[[restoring-bootloader-using-live-disk]]
= 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*.

.Before you start

* Get the Fedora Live ISO from link:https://download.fedoraproject.org/pub/fedora/linux/releases/[getfedora.org].

* Prepare a bootable device using the downloaded ISO, either a CD or a USB.

.Procedure

. 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.
+
----
# fdisk -l
----

. Follow the <<btrfs-steps, BTRFS steps>> (Fedora 33 or newer) or <<lvm-steps, LVM steps>> (older than Fedora 33) to recover your system.

[[btrfs-steps]]
== BTRFS steps

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/`.

Mount the `/root` partition:

* For LUKS:
+
----
# mount /dev/mapper/myvolume /mnt -o subvol=root
----
* For non-LUKS:
+
----
# mount /dev/sda3 /mnt -o subvol=root
----

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 GRUB:

* 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 GRUB should be installed:
+
----
/]# grub2-install /dev/sda
----

Re-generate the GRUB configuration file:

----
/]# grub2-mkconfig -o /boot/grub2/grub.cfg
----

Sync and exit the chroot:

----
/]# sync && exit
----

Reboot the system. 

[[lvm-steps]]
== LVM steps

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
----

. Scan the LVM volumes for the volume group corresponding to the `/root` partition:
+
----
# vgscan
----

. Activate the volume group (e.g. `fedora_localhost-live`):
+
----
# vgchange -ay fedora_localhost-live
----

. Find the logical volume corresponding to `/root`:
+
----
# lvs
----
+
The logical volume will be accessible under `/dev/mapper/`.

Create a `root` directory under `/mnt`:
----
# mkdir -p /mnt/root
----

Mount the logical volume (e.g. `/dev/mapper/fedora_localhost--live-root`) corresponding to the `/root` partition:

----
# mount /dev/mapper/fedora_localhost--live-root /mnt/root
----

Mount the `/boot` partition (e.g. `/dev/sda2`):

----
# mount /dev/sda2 /mnt/root/boot
----

Mount system processes and devices into the `/root` filesystem:

----
# mount -o bind /dev /mnt/root/dev
# mount -o bind /proc /mnt/root/proc
# mount -o bind /sys /mnt/root/sys
# mount -o bind /run /mnt/root/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/root/sys/firmware/efi/efivars
# mount /dev/sda1 /mnt/root/boot/efi
----

Change your filesystem to the one mounted under `/mnt/root`:

----
# chroot /mnt/root/
----

Re-install GRUB and re-generate the GRUB configuration file:

* On UEFI systems, several packages are required:
+
----
/]# dnf reinstall shim-* grub2-efi-* grub2-common
/]# grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg
----
* On BIOS systems, specify the disk (e.g. `/dev/sda`) where GRUB should be installed:
+
----
/]# grub2-install /dev/sda
/]# grub2-mkconfig -o /boot/grub2/grub.cfg
----

Sync and exit the chroot:

----
/]# sync && exit
----

Reboot the system.