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Revision as of 08:35, 21 July 2018 by Rathann (talk | contribs) (Document my findings about using serial console in UEFI environment.)


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 on BIOS systems

The grub2 packages contain commands for installing a bootloader and for creating a bootloader configuration file.

grub2-install will install the bootloader - usually in the MBR, in free unpartioned space, and as files in /boot. The bootloader is installed with something like:

grub2-install /dev/sda

grub2-mkconfig will create a new configuration based on the currently running system, what is found in /boot, what is set in /etc/default/grub, and the customizable scripts in /etc/grub.d/ . A new configuration file is created with:

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

The configuration format has evolved over time, and a new configuration file might be slightly incompatible with the old bootloader. It is therefore a good idea to first run grub2-install whenever you would need to run grub2-mkconfig.

The Fedora installer, anaconda, will run these grub2 commands and there is usually no reason to run them manually.

It is generally safe to directly edit /boot/grub2/grub.cfg in Fedora. 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. (Other distributions, in particular Debian and Debian-derived distributions provide a software patch that adds an update-grub command which is neither included nor needed in Fedora.) Manual changes might however be overwritten with grub2-mkconfig next time the system is upgraded with anaconda. Some customizations can be placed in /etc/grub.d/40_custom or /boot/grub2/custom.cfg and will survive running grub2-mkconfig.

Updating GRUB 2 configuration on UEFI systems

To install or fix GRUB 2 on a UEFI system on Fedora 18 or newer, you need to do four things:

Create an ESP

UEFI firmware, in general, likes to boot from an EFI System Partition on a disk with a GPT label. In gdisk, it looks something like this:

Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048          264191   128.0 MiB   EF00  EFI System

That partition should be formatted as FAT. If in doubt, FAT32 is a good dialect of FAT to choose.

Fedora expects this partition to be mounted at /boot/efi.

Install the bootloader files

If you don't already have the relevant packages installed, do for Fedora 22 and later versions with DNF or with YUM for older Fedora releases:

dnf install grub2-efi shim
yum install grub2-efi shim

If you do, then try:

dnf reinstall grub2-efi shim
yum reinstall grub2-efi shim


Make sure that /boot/efi is mounted when you do this.

This installs the signed shim and the GRUB 2 binary.

Create a GRUB 2 configuration

Under EFI, GRUB 2 looks for its configuration in /boot/efi/EFI/fedora/grub.cfg. For newly installed kernels to work, grubby expects /etc/grub2-efi.cfg to be a symlink to the real grub.cfg (i.e. /boot/efi/EFI/fedora/grub.cfg).

If you already have a grub 2 EFI config file, you should be okay. If not, grub2-mkconfig can help, but your mileage may vary.

   grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg
Stop (medium size).png
grub2-install shouldn't be used on EFI systems. The grub2-efi package installs a prebaked grubx64.efi on the EFI System partition, which looks for grub.cfg on the ESP in /EFI/fedora/ whereas the grub2-install command creates a custom grubx64.efi, deletes the original installed one, and looks for grub.cfg in /boot/grub2/.

Create a boot menu entry

TL;DR: This should happen automatically. If it doesn't, read on.

When you power on your system, your firmware will look for EFI variables that tell it how to boot. If you're already booted in EFI mode and EFI runtime services are working correctly, you can configure your boot menu with efibootmgr. If not, you'll have to bootstrap the process.

Fortunately, 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, if such entries do not already appear to exist. shim provides a BOOT.CSV file that will add an entry for grub2-efi for you. So just using the EFI Shell to invoke fallback.efi should do the trick. You can do this with commands like:

> fs0:
> 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, invoke fallback.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 GRUB 2 menu

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 .

Setting default entry

Please look to (default) kernel sysconfig options.
if file /etc/sysconfig/kernel have
in every kernel update the grub entry is update to last entry, if you don't want that please set:
(write "no" in lower case)
Some parts of this section is wrong or outdated for F17 and later releases.
Be also aware of [Bug 768106 - grubby does not support grub2 set default="${saved_entry}" and replaces with "0"]. version grubby-8.28-1.fc19 has fixed issues with "Default Menuentry" as noted in the linked bug-report

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:


and ensure this line not exists:


or ensure this line exists:

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

Stop (medium size).png
This is not the recommended method
This will not survive grub2-mkconfig. It might not even survive a kernel update.
set default="0" 


set default="5"

Encountering the dreaded GRUB 2 boot prompt

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).

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

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

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_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.

Further Reading