systems:
** Find the device node the <code>/boot/</code> directory is located on:
<pre>
# mount | grep "/boot "
/dev/sda4 on /boot type ext4 (rw,relatime,seclabel)
</pre>


The device node is <code>/dev/sda4</code>.
To discover what firmware your machine uses, run the following command:


** Reinstall the bootloader while specifying the device node without the number:
<pre># [ -d /sys/firmware/efi ] && echo UEFI || echo BIOS</pre>
<pre>
# grub2-install /dev/sda
Installing for i386-pc platform.
Installation finished. No error reported.
</pre>


{{admon/warning| Do not use the <code>grub2-install</code> command on UEFI systems. On those systems, bootloaders are in the <code>shim</code> and <code>grub-efi</code> packages. By reinstalling those packages, the bootloaders are reinstalled to their proper location in <code>/boot/efi/</code> (the EFI system partition). }}
The output returns only UEFI or BIOS, depending on the firmware your machine runs.


= Appendix =
== Instructions for UEFI-based systems ==


== Enabling serial console in GRUB 2 ==
* Learn what firmware your machine is running. See [https://fedoraproject.org/wiki/GRUB_2#Discovering_the_firmware_type Discovering the firmware type] section.


To enable serial console for usage on virtual environments you need to run the following command:
* Systems with UEFI firmware have the shim and GRUB RPM packages updated, which in turn updates the bootloader files found on the EFI System volume. Reasons for reinstallation include troubleshooting early boot problems, and following inadvertent use of the `grub2-install` command, which results in an unsupported configuration on UEFI systems.


* Remove the following files:
<pre>
<pre>
# grubby --args="systemd.journald.forward_to_console=1 console=ttyS0,38400 console=tty1" --update-kernel=/boot/vmlinuz-5.11.16-300.fc34.x86_64
# rm /boot/efi/EFI/fedora/grub.cfg
# grubby --set-default=/boot/vmlinuz-5.11.16-300.fc34.x86_64
# rm /boot/grub2/grub.cfg
</pre>  
</pre>


The first command specifies the baud rate, console forwarding for <code>systemd</code>, what console to use (<code>tty1</code>) and on what kernel such changes should be applied. The second command ensures the specified kernel is going to be loaded by default on next reboot.
* Reinstall the following packages:
<pre># dnf reinstall shim-* grub2-efi-* grub2-common</pre>


For instructions on how to enable serial consol in GRUB 2 for baremetal machines, see [https://www.gnu.org/software/grub/manual/grub/html_node/Serial-terminal.html Using GRUB via a serial line].
{{admon/warning| Do not use the <code>grub2-install</code> command on UEFI systems. On those systems, bootloaders are in the <code>shim</code> and <code>grub-efi</code> RPM packages. By reinstalling those packages, the bootloaders are reinstalled to their proper location in <code>/boot/efi/</code> on the EFI System volume. }}


In UEFI boot environment, use <code>efi0</code> instead of <code>--unit=0</code>. If it does not work, check that your serial port is visible in your UEFI environment, e.g. by running <code>devtree</code> or <code>dh -p SerialIO</code> in EFI Shell. See [https://lists.gnu.org/archive/html/help-grub/2017-01/msg00007.html Grub2 UEFI boot and serial console output] for more information.
{{admon/note|The removal of the two `grub.cfg` files will trigger a script in `grub2-common` to recreate these files.|}}


== Fixing a damaged GRUB 2 configuration file using a plaintext stub file ==
== Instructions for BIOS-based systems ==


On Fedora 34 and later, you can repair a malfunctioning <code>grub.cfg</code> configuration file by creating a stub file with the following content.
* Learn what firmware your machine is running. See [https://fedoraproject.org/wiki/GRUB_2#Discovering_the_firmware_type Discovering the firmware type] section.


# Discover the Universally Unique Identifier (UUID) for the <code>/boot/</code> mount point:
* Systems with the BIOS firmware have the GRUB RPM packages updated. However, the installed or embedded bootloader is never updated automatically. It is a good idea to update it between Fedora release versions.


