Grub2

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set default="5"
 
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== Updating Grub configuration ==
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It is safe to directly edit /boot/grub2/grub.cfg in Fedora.  Grubby in Fedora modifies the configuration when a kernel update is performed but does so in a safe manner.  Other distributions, in particular Debian and Debian derives distributions have a distribution specific patch that adds a update-grub command.  This is not included in Fedora and not required either.
  
 
== Encountering the dreaded Grub2 boot prompt ==
 
== Encountering the dreaded Grub2 boot prompt ==

Revision as of 03:57, 9 November 2011

Contents

Grub2

Starting with Fedora 16, grub2 is used for new installs 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.

Tasks / Common issues

Adding Other operating systems to the grub2 menu

If you have other operating systems setup and wish to boot them via grub2:

Note.png
Note
As of the F16 Beta, os-prober should get pulled in automatically during install and used to auto-generate entries for your other installed operating systems.
# yum install os-prober

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

Setting default entry

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. To do this, we set two things: First, we set
GRUB_DEFAULT=saved
in /etc/default/grub. Second, we call
grub2-set-default <title or number>
with the title of the newly installed Fedora's entry. From that point on you can change the default by calling
grub2-set-default <title or number>
or view it by running
grub2-editenv list


Note.png
Note
There are other, simpler, ways of accomplishing this, 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" 

to

set default="5"

Updating Grub configuration

It is safe to directly edit /boot/grub2/grub.cfg in Fedora. Grubby in Fedora modifies the configuration when a kernel update is performed but does so in a safe manner. Other distributions, in particular Debian and Debian derives distributions have a distribution specific patch that adds a update-grub command. This is not included in Fedora and not required either.

Encountering the dreaded Grub2 boot prompt

You would swear that drive should boot and everything is ok, but then you see the Grub2 Command Prompt what to do and how to recover

1. List the drives which Grub2 saw

grub2> ls

2. The output for a dos partition table /dev/sda with three partitons will look something like

(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

(hd0) (hd0,gpt4) (hd0,gpt3)  (hd0,gpt2) (hd0,gpt1)

4. With this information you can now interrogate each partition of the drive and locate your vmlinuz and initramfs files

ls (hd0,1)/ 

Would list the files on /dev/sda1 and if it was the /boot would give you the full name of vmlnuz and initramfs

5. Armed with the location and full name of vmlinuz and initramfs you can "bootstrap" boot your system

5a. Declare your root partition first

grub> set root=(hd0,3)

5b. Declare the kernel you want used

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 Grub2 to now please boot the chosen files

grub> boot

6. Now when your system boots 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

Other issues

Other grub2 issues: it refuses to install on partition's boot sector (maybe can be forced?) and, !!! this is important !!!, it fails to install if for whatever reason your floppy controller is activated in BIOS and there's no drive connected or floppy disk inserted. The workaround is to run (post OS install) from rescue mode:

grub2-install <target device> --no-floppy

Further Reading

http://www.gnu.org/software/grub/manual/grub.html

http://fedoraproject.org/wiki/Features/Grub2

http://fedoraproject.org/wiki/Anaconda/Features/Grub2Migration