Le GRUB2 Bootloader - Installation et Configuration
GRUB2 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. In Fedora, the kernel is Linux. The kernel then initializes the rest of the operating system.
GRUB2 est la version suivante de GRUB (version 0.9x). La version originale est disponible sous le nom GRUB Legacy.
Since Fedora 16, GRUB2 has been the default bootloader on x86 BIOS systems. For upgrades of BIOS systems, the default is also to install GRUB2, but you can opt to skip bootloader configuration entirely.
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.
Installing GRUB2 on a BIOS system
Normally, GRUB2 will be installed and set up by the installer, Anaconda, during the installation process. You will probably never have to deal with manual installation of GRUB2. However, in certain situations , you will want to install GRUB2 manually, especially if you need to repair the existing GRUB2 installation or you want to change its configuration.
This procedure shows the steps to install GRUB2 on your Master Boot Record (MBR) of your primary hard disk.
-
Make sure you have the the GRUB2 packages and the os-prober package installed in your system.
$ dnf list installed | grep grub
-
To automatically collect information about your disks and operating systems installed on them, the
os-prober
package needs to be installed on your system.
-
List block devices available on the system.
$ lsblk
-
Identify the primary hard disk. Usually, it is the
sda
device. -
Install GRUB2 in the MBR of the primary hard disk.
# grub2-install /dev/sda
-
Create a configuration file for GRUB2.
# grub2-mkconfig -o /boot/grub2/grub.cfg
-
Reboot your computer to boot with the newly installed bootloader.
-
The
grub2-mkconfig
command creates a new configuration based on the currently running system. It collects information from the/boot
partition (or directory), from the/etc/default/grub
file, and the customizable scripts in/etc/grub.d
. -
The configuration format is changing with time, and a new configuration file can become slightly incompatible with the older versions of the bootloader. Always run
grub2-install
before you create the configuration file withgrub2-mkconfig
. -
In Fedora, it is generally safe to edit
/boot/grub2/grub.cfg
manually. Grubby in Fedora patches the configuration when a kernel update is performed and will try to not make any other changes than what is necessary. Manual changes can be overwritten withgrub2-mkconfig
when the system gets upgraded with Anaconda. Customizations placed in/etc/grub.d/40_custom
or/boot/grub2/custom.cfg
files will survive running thegrub2-mkconfig
command.
Installing GRUB2 on a UEFI system
Normally, GRUB2 will be installed and set up by the installer, Anaconda, during the installation process. You will probably never have to deal with manual installation of GRUB2. However, in certain situations , you will want to install GRUB2 manually, especially if you need to repair the existing GRUB2 installation or you want to change its configuration.
This procedure shows the steps to install GRUB2 on a UEFI system on Fedora 18 or newer. The procedure consists of four parts.
Creating an EFI System Partition
The UEFI firmware requires to boot from an EFI System Partition on a disk with a GPT label. To create such a partition:
-
List available block devices to find a place to create your ESP.
$ lsblk -f -p
-
Create at least a 128 MiB disk partition using a GPT label on the primary hard disk.
# gdisk /dev/sda
For the sake of this procedure, we assume that the created partition is recognized as
/dev/sda1
. -
Format the partition with the FAT32 file system.
# mkfs.vfat /dev/sda1
-
Create the
/boot/efi
directory as a mount point for the new partition.# mkdir /boot/efi
-
Mount the partition to the
/boot/efi
mount point.# mount /dev/sda1 /boot/efi
-
Proceed to the next part.
Install the bootloader files
In order to use GRUB2 with on the UEFI systems, you need to install or re-install appropriate packages:
-
Re-install the necessary packages.
# dnf reinstall grub2-efi grub2-efi-modules shim-\*
-
If the above command ends with an error, install the packages.
# dnf install grub2-efi grub2-efi-modules shim-\*
-
This installs the signed shim and the GRUB2 binary.
Create a GRUB2 configuration
If you already have a working GRUB2 EFI configuration file, you do not need to do anything else.
Otherwise, create the configuration file using the grub2-mkconfig
command.
# grub2-mkconfig -o /boot/grub2/grub.cfg
-
Under EFI, GRUB2 looks for its configuration in
/boot/efi/EFI/fedora/grub.cfg
, however the postinstall script ofgrub2-common
installs a small shim which chains to the standard configuration at/boot/grub2/grub.cfg
which is generated above. To reset this shim to defaults, delete the existing/boot/efi/EFI/fedora/grub.cfg
and then reinstallgrub2-common
.# rm -f /boot/efi/EFI/fedora/grub.cfg # dnf reinstall grub2-common
-
For newly installed kernels to work,
grubby
expects/etc/grub2-efi.cfg
to be a symlink to the realgrub.cfg
(for example/boot/grub2/grub.cfg
).
