Description: Candidates should be able to select, install and configure a boot manager.
Key Knowledge Areas:
Providing alternative boot locations and backup boot options
Install and configure a boot loader such as GRUB Legacy
Perform basic configuration changes for GRUB 2
Interact with the boot loader
The following is a partial list of the used files, terms and utilities:
menu.lst, grub.cfg and grub.conf
grub-install
grub-mkconfig
MBR
We have talked about linux boot process and boot loaders. We got introduced to LILO as a very old boot loader which was replaced by GRUB in late 1990s. In this course we take a closer look at GRUB and GRUB2 boot loaders .
GRUB
GRUB (GRand Unified Bootloader) is a boot loader package developed to support multiple operating systems and allow the user to select among them during boot-up.
GRUB versions
GRUB was created by Erich Stefan Boleyn and has been further developed under the GNU project as GNU GRUB. The original package is still available for download but no longer being developed.
GRUB2 has replaced what was formerly known as GRUB (i.e. version 0.9x), which has, in turn, become GRUB Legacy. Enhancements to GRUB2 are still being made, but the current released versions are quite usable for normal operation.
How does GRUB work?
When a computer boots, the BIOS transfers control to the first boot device, which can be a hard disk, a floppy disk, a CD-ROM, or any other BIOS-recognized device.
MBR
The first sector on a hard is called the Master Boot Record (MBR). This sector is only 512 bytes long and contains a small piece of code (446 bytes) called the primary boot loader and the partition table (64 bytes) describing the primary and extended partitions.
GRUB replaces the default MBR with its own code:
[root@centos7-1 ~]# xxd -l 512 /dev/sda
By default, MBR code looks for the partition marked as active and once such a partition is found, it loads its boot sector into memory and passes control to it.
[root@centos7-1 ~]# fdisk -l /dev/sda
Disk /dev/sda: 53.7 GB, 53687091200 bytes, 104857600 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk label type: dos
Disk identifier: 0x000a1a5f
Device Boot Start End Blocks Id System
/dev/sda1 * 2048 2099199 1048576 83 Linux
/dev/sda2 2099200 104857599 51379200 8e Linux LVM
Furthermore, GRUB works in stages.
Stage 1 is located in the MBR and mainly points to Stage 2, since the MBR is too small to contain all of the needed data.
Stage 2 points to its configuration file, which contains all of the complex user interface and options we are normally familiar with when talking about GRUB. Stage 2 can be located anywhere on the disk. If Stage 2 cannot find its configuration table, GRUB will cease the boot sequence and present the user with a command line for manual configuration.
The Stage architecture allows GRUB to be large (~20-30K) and therefore fairly complex and highly configurable, compared to most bootloaders, which are sparse and simple to fit within the limitations of the Partition Table.
GRUB Legacy vs GRUB2
Lets draw a big picture:
/boot/grub/menu.lst in GRUB Legacy has been replaced by /boot/grub/grub.cfg in GRUB2.
In order to show differences between GRUB Legacy and GRUB2, lets change timeout parameter in Two systems (CentsOS5 and Centos7) :
menu.lst (GRUB Legacy)
[root@centos5-1 ~]# cat /boot/grub/menu.lst
# grub.conf generated by anaconda
#
# Note that you do not have to rerun grub after making changes to this file
# NOTICE: You have a /boot partition. This means that
# all kernel and initrd paths are relative to /boot/, eg.
# root (hd0,0)
# kernel /vmlinuz-version ro root=/dev/sda2
# initrd /initrd-version.img
#boot=/dev/sda
default=0
timeout=5
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
title CentOS (2.6.18-398.el5)
root (hd0,0)
kernel /vmlinuz-2.6.18-398.el5 ro root=LABEL=/ rhgb quiet
initrd /initrd-2.6.18-398.el5.img
just make a change in menu.lst and save it and that is all.
grub.cfg (GRUB2)
grub.cfg is overwritten by certain Grub 2 package updates, whenever a kernel is added or removed, or when the user runs update-grub.Do not edit grub.cfg directly!!
