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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:
Line 25: Line 17:
<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>


Line 37: Line 29:
<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:


== Adding Other operating systems to the GRUB 2 menu ==
<code>sudo grub2-mkconfig -o /etc/grub2.cfg</code>
 
grub2-mkconfig will add entries for other operating systems it can find. That will be done based on the output of the os-prober tool.
 
That might however not work so well, especially not for booting other Linux operating systems, and especially not on UEFI systems. See http://www.gnu.org/software/grub/manual/grub.html#Multi_002dboot-manual-config .
 
== Setting default entry ==
 
{{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) |}}
 
{{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:
<code>sudo grub2-mkconfig -o /etc/grub2-efi.cfg</code>


Edit /boot/grub2/grub.cfg, and change the line
<code>sudo grub2-mkconfig -o /boot/grub2/grub.cfg</code> - Legacy boot method for grub update.


{{admon/caution|This is not the recommended method|This will not survive grub2-mkconfig. It might not even survive a kernel update.}}
These commands use information provided by the <code>os-prober</code> utility to add entries for other Linux distributions and Windows.


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


to
= I have a `grub>` prompt! Now what?  =
<pre>
set default="5"
</pre>


== Encountering the dreaded GRUB 2 boot prompt ==
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/tip|XFS and LVM Note|Since recent versions of Fedora and even RHEL/CentOS have started using ''xfs'' and ''lvm'' by default,  
it's worth noting that these steps outlined here may need to be preceded with the necessary step to load the XFS and/or LVM module(s).}}


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].


If improperly configured, GRUB 2 may fail to load and subsequently drop to a boot prompt. To address this issue, proceed as follows:
After completing steps specified in the previous link, run the following command to mount the root partition:


0. Load the XFS and LVM modules
<pre># chroot /mnt/sysimage</pre>
<pre>
insmod xfs
insmod lvm
</pre>


1. List the drives which GRUB 2 sees:
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].
<pre>
grub2> ls
</pre>


2. The output for a dos partition table /dev/sda with three partitons will look something like this:
= Reinstalling GRUB =
<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:
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.
<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:
== Discovering the firmware type ==
<pre>
ls (hd0,1)/
</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.
To discover what firmware your machine uses, run the following command:


5a. Declare your root partition:
<pre># [ -d /sys/firmware/efi ] && echo UEFI || echo BIOS</pre>
<pre>
grub> set root=(hd0,3)
</pre>


5b. Declare the kernel you wish to use:
The output returns only UEFI or BIOS, depending on the firmware your machine runs.
<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 UEFI-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 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.


7. Issue the grub2-mkconfig command to re-create the grub.cfg file grub2 needed to boot your system:
* Remove the following files:
<pre>
<pre>
grub2-mkconfig -o /boot/grub2/grub.cfg
# rm /boot/efi/EFI/fedora/grub.cfg
# rm /boot/grub2/grub.cfg
</pre>
</pre>


8. Issue the grub2-install command to install grub2 to your hard drive and make use of your config:
* Reinstall the following packages:
<pre>
<pre># dnf reinstall shim-* grub2-efi-* grub2-common</pre>
grub2-install --boot-directory=/boot /dev/sda
# Note: your drive may have another device name. Check for it with mount command output.
</pre>


== Additional Scenario ==
{{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. }}
It's also possible to boot into a ''configfile'' that's located on another partition. If the user is
faced with such a scenario, as is often the case with multi-boot systems containing Ubuntu and Fedora,
the following steps in the grub rescue shell might become useful to know:


<pre>
{{admon/note|The removal of the two `grub.cfg` files will trigger a script in `grub2-common` to recreate these files.|}}
insmod part_msdos
insmod xfs
insmod lvm
set root='hd0,msdos1'
configfile /grub2/grub.cfg
</pre>


Where, '''hd0,msdos1''' is the pertinent ''boot'' partition, which holds the grub.cfg file.
== Instructions for BIOS-based systems ==


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


''' 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:
* 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>
grub2-install <target device> --no-floppy
# mount | grep "/boot "
/dev/sda4 on /boot type ext4 (rw,relatime,seclabel)
</pre>
</pre>


== Setting a password for interactive edit mode ==
The device node is <code>/dev/sda4</code>.  
 
If you wish to password-protect GRUB2's interactive edit mode '''but''' you do not want to require users to enter a password to do a plain, simple, ordinary boot, create /etc/grub.d/01_users with the following lines:


* Reinstall the bootloader while specifying the device node without the number:
<pre>
<pre>
cat << EOF
# grub2-install /dev/sda
set superusers="root"
Installing for i386-pc platform.
export superusers
Installation finished. No error reported.
password root secret
EOF
</pre>
</pre>


To apply your changes run:
= Enabling serial console in GRUB 2 =
 
<pre>
grub2-mkconfig -o /boot/grub2/grub.cfg
</pre>


You can encrypt the password by using pbkdf2. Use grub2-mkpasswd-pbkdf2 to encrypt the password, then replace the password line with:
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>
password_pbkdf2 root grub.pbkdf2.sha512.10000.1B4BD9B60DE889A4C50AA9458C4044CBE129C9607B6231783F7E4E7191D8254C0732F4255178E2677BBE27D03186E44815EEFBAD82737D81C87F5D24313DDDE7.E9AEB53A46A16F30735E2558100D8340049A719474AEEE7E3F44C9C5201E2CA82221DCF2A12C39112A701292BF4AA071EB13E5EC8C8C84CC4B1A83304EA10F74
# 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>


More details can be found at [https://help.ubuntu.com/community/Grub2/Passwords Ubuntu Help: GRUB2 Passwords].
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.
 
Starting from atleast Fedora 21, the <code>--md5pass</code> kickstart option must be set using output from grub2-mkpasswd-pbkdf2.
 
== Using old graphics modes in bootloader ==
Terminal device is chosen with GRUB_TERMINAL; additional quote from http://www.gnu.org/software/grub/manual/grub.html#Simple-configuration
 
<code>
Valid terminal output names depend on the platform, but may include ‘console’ (PC BIOS and EFI consoles), ‘serial’ (serial terminal), ‘gfxterm’ (graphics-mode output), ‘ofconsole’ (Open Firmware console), or ‘vga_text’ (VGA text output, mainly useful with Coreboot).
 
The default is to use the platform's native terminal output.
</code>
 
The default in Fedora is gfxterm and to get the legacy graphics modes you need to set GRUB_TERMINAL to right variable from the description above in /etc/default/grub
 
== Enable Serial Console in Grub ==
 
To enable Serial console in grub add the following entry's to /etc/default/grub
 
( Adjust baudrate/parity/bits/flow control to fit your environment and cables)


<pre>
<pre>
GRUB_CMDLINE_LINUX='console=tty0 console=ttyS0,115200n8'
# grubby --set-default=/boot/vmlinuz-5.11.16-300.fc34.x86_64
GRUB_TERMINAL=serial
</pre>  
GRUB_SERIAL_COMMAND="serial --speed=115200 --unit=0 --word=8 --parity=no --stop=1"
</pre>
 
And re-generate grub
 
<code>
grub2-mkconfig -o /boot/grub2/grub.cfg
</code>
 
'''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 this discussion] for more information.


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


* 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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