Solving problems with UEFI bootloader
When you power on your system, your firmware will look for EFI variables that tell it how to boot. On running systems, which have booted into the EFI mode and their EFI runtime services are working correctly, you can configure your boot menu with efibootmgr
.
If not, shim
can help you bootstrap. The EFI program /boot/efi/EFI/BOOT/fallback.efi
will look for files called BOOT.CSV
in your ESP and will add boot entries corresponding to them. The shim
command provides its own BOOT.CSV
file that will add an entry for grub2-efi
.
During the boot process, you can use the EFI Shell to invoke the fallback.efi
profile to boot the system:
-
Enter the boot partition.
fs0:
-
Navigate into the
EFI\BOOT
directory.> cd EFI\BOOT
-
Invoke the
fallback.efi
profile.> fallback.efi
-
If you have no boot entries at all, then just booting off your disk in UEFI mode should automatically invoke
/boot/efi/EFI/BOOT/BOOTX64.EFI
, which will, in turn, invokefallback.efi
. -
If you already have incorrect boot entries, you’ll either need to delete them or to modify
BOOT.CSV
to create new entries with different names.
Adding other operating systems to the GRUB2 menu
Normally, GRUB2 is preset to boot multiple operating systems during the Fedora installation process. If you can, it is advisable to install non-Linux operating systems first. Then, during the installation process, all those operating systems and their locations will be discovered and properly set.
Adding other records into the GRUB2 menu only means to run grub2-mkconfig
command to regenerate the configuration files. During this process, all operating systems known to the system will be added into the configuration. By reinstalling GRUB2, this configuration will be used for further boots.
-
Make sure that the operating systems are on disks, connected to the system.
-
You have the
os-prober
package installed.
-
Recreate the GRUB2 configuration file.
# grub2-mkconfig -o /boot/grub2/grub.cfg
-
Install GRUB2.
-
On UEFI systems.
# dnf reinstall shim-\* grub2-efi-\* grub2-common
-
On BIOS systems, specify the disk where the bootloader should be installed.
# grub2-install /dev/sda
-
-
The
grub2-mkconfig
command will add entries for all operating systems it can find. -
When problems appear, see the GRUB manual to solve issues with booting secondary operating systems.
Setting default entry for GRUB2
Since grub2-mkconfig
(and os-prober) cannot estimate which operating system, of those it finds, is to be marked as default, we usually are unable to predict the order of the entries in /boot/grub2/grub.cfg
. To change the default layout, we need to set the default based on the name
or title
.
-
Open
/etc/default/grub
and make sure these lines exist in the file.GRUB_DEFAULT=saved GRUB_SAVEDEFAULT=false
-
If you needed to change the content of the
/etc/default/grub
, apply the changes togrub.cfg
.# grub2-mkconfig -o /boot/grub2/grub.cfg
-
List all possible menu entries.
# grep -P "^menuentry" /boot/grub2/grub.cfg | cut -d "'" -f2
-
Select one of the displayed options and use it as an argument to set the default menu entry.
# grub2-set-default <menuentry>
-
Verify the default menu entry
# grub2-editenv - list
-
Regenerate the GRUB2 configuration file and reinstall the bootloader into the MBR, as described in Adding other operating systems to the GRUB2 menu.
If you understand the risks involved, you can manually modify the /boot/grub2/grub.cfg
file. In that case, set the number of the default operating system using the set default
variable.
Par exemple :
set default="5"
If you edit the configuration file manually, the settings will be overwritten each time the |
Restoring the bootloader using the Live disk
Sometimes, especially after a secondary operating systems has been installed, the master boot record gets damaged which then prevents the original Linux system from booting.
If this happens, it is necessary to reinstall GRUB2 to recreate the original settings. The process not only discovers all installed operating systems, but usually adds them to the GRUB2 configuration files, so they will all become bootable by GRUB2.
-
Get the Fedora Live ISO from fedoraproject.org.
-
Prepare a bootable device using the downloaded ISO, either a CD or a USB.
-
Boot the Fedora live system from the bootable device you have created.
-
Open the terminal.
-
Examine the partition layout and identify the
/boot
and the/root
partition.# lsblk -f -p
-
If your
/root
partition is encrypted by LUKS, it must be decrypted.-
Make sure the crypt module is in use.
# modprobe dm-crypt
-
Decrypt the
/root
partition (e.g./dev/sda3
).# cryptsetup luksOpen /dev/sda3 myvolume
The decrypted device (i.e.
myvolume
) will be accessible under/dev/mapper
.
-
-
Follow the BTRFS steps (used by default in Fedora 33 or newer).
-
Mount the
/root
partition.-
For LUKS.
# mount /dev/mapper/myvolume /mnt -o subvol=root
-
For non-LUKS.