[root@centos7-1 ~]# cat /boot/grub2/grub.cfg
#
# DO NOT EDIT THIS FILE
#
# It is automatically generated by grub2-mkconfig using templates
# from /etc/grub.d and settings from /etc/default/grub
#
### BEGIN /etc/grub.d/00_header ###
set pager=1
if [ -s $prefix/grubenv ]; then
load_env
fi
if [ "${next_entry}" ] ; then
set default="${next_entry}"
set next_entry=
save_env next_entry
set boot_once=true
else
set default="${saved_entry}"
fi
if [ x"${feature_menuentry_id}" = xy ]; then
menuentry_id_option="--id"
else
menuentry_id_option=""
fi
export menuentry_id_option
if [ "${prev_saved_entry}" ]; then
set saved_entry="${prev_saved_entry}"
save_env saved_entry
set prev_saved_entry=
save_env prev_saved_entry
set boot_once=true
fi
function savedefault {
if [ -z "${boot_once}" ]; then
saved_entry="${chosen}"
save_env saved_entry
fi
}
function load_video {
if [ x$feature_all_video_module = xy ]; then
insmod all_video
else
insmod efi_gop
insmod efi_uga
insmod ieee1275_fb
insmod vbe
insmod vga
insmod video_bochs
insmod video_cirrus
fi
}
terminal_output console
if [ x$feature_timeout_style = xy ] ; then
set timeout_style=menu
set timeout=5
# Fallback normal timeout code in case the timeout_style feature is
# unavailable.
else
set timeout=5
fi
### END /etc/grub.d/00_header ###
### BEGIN /etc/grub.d/00_tuned ###
set tuned_params=""
set tuned_initrd=""
### END /etc/grub.d/00_tuned ###
### BEGIN /etc/grub.d/01_users ###
if [ -f ${prefix}/user.cfg ]; then
source ${prefix}/user.cfg
if [ -n "${GRUB2_PASSWORD}" ]; then
set superusers="root"
export superusers
password_pbkdf2 root ${GRUB2_PASSWORD}
fi
fi
### END /etc/grub.d/01_users ###
### BEGIN /etc/grub.d/10_linux ###
menuentry 'CentOS Linux (3.10.0-693.el7.x86_64) 7 (Core)' --class centos --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-3.10.0-693.el7.x86_64-advanced-162b8754-3820-4229-9ab4-810de8ab8558' {
load_video
set gfxpayload=keep
insmod gzio
insmod part_msdos
insmod xfs
set root='hd0,msdos1'
if [ x$feature_platform_search_hint = xy ]; then
search --no-floppy --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1 --hint='hd0,msdos1' 985589ee-8d6c-4dbb-9463-90576a5a1ff4
else
search --no-floppy --fs-uuid --set=root 985589ee-8d6c-4dbb-9463-90576a5a1ff4
fi
linux16 /vmlinuz-3.10.0-693.el7.x86_64 root=/dev/mapper/centos-root ro crashkernel=auto rd.lvm.lv=centos/root rd.lvm.lv=centos/swap rhgb quiet LANG=en_US.UTF-8
initrd16 /initramfs-3.10.0-693.el7.x86_64.img
}
menuentry 'CentOS Linux (0-rescue-1cf30b938dc94f2bb08fb045c7a0734e) 7 (Core)' --class centos --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-0-rescue-1cf30b938dc94f2bb08fb045c7a0734e-advanced-162b8754-3820-4229-9ab4-810de8ab8558' {
load_video
insmod gzio
insmod part_msdos
insmod xfs
set root='hd0,msdos1'
if [ x$feature_platform_search_hint = xy ]; then
search --no-floppy --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1 --hint='hd0,msdos1' 985589ee-8d6c-4dbb-9463-90576a5a1ff4
else
search --no-floppy --fs-uuid --set=root 985589ee-8d6c-4dbb-9463-90576a5a1ff4
fi
linux16 /vmlinuz-0-rescue-1cf30b938dc94f2bb08fb045c7a0734e root=/dev/mapper/centos-root ro crashkernel=auto rd.lvm.lv=centos/root rd.lvm.lv=centos/swap rhgb quiet
