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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.


= Tasks / Common issues =
= Changing kernel command-line parameters with <code>grubby</code> =


== Updating GRUB 2 configuration using grubby ==
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.  
 
The GRUB 2 packages contain commands for installing a bootloader and for creating a bootloader 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. You can install the GRUB 2 bootloader with:
 
<pre># grub2-install /dev/sda</pre>
 
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.  


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 the GRUB configuration file =


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:


== Updating and repairing the GRUB 2 main configuration file ==
<code>sudo grub2-mkconfig -o /etc/grub2.cfg</code>


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.
<code>sudo grub2-mkconfig -o /etc/grub2-efi.cfg</code>


* To discover what firmware your machine is using, run the following command:
<code>sudo grub2-mkconfig -o /boot/grub2/grub.cfg</code> - Legacy boot method for grub update.


** On UEFI system:
These commands use information provided by the <code>os-prober</code> utility to add entries for other Linux distributions and Windows.
<pre># ls -ld /sys/firmware/efi</pre>


** On BIOS system:
{{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.|}}
<pre># ls -lrt /etc/grub2.cfg</pre>


= I have a `grub>` prompt! Now what?  =


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.
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.


{{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>. |}}
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].


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


<pre># chroot /mnt/sysimage</pre>


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].


= Reinstalling GRUB =


== Setting default entry ==
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.


{{admon/warning|Please 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 please set:  <pre>UPDATEDEFAULT=no</pre> (write "no" in lower case) |}}
== Discovering the firmware type ==


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


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.
<pre># [ -d /sys/firmware/efi ] && echo UEFI || echo BIOS</pre>


Open <code>/etc/default/grub</code> and ensure this line exists:
The output returns only UEFI or BIOS, depending on the firmware your machine runs.


<pre>GRUB_DEFAULT=saved</pre>
== Instructions for UEFI-based systems ==


and ensure this line not exists:
* Learn what firmware your machine is running. See [https://fedoraproject.org/wiki/GRUB_2#Discovering_the_firmware_type Discovering the firmware type] section.


<pre>GRUB_SAVEDEFAULT=true</pre>
* 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.


or ensure this line exists:
* Remove the following files:
 
<pre>
<pre>GRUB_SAVEDEFAULT=false</pre>  
# rm /boot/efi/EFI/fedora/grub.cfg
 
# rm /boot/grub2/grub.cfg
{{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.}}
</pre>
 
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>
* Reinstall the following packages:
<pre># dnf reinstall shim-* grub2-efi-* grub2-common</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/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. }}


{{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.}}
{{admon/note|The removal of the two `grub.cfg` files will trigger a script in `grub2-common` to recreate these files.|}}


If you understand the risks involved and still want to directly modify /boot/grub2/grub.cfg, here's how you can do it:
== Instructions for BIOS-based systems ==


Edit /boot/grub2/grub.cfg, and change the line
* Learn what firmware your machine is running. See [https://fedoraproject.org/wiki/GRUB_2#Discovering_the_firmware_type Discovering the firmware type] section.


{{admon/caution|This is not the recommended method|This will not survive grub2-mkconfig. It might not even survive a kernel update.}}
* 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>
set default="0"  
# mount | grep "/boot "
/dev/sda4 on /boot type ext4 (rw,relatime,seclabel)
</pre>
</pre>


to
The device node is <code>/dev/sda4</code>.
 
* Reinstall the bootloader while specifying the device node without the number:
<pre>
<pre>
set default="5"
# grub2-install /dev/sda
Installing for i386-pc platform.
Installation finished. No error reported.
</pre>
</pre>


= Enabling serial console in GRUB 2 =


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


== Enabling serial console in GRUB 2 ==
<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
</pre>


To enable serial console for usage on virtual environments you need to run the following command:
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>
# grubby --args="systemd.journald.forward_to_console=1 console=ttyS0,38400 console=tty1" --update-kernel=/boot/vmlinuz-5.11.16-300.fc34.x86_64
# grubby --set-default=/boot/vmlinuz-5.11.16-300.fc34.x86_64
# grubby --set-default=/boot/vmlinuz-5.11.16-300.fc34.x86_64
</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.
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 [https://www.gnu.org/software/grub/manual/grub/html_node/Serial-terminal.html Using GRUB via a serial line]
 
 
{{admon/note|In UEFI boot environment, use <code>efi0</code> instead of <code>--unit=0</code>. If that doesn't 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 for more information. |}}
 


* 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.

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
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.
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.

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