Introduction
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.
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.
Tasks / Common issues
Updating GRUB 2 configuration using grubby
The GRUB 2 packages contain commands for installing a bootloader and for creating a bootloader configuration file.
The grub2-install 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 /boot/ directory. You can install the GRUB 2 bootloader with:
# grub2-install /dev/sda
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 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 paramters 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
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
if file
/etc/sysconfig/kernel have UPDATEDEFAULT=yesin every kernel update the grub entry is update to last entry, if you don't want that please set:
UPDATEDEFAULT=no(write "no" in lower case)
Due to grub2-mkconfig (and os-prober) we cannot predict the order of the entries in /boot/grub2/grub.cfg, so we set the default by name/title instead.
Open /etc/default/grub and ensure this line exists:
GRUB_DEFAULT=saved
and ensure this line not exists:
GRUB_SAVEDEFAULT=true
or ensure this line exists:
GRUB_SAVEDEFAULT=false
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 grub.cfg by running:
grub2-mkconfig -o /boot/grub2/grub.cfg
Now list all possible menu entries
grep -P "submenu|^menuentry" /boot/grub2/grub.cfg | cut -d "'" -f2
Now set the desired default menu entry
grub2-set-default "<submenu title><menu entry title>"
Verify the default menu entry
grub2-editenv list
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.
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:
Edit /boot/grub2/grub.cfg, and change the line
.png){kind=small}
This will not survive grub2-mkconfig. It might not even survive a kernel update.
set default="0"
to
set default="5"
Encountering the dreaded GRUB 2 boot prompt
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).
If improperly configured, GRUB 2 may fail to load and subsequently drop to a boot prompt. To address this issue, proceed as follows:
0. Load the XFS and LVM modules
insmod xfs insmod lvm
1. List the drives which GRUB 2 sees:
grub2> ls
2. The output for a dos partition table /dev/sda with three partitons will look something like this:
(hd0) (hd0,msdos3) (hd0,msdos2) (hd0,msdos1)
3. While the output for a gpt partition table /dev/sda with four partitions will look something like this:
(hd0) (hd0,gpt4) (hd0,gpt3) (hd0,gpt2) (hd0,gpt1)
4. With this information you can now probe each partition of the drive and locate your vmlinuz and initramfs files:
ls (hd0,1)/
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.
5a. Declare your root partition:
grub> set root=(hd0,3)
5b. Declare the kernel you wish to use:
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
5c. Declare the initrd to use:
grub> initrd (hd0,1)/initramfs-3.0.0-1.fc16.i686.img # NOTE : change the numbers to match your system
5d. Instruct GRUB 2 to boot the chosen files:
grub> boot
6. After boot, open a terminal.
7. Issue the grub2-mkconfig command to re-create the grub.cfg file grub2 needed to boot your system:
grub2-mkconfig -o /boot/grub2/grub.cfg
8. Issue the grub2-install command to install grub2 to your hard drive and make use of your config:
grub2-install --boot-directory=/boot /dev/sda # Note: your drive may have another device name. Check for it with mount command output.
Additional Scenario
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:
insmod part_msdos insmod xfs insmod lvm set root='hd0,msdos1' configfile /grub2/grub.cfg
Where, hd0,msdos1 is the pertinent boot partition, which holds the grub.cfg file.
Other GRUB 2 issues
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:
grub2-install <target device> --no-floppy
Setting a password for interactive edit mode
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:
cat << EOF set superusers="root" export superusers password root secret EOF
To apply your changes run:
grub2-mkconfig -o /boot/grub2/grub.cfg
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
More details can be found at Ubuntu Help: GRUB2 Passwords.
Starting from atleast Fedora 21, the --md5pass 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
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.
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)
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"
And re-generate grub
grub2-mkconfig -o /boot/grub2/grub.cfg
NOTE: in UEFI boot environment, use efi0 instead of --unit=0. If that doesn't work, check that your serial port is visible in your UEFI environment, e.g. by running devtree or dh -p SerialIO in EFI Shell. See this discussion for more information.