initrd16 /initramfs-0-rescue-1cf30b938dc94f2bb08fb045c7a0734e.img
}
### END /etc/grub.d/10_linux ###
### BEGIN /etc/grub.d/20_linux_xen ###
### END /etc/grub.d/20_linux_xen ###
### BEGIN /etc/grub.d/20_ppc_terminfo ###
### END /etc/grub.d/20_ppc_terminfo ###
### BEGIN /etc/grub.d/30_os-prober ###
### END /etc/grub.d/30_os-prober ###
### BEGIN /etc/grub.d/40_custom ###
# This file provides an easy way to add custom menu entries. Simply type the
# menu entries you want to add after this comment. Be careful not to change
# the 'exec tail' line above.
### END /etc/grub.d/40_custom ###
### BEGIN /etc/grub.d/41_custom ###
if [ -f ${config_directory}/custom.cfg ]; then
source ${config_directory}/custom.cfg
elif [ -z "${config_directory}" -a -f $prefix/custom.cfg ]; then
source $prefix/custom.cfg;
fi
### END /etc/grub.d/41_custom ###
In order to make any changes in grub.cfg two steps are required.
save chanages in /etc/default/grub. The configurations are written to /boot/grub2/grub.cfg using grub-mkconfig command
2- grub-mkconfig
grub-mkconfig Generate a GRUB configuration file.
[root@centos7-1 ~]# grub2-mkconfig
Generating grub configuration file ...
#
# DO NOT EDIT THIS FILE
#
# It is automatically generated by grub2-mkconfig using templates
# from /etc/grub.d and settings from /etc/default/grub
#
### BEGIN /etc/grub.d/00_header ###
set pager=1
if [ -s $prefix/grubenv ]; then
load_env
fi
if [ "${next_entry}" ] ; then
set default="${next_entry}"
set next_entry=
save_env next_entry
set boot_once=true
else
set default="${saved_entry}"
fi
if [ x"${feature_menuentry_id}" = xy ]; then
menuentry_id_option="--id"
else
menuentry_id_option=""
fi
export menuentry_id_option
if [ "${prev_saved_entry}" ]; then
set saved_entry="${prev_saved_entry}"
save_env saved_entry
set prev_saved_entry=
save_env prev_saved_entry
set boot_once=true
fi
function savedefault {
if [ -z "${boot_once}" ]; then
saved_entry="${chosen}"
save_env saved_entry
fi
}
function load_video {
if [ x$feature_all_video_module = xy ]; then
insmod all_video
else
insmod efi_gop
insmod efi_uga
insmod ieee1275_fb
insmod vbe
insmod vga
insmod video_bochs
insmod video_cirrus
fi
}
terminal_output console
if [ x$feature_timeout_style = xy ] ; then
set timeout_style=menu
set timeout=5
# Fallback normal timeout code in case the timeout_style feature is
# unavailable.
else
set timeout=5
fi
### END /etc/grub.d/00_header ###
### BEGIN /etc/grub.d/00_tuned ###
set tuned_params=""
set tuned_initrd=""
### END /etc/grub.d/00_tuned ###
### BEGIN /etc/grub.d/01_users ###
if [ -f ${prefix}/user.cfg ]; then
source ${prefix}/user.cfg
if [ -n "${GRUB2_PASSWORD}" ]; then
set superusers="root"
export superusers
password_pbkdf2 root ${GRUB2_PASSWORD}
fi
fi
### END /etc/grub.d/01_users ###
### BEGIN /etc/grub.d/10_linux ###
Found linux image: /boot/vmlinuz-3.10.0-693.el7.x86_64
Found initrd image: /boot/initramfs-3.10.0-693.el7.x86_64.img
menuentry 'CentOS Linux (3.10.0-693.el7.x86_64) 7 (Core)' --class centos --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-3.10.0-693.el7.x86_64-advanced-162b8754-3820-4229-9ab4-810de8ab8558' {
load_video
set gfxpayload=keep
insmod gzio
insmod part_msdos
insmod xfs
set root='hd0,msdos1'
if [ x$feature_platform_search_hint = xy ]; then
search --no-floppy --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1 --hint='hd0,msdos1' 985589ee-8d6c-4dbb-9463-90576a5a1ff4
else
search --no-floppy --fs-uuid --set=root 985589ee-8d6c-4dbb-9463-90576a5a1ff4
