{{parent page="Linux"}}

**Sub topics**
==[[LinuxBootParameters Useful boot time parameters]]==
==[[LinuxGrub2 Grub2]]==

===Backup MBR===
The first 512b contains partition table and the MBR. First 448b contains the MBR only.
ref http://www.tuxation.com/mbr-tricks-with-linux.html

%%
dd if=/dev/sda of=mbr.img bs=448 count=1
%%


===Rescue / emergency boot disc===
It is a good idea to make such a boot disc especially when you have custom kernel with extra filesystem and storage controllers enabled. On your running system, run the following to create an iso, which can then be cut to a cd disc:

%%
mkbootdisk --device /rescue.iso --verbose --iso 2.6.28.1
%%

===Fixing boot record===
Sometimes, MBR (master boot record) may be messed up if you have a dual-boot system. Do the followings and get your boot loader fixed:

%%(bash)
# boot up your system with your installation CD, enter **linux rescue** at boot prompt. Most Linux LiveCD (like Ubunto) will do.
mkdir /mnt/mysys

# My root partition is on /dev/vg0/lv_root
mount -t ext3 /dev/vg0/lv_root /mnt/mysys
mount -t proc none /mnt/mysys/proc
mount -o bind /dev /mnt/mysys/dev

# chroot to the mysys mount point
chroot /mnt/mysys /bin/bash

# mount the boot partition
mount -t ext3 /dev/hda2 /boot

# run grub install to fix the boot loader
grub-install /dev/hda
grub-install --recheck /dev/dev/hda

# Reboot
%%

==Fix grub in grub prompt==
%%
[root@www root]# grub

grub> find /boot/grub/stage1
(hd0,0)

grub> root (hd0,0)
Filesystem type is ext2fs, partition type 0x83

grub> setup (hd0)
%%

==Boot from grub prompt==
The key is to point grub to the right partition. If the root parameter points to a partition other than /, system will halt with a kernel panic error.

%%
grub> root (hd0,1)
grub> kernel /vmlinuz root=/dev/cciss/c0d0p1 ro vga=791
grub> boot
%%

===Setting up a boot-once grub session===
It's very useful to setup grub to boot to a specific kernel only once, to perhaps test out a newly compiled kernel. Setup your grub.conf like this
%%
default saved # This is important!!!
timeout 10

title the old kernel
root (hd0,0)
kernel /old_kernel
savedefault

title the new kernel
root (hd0,0)
kernel /new_kernel
savedefault 0 # This is important!!!
%%

Essentially, when entry #1 is booted, it will instruct grub to boot from entry #0 on next boot. Save grub.conf and then do a
%%(bash)
grub-set-default 1
%%

You can also configure grub to do fallback boot. See Grub's manual http://www.gnu.org/software/grub/manual/grub.html#Booting-once_002donly

===Messing with initrd===
If you clone / migrate your system to a different box which uses a different storage card, you may need to edit the initrd file to add load the necessary modules.

%%
# back it up first!!
cp initrd-2.x.x.img initrd-2.x.x.img-bak
# extract the image to temp
mkdir temp
cd temp
zcat ../initrd-2x.x.img | cpio -idmv
# do what you have to. e.g. edit init to load a certain module, copy that module to temp/lib
# then pack the image
find . | cpio --create --format=newc > /boot/initrd-2.x.x-new.img
%%