# mount /dev/sda3 /mnt -o subvol=root
-
-
-
Follow the LVM steps (used by default before Fedora 33).
-
Scan the LVM volumes for the volume group corresponding to the
/root
partition.# vgscan
-
Activate the volume group (e.g.
fedora
).# vgchange -ay fedora
-
Find the logical volume corresponding to
/root
.# lvs
The logical volume will be accessible under
/dev/mapper
. -
Mount the logical volume (e.g.
/dev/mapper/fedora-root
) corresponding to the/root
partition.# mount /dev/mapper/fedora-root /mnt
-
-
Mount the
/boot
partition (e.g./dev/sda2
).# mount /dev/sda2 /mnt/boot
-
Mount system processes and devices into the
/root
filesystem.# mount -o bind /dev /mnt/dev # mount -o bind /proc /mnt/proc # mount -o bind /sys /mnt/sys # mount -o bind /run /mnt/run
-
On UEFI systems, bind the
efivars
directory and mount the EFI system partition (e.g./dev/sda1
).# mount -o bind /sys/firmware/efi/efivars /mnt/sys/firmware/efi/efivars # mount /dev/sda1 /mnt/boot/efi
-
Change your filesystem to the one mounted under
/mnt
.# chroot /mnt
-
Re-install GRUB2.
-
On UEFI systems, several packages are required.
# dnf reinstall shim-\* grub2-efi-\* grub2-common
-
On BIOS systems, specify the disk (e.g.
/dev/sda
) where GRUB2 should be installed.# grub2-install /dev/sda
-
-
Re-generate the GRUB2 configuration file.
# grub2-mkconfig -o /boot/grub2/grub.cfg
-
Sync and exit the chroot.
# sync && exit
-
Reboot the system.
Using the GRUB2 boot prompt
If improperly configured, GRUB2 may fail to load and subsequently drop to a boot prompt. To boot into the system, follow the steps below.
-
Load the necessary modules to read your system’s partitions (you will also need to load
part_msdos
orpart_gpt
, depending on your partition table).-
For BTRFS filesystems (Fedora 33 or newer).
grub> insmod btrfs
-
For LVM filesystems (older than Fedora 33).
grub> insmod xfs grub> insmod lvm
-
-
List the drives which GRUB2 sees.
grub> ls
-
Examine the output to understand the partition table of the
/dev/sda
device. The following example shows a DOS partition table with three partitions.(hd0) (hd0,msdos3) (hd0,msdos2) (hd0,msdos1)
A GPT partition table of the
/dev/sda
device with four partitions could look like this.(hd0) (hd0,gpt4) (hd0,gpt3) (hd0,gpt2) (hd0,gpt1)
-
Probe each partition of the drive and locate your
vmlinuz
andinitramfs
files.grub> ls (hd0,1)/
The outcome of the previous command will list the files on
/dev/sda1
. The partition that contains the/boot
directory is the correct one. There you will search for the full names of thevmlinuz
andinitramfs
files. -
Follow the Pre-boot setup for BTRFS or the Pre-boot setup for LVM to recover your system.
-
After the pre-boot setup, boot the system.
grub> boot
-
To restore the bootloader’s functionality, regenerate the GRUB2 configuration file and reinstall the bootloader, as described in Adding other operating systems to the GRUB2 menu.
Pre-boot setup for BTRFS filesystems
-
On BIOS systems.
-
Set GRUB2 root to your
/boot
partition. If your/boot
partition is(hd0,msdos1)
, the command will be.grub> set root=(hd0,msdos1)
-
Next, select the desired kernel. Set the
/root
partition (e.g./dev/sda2
).grub> linux /vmlinuz-5.14.10-300.fc35.x86_64 root=/dev/sda2 ro rootflags=subvol=root
-
-
On UEFI systems.
-
Set GRUB2 root to your EFI system partition. If your EFI system partition is
(hd0,gpt1)
, use this command.grub> set root=(hd0,gpt1)
-
Next, select the desired kernel. Find the path to
vmlinuz
and set the/root
partition (e.g./dev/sda3
).grub> linux (hd0,gpt2)/vmlinuz-5.14.10-300.fc35.x86_64 root=/dev/sda3 ro rootflags=subvol=root
-
Select the RAM filesystem that will be loaded.
grub> initrd (hd0,gpt2)/initramfs-5.14.10-300.fc35.x86_64.img
-
Pre-boot setup for LVM filesystems
-
On BIOS systems.
-
Set GRUB2 root to your
/boot
partition. If your/boot
partition is(hd0,msdos1)
, use this command.grub> set root=(hd0,msdos1)
-
Next, select the desired kernel. Set
root
to the logical volume that corresponds to the/root
directory.grub> linux /vmlinuz-3.0.0-1.fc16.i686 root=/dev/mapper/fedora-root
-
Select the RAM filesystem that will be loaded.
grub> initrd /initramfs-3.0.0-1.fc16.i686.img
-
-
On UEFI systems.