fi
linux16 /vmlinuz-3.10.0-693.el7.x86_64 root=/dev/mapper/centos-root ro crashkernel=auto rd.lvm.lv=centos/root rd.lvm.lv=centos/swap rhgb quiet
initrd16 /initramfs-3.10.0-693.el7.x86_64.img
}
Found linux image: /boot/vmlinuz-0-rescue-1cf30b938dc94f2bb08fb045c7a0734e
Found initrd image: /boot/initramfs-0-rescue-1cf30b938dc94f2bb08fb045c7a0734e.img
menuentry 'CentOS Linux (0-rescue-1cf30b938dc94f2bb08fb045c7a0734e) 7 (Core)' --class centos --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-0-rescue-1cf30b938dc94f2bb08fb045c7a0734e-advanced-162b8754-3820-4229-9ab4-810de8ab8558' {
load_video
insmod gzio
insmod part_msdos
insmod xfs
set root='hd0,msdos1'
if [ x$feature_platform_search_hint = xy ]; then
search --no-floppy --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1 --hint='hd0,msdos1' 985589ee-8d6c-4dbb-9463-90576a5a1ff4
else
search --no-floppy --fs-uuid --set=root 985589ee-8d6c-4dbb-9463-90576a5a1ff4
fi
linux16 /vmlinuz-0-rescue-1cf30b938dc94f2bb08fb045c7a0734e root=/dev/mapper/centos-root ro crashkernel=auto rd.lvm.lv=centos/root rd.lvm.lv=centos/swap rhgb quiet
initrd16 /initramfs-0-rescue-1cf30b938dc94f2bb08fb045c7a0734e.img
}
if [ "x$default" = 'CentOS Linux (3.10.0-693.el7.x86_64) 7 (Core)' ]; then default='Advanced options for CentOS Linux>CentOS Linux (3.10.0-693.el7.x86_64) 7 (Core)'; fi;
### END /etc/grub.d/10_linux ###
### BEGIN /etc/grub.d/20_linux_xen ###
### END /etc/grub.d/20_linux_xen ###
### BEGIN /etc/grub.d/20_ppc_terminfo ###
### END /etc/grub.d/20_ppc_terminfo ###
### BEGIN /etc/grub.d/30_os-prober ###
### END /etc/grub.d/30_os-prober ###
### BEGIN /etc/grub.d/40_custom ###
# This file provides an easy way to add custom menu entries. Simply type the
# menu entries you want to add after this comment. Be careful not to change
# the 'exec tail' line above.
### END /etc/grub.d/40_custom ###
### BEGIN /etc/grub.d/41_custom ###
if [ -f ${config_directory}/custom.cfg ]; then
source ${config_directory}/custom.cfg
elif [ -z "${config_directory}" -a -f $prefix/custom.cfg ]; then
source $prefix/custom.cfg;
fi
### END /etc/grub.d/41_custom ###
done
It reads main grub script files from /etc/grub.d and setting from /etc/default/grub and print out generated configuration.
[root@centos7-1 ~]# grub2-mkconfig --help
Usage: grub2-mkconfig [OPTION]
Generate a grub config file
-o, --output=FILE output generated config to FILE [default=stdout]
-h, --help print this message and exit
-v, --version print the version information and exit
Report bugs to <bug-grub@gnu.org>.
For saving generated configuration we use -o option (it is recommanded to take a backup from /boot/grub2/grub.cfg file, how ever this is not touching Master Boot Record and just re touches GRUB menu):
[root@centos7-1 ~]# grub2-mkconfig -o /boot/grub2/grub.cfg
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-3.10.0-693.el7.x86_64
Found initrd image: /boot/initramfs-3.10.0-693.el7.x86_64.img
Found linux image: /boot/vmlinuz-0-rescue-1cf30b938dc94f2bb08fb045c7a0734e
Found initrd image: /boot/initramfs-0-rescue-1cf30b938dc94f2bb08fb045c7a0734e.img
done
We where able to use grub2-mkconfig > /boot/grub2/grub.cfg command too,and GRUB2 configuration steps finish here.
grub-update
When you run the update-grub command, GRUB automatically combines the settings from the /etc/default/grub file, the scripts from the /etc/grub.d/ directory, and everything else, creating a /boot/grub/grub.cfg file that’s read at boot.
