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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> =


== Adding Other operating systems to the GRUB 2 menu ==
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.


If you have other operating systems installed and wish to boot them via GRUB 2:  
Following are some of the selected illustrations of <code>grubby</code> usage:


<pre>
* To add one kernel parameter to a single boot entry:
# yum install os-prober
<pre># grubby --args=<NEW_PARAMETER> --update-kernel=/boot/vmlinuz-5.11.14-300.fc34.x86_64</pre>
 
* 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>
 
* To add one kernel parameter to all currently existing and future boot entries:
<pre># grubby --args=<NEW_PARAMETER> --update-kernel=ALL</pre>
 
* To remove one kernel parameter from all currently existing and future boot entries:
<pre># grubby --remove-args=<PARAMETER_TO_REMOVE> --update-kernel=ALL</pre>
 
* To set the default kernel:
<pre># grubby --set-default=/boot/vmlinuz-5.11.12-300.fc34.x86_64</pre>


# grub2-mkconfig -o /boot/grub2/grub.cfg
= Updating the GRUB configuration file =
</pre>


{{admon/note|Note|As of the F16 Beta, os-prober should get pulled in automatically during install and used to auto-generate entries for your other installed operating systems.}}
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:


== Setting default entry ==
<code>sudo grub2-mkconfig -o /etc/grub2.cfg</code>


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


Open <code>/etc/default/grub</code> and ensure this line exists:
<code>sudo grub2-mkconfig -o /boot/grub2/grub.cfg</code> - Legacy boot method for grub update.


<pre>GRUB_DEFAULT=saved</pre>
These commands use information provided by the <code>os-prober</code> utility to add entries for other Linux distributions and Windows.


Apply the change to <code>grub.cfg</code> by running: <pre>grub2-mkconfig -o /boot/grub2/grub.cfg</pre>
{{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.|}}


Now list all possible menu entries
= I have a `grub>` prompt! Now what?  =
<pre>grep ^menuentry /boot/grub2/grub.cfg | cut -d "'" -f2</pre>  


Now set the desired default menu entry <pre>grub2-set-default <menu entry title></pre>
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.


Verify the default menu entry <pre>grub2-editenv list</pre>
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].


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


If you understand the risks involved and still want to directly modify /boot/grub2/grub.cfg, here's how you can do it:
<pre># chroot /mnt/sysimage</pre>


Edit /boot/grub2/grub.cfg, and change the line
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].


{{admon/caution|This is not the recommended method|This will not survive grub2-mkconfig. It might not even survive a kernel update.}}
= Reinstalling GRUB =


<pre>
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.
set default="0"
</pre>


to
== Discovering the firmware type ==
<pre>
set default="5"
</pre>


== Updating GRUB 2 configuration ==
To discover what firmware your machine uses, run the following command:


It is safe to directly edit /boot/grub2/grub.cfg in Fedora. Grubby in Fedora modifies the configuration when a kernel update is performed but does so in a safe manner. Other distributions, in particular Debian and Debian-derived distributions provide a software patch that adds an {{command|update-grub}} command which is neither included nor needed in Fedora.
<pre># [ -d /sys/firmware/efi ] && echo UEFI || echo BIOS</pre>


== Encountering the dreaded GRUB 2 boot prompt ==
The output returns only UEFI or BIOS, depending on the firmware your machine runs.
If improperly configured, GRUB 2 may fail to load and subsequently drop to a boot prompt. To address this issue, proceed as follows:


1. List the drives which GRUB 2 sees:
== Instructions for UEFI-based systems ==
<pre>
grub2> ls
</pre>


2. The output for a dos partition table /dev/sda with three partitons will look something like this:
* Learn what firmware your machine is running. See [https://fedoraproject.org/wiki/GRUB_2#Discovering_the_firmware_type Discovering the firmware type] section.
<pre>
(hd0) (hd0,msdos3) (hd0,msdos2) (hd0,msdos1)
</pre>


3. While the output for a gpt partition table /dev/sda with four partitions will look something like this:
* 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.
<pre>
(hd0) (hd0,gpt4) (hd0,gpt3)  (hd0,gpt2) (hd0,gpt1)
</pre>


4. With this information you can now probe each partition of the drive and locate your vmlinuz and initramfs files:
* Remove the following files:
<pre>
<pre>
ls (hd0,1)/  
# rm /boot/efi/EFI/fedora/grub.cfg
# rm /boot/grub2/grub.cfg
</pre>
</pre>
Will list the files on /dev/sda1. If this partition contains /boot, the output will show the full name of vmlinuz and initramfs.


5. Armed with the location and full name of vmlinuz and initramfs you can now boot your system.
* Reinstall the following packages:
<pre># dnf reinstall shim-* grub2-efi-* grub2-common</pre>


5a. Declare your root partition:
{{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. }}
<pre>
grub> set root=(hd0,3)
</pre>


5b. Declare the kernel you wish to use:
{{admon/note|The removal of the two `grub.cfg` files will trigger a script in `grub2-common` to recreate these files.|}}
<pre>
grub> linux (hd0,1)/vmlinuz-3.0.0-1.fc16.i686 root=/dev/sda3 rhgb quiet selinux=0
# NOTE : add other kernel args if you have need of them
# NOTE : change the numbers to match your system
</pre>


5c. Declare the initrd to use:
== Instructions for BIOS-based systems ==
<pre> 
grub> initrd (hd0,1)/initramfs-3.0.0-1.fc16.i686.img
# NOTE : change the numbers to match your system
</pre>


5d. Instruct GRUB 2 to boot the chosen files:
* 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> boot
</pre>


6. After boot, open a terminal.
* 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.


7. Issue the grub2-mkconfig command to re-create the grub.cfg file grub2 needed to boot your system:
* Find the device node the <code>/boot/</code> directory is located on:
<pre>
<pre>
grub2-mkconfig -o /boot/grub2/grub.cfg
# mount | grep "/boot "
/dev/sda4 on /boot type ext4 (rw,relatime,seclabel)
</pre>
</pre>


8. Issue the grub2-install command to install grub2 to your hard drive and make use of your config:
The device node is <code>/dev/sda4</code>.  
 
* Reinstall the bootloader while specifying the device node without the number:
<pre>
<pre>
grub2-install --boot-directory=/boot /dev/sda
# grub2-install /dev/sda
# Note: your drive may have another device name. Check for it with mount command output.
Installing for i386-pc platform.
Installation finished. No error reported.
</pre>
</pre>


== Other GRUB 2 issues ==
= Enabling serial console in GRUB 2 =


''' Absent Floppy Disk ''': It has been reported by some users that GRUB 2 may fail to install on a partition's boot sector if the computer floppy controller is activated in BIOS without an actual floppy disk drive being present. A possible workaround is to run (post OS install) from rescue mode:
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>
grub2-install <target device> --no-floppy
# 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>
</pre>


= Further Reading =
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.


http://www.gnu.org/software/grub/manual/grub.html
<pre>
# grubby --set-default=/boot/vmlinuz-5.11.16-300.fc34.x86_64
</pre>


http://fedoraproject.org/wiki/Features/Grub2
The second command ensures the specified kernel is going to be loaded by default on next reboot.


http://fedoraproject.org/wiki/Anaconda/Features/Grub2Migration
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].

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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