-
Set GRUB2 root to your EFI system partition. If your EFI system partition is
(hd0,gpt1)
, use this command.set root=(hd0,gpt1)
-
Next, select the desired kernel. Find the path to
vmlinuz
and setroot
to the logical volume that corresponds to the/root
directory.linux (hd0,gpt2)/vmlinuz-3.0.0-1.fc16.i686 root=/dev/mapper/fedora-root
-
Select the RAM filesystem that will be loaded.
initrd (hd0,gpt2)/initramfs-3.0.0-1.fc16.i686.img
-
Booting the system using a configuration file on a different partition
If you end up in GRUB2 boot prompt, it is also possible to boot using a configfile that’s located on another partition, as is often the case with multi-boot systems containing Ubuntu and Fedora. Follow the below procedure if you need to boot from a configuration file on a different partition.
-
Load the necessary modules to read your system’s partitions (you will also need to load
part_msdos
orpart_gpt
, depending on your partition table).-
For BTRFS filesystems.
grub> insmod btrfs
-
For LVM filesystems.
grub> insmod xfs grub> insmod lvm
-
-
Set GRUB2 root to your
/boot
partition. On UEFI systems, you should set GRUB2 root to the EFI system partition.grub> set root=(hd0,msdos1)
-
Set the path to the configuration file.
grub> configfile /grub2/grub.cfg
-
The hd0,msdos1 line shows the pertinent
/boot
partition, which holds thegrub.cfg
file. The setting may be different on your system. See also Using the GRUB2 boot prompt for more information.
Setting a password for interactive edit mode
If you wish to protect the GRUB2 interactive edit mode with a password, but allow ordinary users to boot the computer, use the grub2-set-password
command. You will be prompted for the password, and then will have to confirm it. The encrypted password will be stored in /boot/grub2/user.cfg.
To remove password protection, simply delete the user.cfg file.
Alternately, you can set this up manually:
-
Create the
/etc/grub.d/01_users
file and write the following lines into the file.cat << EOF set superusers="root" export superusers password root <password> EOF
-
Regenerate the GRUB2 configuration file, as described in Adding other operating systems to the GRUB2 menu.
You can encrypt the password by using pbkdf2. Use grub2-mkpasswd-pbkdf2
to encrypt the password, then replace the password line with:
password_pbkdf2 root grub.pbkdf2.sha512.10000.1B4BD9B60DE889A4C50AA9458C4044CBE129C9607B6231783F7E4E7191D8254C0732F4255178E2677BBE27D03186E44815EEFBAD82737D81C87F5D24313DDDE7.E9AEB53A46A16F30735E2558100D8340049A719474AEEE7E3F44C9C5201E2CA82221DCF2A12C39112A701292BF4AA071EB13E5EC8C8C84CC4B1A83304EA10F74
To remove password protection, simple remove the changes you made to the /etc/grub.d/01_users
file and regenerate the GRUB2 configuration file, as before.
More details can be found at Ubuntu Help: GRUB2 Passwords.
Starting from Fedora 15, the |
Dealing with the "Absent Floppy Disk" Error
It has been reported by some users that GRUB2 may fail to install on a partition’s boot sector if the computer’s floppy controller is activated in BIOS without an actual floppy disk drive being present. Such situations resulted in an Absent Floppy Disk error.
To workaround this issue, go into the rescue mode and follow the procedure in Installing GRUB2 on a BIOS system GRUB2, but use the --no-floppy
option with the grub2-install
command.
# grub2-install <target device> --no-floppy
Using old graphics modes in bootloader
The terminal device is chosen with GRUB_TERMINAL. For more information, see the Grub manual.
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.
In Fedora, gfxterm
is the default options. To get the legacy graphics modes:
-
Edit the
/etc/default/grub
file. -
Set the
GRUB_TERMINAL
variable to one of the above mentioned options. -
Regenerate the GRUB2 configuration file and reinstall the bootloader into the MBR, as described in Adding other operating systems to the GRUB2 menu.
Enabling Serial Console in GRUB2
To enable Serial console in grub:
-
Edit the
/etc/default/grub
file. -
Adjust
baudrate
,parity
,bits
, andflow
controls to fit your environment and cables, see the example.GRUB_CMDLINE_LINUX='console=tty0 console=ttyS0,115200n8' GRUB_TERMINAL=serial GRUB_SERIAL_COMMAND="serial --speed=115200 --unit=0 --word=8 --parity=no --stop=1"
-
Regenerate the GRUB2 configuration file and reinstall the bootloader into the MBR, as described in Adding other operating systems to the GRUB2 menu.
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