* Find the device node the <code>/boot/</code> directory is located on:
<pre>
<pre>
# lsblk --fs
# mount | grep "/boot "
NAME  FSTYPE  FSVER            LABEL                UUID                                FSAVAIL FSUSE% MOUNTPOINT
/dev/sda4 on /boot type ext4 (rw,relatime,seclabel)
sr0    iso9660 Joliet Extension Fedora-WS-Live-34-1-2 2021-04-23-11-17-40-00                    0  100% /run/media/jdoe/Fedora-WS-Live-34-1-2
zram0                                                                                                    [SWAP]
vda                                                                                                     
├─vda1 ext4   1.0                                    dc29837b-22dc-4469-be85-fc9acf3009fd  699.8M    21% /boot
└─vda2 btrfs                    fedora_localhost-live c58f3698-5587-40f2-b920-64d46c43161d  23.7G    14% /home
</pre>
</pre>


The UUID of <code>/boot/</code> is <code>dc29837b-22dc-4469-be85-fc9acf3009fd</code>.
The device node is <code>/dev/sda4</code>.  


# Create a custom grub.cfg file with the following content:
* Reinstall the bootloader while specifying the device node without the number:
<pre>
<pre>
search --no-floppy --fs-uuid --set=dev dc29837b-22dc-4469-be85-fc9acf3009fd
# grub2-install /dev/sda
set prefix=($dev)/grub2
Installing for i386-pc platform.
 
Installation finished. No error reported.
export $prefix
configfile $prefix/grub.cfg
</pre>
</pre>


At the end of first line (<code>--set=dev</code>), there is the UUID value of the <code>/boot/</code> mount point.
= Enabling serial console in GRUB 2 =


The above example assumes default partitioning, where a separate ext4 file system is mounted at <code>/boot/</code>. In case of other configurations, you need to insert <code>/boot/</code> into line 2. For example:
On Fedora 34 and later, you can enable serial console for usage on virtual environments. The following procedure explains how to achieve this goal.


<pre>
<pre>
...
# grubby --args="systemd.journald.forward_to_console=1 console=ttyS0,38400 console=tty1" --update-kernel=/boot/vmlinuz-5.11.16-300.fc34.x86_64
set prefix=($dev)/boot/grub2
</pre>
...
</pre>
 
# Move or copy the custom grub.cfg file you created in the previous step to /boot/efi/EFI/fedora/grub.cfg
 
== Setting default entry ==
 
{{admon/warning|Look to (default) kernel sysconfig options. |if file <code>/etc/sysconfig/kernel</code> have <pre>UPDATEDEFAULT=yes</pre> in every kernel update the grub entry is update to last entry, if you don't want that set:  <pre>UPDATEDEFAULT=no</pre> (write "no" in lower case) |}}
 
{{admon/warning|Some parts of this section is wrong or outdated for F17 and later releases. | <s>Be also aware of [[https://bugzilla.redhat.com/show_bug.cgi?id=768106 Bug 768106 - grubby does not support grub2 set default="${saved_entry}" and replaces with "0"]].</s>  version grubby-8.28-1.fc19 has fixed issues with "Default Menuentry" as noted in the linked bug-report|}}
 
Due to <code>grub2-mkconfig</code> (and os-prober) we cannot predict the order of the entries in <code>/boot/grub2/grub.cfg</code>, so we set the default by name/title instead.
 
Open <code>/etc/default/grub</code> and ensure this line exists:
 
<pre>GRUB_DEFAULT=saved</pre>
 
and ensure this line not exists:
 
<pre>GRUB_SAVEDEFAULT=true</pre>
 
or ensure this line exists:
 
<pre>GRUB_SAVEDEFAULT=false</pre>
 
{{admon/note|Note|If GRUB_SAVEDEFAULT is set to true, then, when an entry is selected, save it as a new default entry for use by future runs of GRUB. So, maybe, you need be sure that GRUB_SAVEDEFAULT is not set to true. GRUB_SAVEDEFAULT is only useful if GRUB_DEFAULT is saved.}}
 
Apply the change to <code>grub.cfg</code> by running: <pre>grub2-mkconfig -o /boot/grub2/grub.cfg</pre>
 
Now list all possible menu entries
<pre>grep -P "submenu|^menuentry" /boot/grub2/grub.cfg | cut -d "'" -f2</pre>
 
Now set the desired default menu entry <pre>grub2-set-default "<submenu title><menu entry title>"</pre>
 
Verify the default menu entry <pre>grub2-editenv list</pre>
 
{{admon/note|Note|The above method fails to work on some F20 (and newer) systems due to a missing or improperly linked /boot/grub2/grubenv file. The /boot/grub2/grubenv is symbolic linked to /boot/efi/EFI/fedora/grubenv but /boot is not mounted at the time of booting. So grub2 does not have access to the environment variables. To fix this, change /boot/grub2/grubenv to point to ../efi/EFI/fedora/grubenv instead and your chosen default OS will boot without any problems.}}
 
{{admon/note|Note|There are other, simpler, ways of setting the default entry, but they are prone to error if/when grub2-mkconfig is re-run. These include directly setting the default in /boot/grub2/grub.cfg or setting GRUB_DEFAULT to either a number or an entry title in /etc/default/grub. Neither of these methods is recommended.}}
 
If you understand the risks involved and still want to directly modify /boot/grub2/grub.cfg, here's how you can do it:
 
Edit /boot/grub2/grub.cfg, and change the line


{{admon/caution|This is not the recommended method|This will not survive grub2-mkconfig. It might not even survive a kernel update.}}
The first command specifies the baud rate, console forwarding for <code>systemd</code>, what console to use (<code>tty1</code>) and on what kernel such changes should be applied.


<pre>
<pre>
set default="0"
# grubby --set-default=/boot/vmlinuz-5.11.16-300.fc34.x86_64
</pre>
</pre>  


to  
The second command ensures the specified kernel is going to be loaded by default on next reboot.
<pre>
set default="5"
</pre>


* http://www.gnu.org/software/grub/manual/grub.html
For instructions on how to enable serial consol in GRUB 2 for baremetal machines, see [https://www.gnu.org/software/grub/manual/grub/html_node/Serial-terminal.html Using GRUB via a serial line].
* [[Features/Grub2]]
* [[Anaconda/Features/Grub2Migration]]

Latest revision as of 11:38, 7 May 2022

What is GRUB 2

GRUB 2 is the latest version of GNU GRUB, the GRand Unified Bootloader. A bootloader is the first software program that runs when a computer starts. It is responsible for loading and transferring control to the operating system kernel - Linux in the case of Fedora. The kernel, in turn, initializes the rest of the operating system.

GRUB 2 has replaced GRUB version 0.9x, which has become GRUB Legacy. Upstream refers to GRUB 2 as just GRUB.

GRUB 2 is the bootloader used on x86_64 systems, and on aarch64 if UEFI firmware is also used.

Changing kernel command-line parameters with grubby

The grubby utility updates the bootloader-specific configuration files. The utility is a recommended way for making routine changes to the kernel boot parameters and setting a default kernel.

Following are some of the selected illustrations of grubby usage:

  • To add one kernel parameter to a single boot entry:
# grubby --args=<NEW_PARAMETER> --update-kernel=/boot/vmlinuz-5.11.14-300.fc34.x86_64
  • To add multiple kernel parameters to a single boot entry:
# grubby --args="<NEW_PARAMETER1> <NEW_PARAMETER2 <NEW_PARAMETER_n>" --update-kernel=/boot/vmlinuz-5.11.14-300.fc34.x86_64
  • To add one kernel parameter to all currently existing and future boot entries:
# grubby --args=<NEW_PARAMETER> --update-kernel=ALL
  • To remove one kernel parameter from all currently existing and future boot entries:
# grubby --remove-args=<PARAMETER_TO_REMOVE> --update-kernel=ALL
  • To set the default kernel:
# grubby --set-default=/boot/vmlinuz-5.11.12-300.fc34.x86_64

Updating the GRUB configuration file

The GRUB configuration file is located at /boot/grub2/grub.cfg and is intended to be a static file that does not need updating. In case of disk replacement, or installation of another Linux distribution, /boot/grub2/grub.cfg should be updated. Use the following commands:

sudo grub2-mkconfig -o /etc/grub2.cfg

sudo grub2-mkconfig -o /etc/grub2-efi.cfg

sudo grub2-mkconfig -o /boot/grub2/grub.cfg - Legacy boot method for grub update.

These commands use information provided by the os-prober utility to add entries for other Linux distributions and Windows.

Note.png
Refrain from using grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg going forward. This is a valid location on Fedora 33 and earlier. However on Fedora 34 and later, it is a small stub file that merely forwards to /boot/grub2/grub.cfg. See the Reinstalling GRUB section if you have accidentally overwritten this file.

I have a grub> prompt! Now what?

If you are stuck at a grub> prompt, use a rescue mode to repair the already installed operating system. You can reach the rescue mode on any Fedora edition, spin of Network Installer, or DVD Installer.

For more details see Booting Your Computer in Rescue Mode.

After completing steps specified in the previous link, run the following command to mount the root partition:

# chroot /mnt/sysimage

Next, update the GRUB configuration file as described in the Updating the GRUB configuration file section. Afterwards, continue with the section below for firmware specific instructions on Reinstalling GRUB.

Reinstalling GRUB

GRUB comes in two flavors, BIOS GRUB and UEFI GRUB. The instructions on reinstalling GRUB depend on the firmware type. Systems with UEFI firmware have their GRUB RPM packages updated, which in turn updates the bootloader installed on the EFI System volume.

Discovering the firmware type

To discover what firmware your machine uses, run the following command:

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

The output returns only UEFI or BIOS, depending on the firmware your machine runs.

Instructions for UEFI-based systems

  • Systems with UEFI firmware have the shim and GRUB RPM packages updated, which in turn updates the bootloader files found on the EFI System volume. Reasons for reinstallation include troubleshooting early boot problems, and following inadvertent use of the grub2-install command, which results in an unsupported configuration on UEFI systems.
  • Remove the following files:
# rm /boot/efi/EFI/fedora/grub.cfg
# rm /boot/grub2/grub.cfg
  • Reinstall the following packages:
# dnf reinstall shim-* grub2-efi-* grub2-common
Warning.png
Do not use the grub2-install command on UEFI systems. On those systems, bootloaders are in the shim and grub-efi RPM packages. By reinstalling those packages, the bootloaders are reinstalled to their proper location in /boot/efi/ on the EFI System volume.
Note.png
The removal of the two grub.cfg files will trigger a script in grub2-common to recreate these files.

Instructions for BIOS-based systems

  • Systems with the BIOS firmware have the GRUB RPM packages updated. However, the installed or embedded bootloader is never updated automatically. It is a good idea to update it between Fedora release versions.
  • Find the device node the /boot/ directory is located on:
# mount | grep "/boot "
/dev/sda4 on /boot type ext4 (rw,relatime,seclabel)

The device node is /dev/sda4.

  • Reinstall the bootloader while specifying the device node without the number:
# grub2-install /dev/sda
Installing for i386-pc platform.
Installation finished. No error reported.

Enabling serial console in GRUB 2

On Fedora 34 and later, you can enable serial console for usage on virtual environments. The following procedure explains how to achieve this goal.

# grubby --args="systemd.journald.forward_to_console=1 console=ttyS0,38400 console=tty1" --update-kernel=/boot/vmlinuz-5.11.16-300.fc34.x86_64

The first command specifies the baud rate, console forwarding for systemd, what console to use (tty1) and on what kernel such changes should be applied.

# grubby --set-default=/boot/vmlinuz-5.11.16-300.fc34.x86_64

The second command ensures the specified kernel is going to be loaded by default on next reboot.

For instructions on how to enable serial consol in GRUB 2 for baremetal machines, see Using GRUB via a serial line.