GRUB Interfaces
There are three interfaces in GRUB which all provide different levels of functionality. The Linux kernel can be booted by the users with the help of these interfaces. Details about the interfaces are:
1-Menu Interface
The GRUB is configured by the installation program in the menu interface. It is the default interface available. It contains a list of the operating systems or kernels which is ordered by name. A specific operating system or kernel can be selected using the arrow keys and it can be booted using the enter key.
GRUB Legacy
GRUB2
Missing 'a' hah!
2-Menu Entry Editor Interface
The e key (a key GRUB Legacy) in the boot loader menu is used to access the menu entry editor. All the GRUB commands for the particular menu entry are displayed there and these commands may be altered before loading the operating system.
GRUB Legacy
GRUB2
3-Command Line Interface
This interface is the most basic GRUB interface but it grants the most control to the user. Using the command line interface, any command can be executed by typing it and then pressing enter. This interface also features some advanced shell features.
GRUB Legacy
It is possible to have GRUB Legacy command prompt after rebooting the system and gain information form there.
[root@centos5-1 ~]# grub
Probing devices to guess BIOS drives. This may take a long time.
GNU GRUB version 0.97 (640K lower / 3072K upper memory)
[ Minimal BASH-like line editing is supported. For the first word, TAB
lists possible command completions. Anywhere else TAB lists the possible
completions of a device/filename.]
GRUB2
Note: Partition numbering has changed in GRUB2. The first partition is 1 in GRUB2 rather than 0 (GRUB Legacy). The first device/drive is still hd0 by default (no change).
GRUB Installation
Normally when we setup a system GRUB is installed for us but there are some cases which we might need to install GRUB ourselves. There are different ways to install GRUB.
If we are on old system with GRUB legacy we can run grub-install command or using setup command in GRUB shell.
[root@centos5-1 ~]# grub-install /dev/sdb
If we are on GRUB2 we can use one of GRUB2 utilities.
grub-install installs GRUB onto a device. This includes copying GRUB images into the target directory (generally /boot/grub), and on some platforms may also include installing GRUB onto a boot sector.
[root@centos7-1 ~]# grub2-install --help
Usage: grub2-install [OPTION...] [OPTION] [INSTALL_DEVICE]
Install GRUB on your drive.
--compress[=no,xz,gz,lzo] compress GRUB files [optional]
-d, --directory=DIR use images and modules under DIR
[default=/usr/lib/grub/<platform>]
--fonts=FONTS install FONTS [default=unicode]
--install-modules=MODULES install only MODULES and their dependencies
[default=all]
-k, --pubkey=FILE embed FILE as public key for signature checking
--locale-directory=DIR use translations under DIR
[default=/usr/share/locale]
--locales=LOCALES install only LOCALES [default=all]
--modules=MODULES pre-load specified modules MODULES
--themes=THEMES install THEMES [default=starfield]
-v, --verbose print verbose messages.
--allow-floppy make the drive also bootable as floppy (default
for fdX devices). May break on some BIOSes.
--boot-directory=DIR install GRUB images under the directory DIR/grub2
instead of the boot/grub2 directory
--bootloader-id=ID the ID of bootloader. This option is only
available on EFI and Macs.
--core-compress=xz|none|auto
choose the compression to use for core image
--disk-module=MODULE disk module to use (biosdisk or native). This
option is only available on BIOS target.
--efi-directory=DIR use DIR as the EFI System Partition root.
--force install even if problems are detected
--force-file-id use identifier file even if UUID is available
--label-bgcolor=COLOR use COLOR for label background
--label-color=COLOR use COLOR for label
--label-font=FILE use FILE as font for label
--macppc-directory=DIR use DIR for PPC MAC install.
--no-bootsector do not install bootsector
--no-nvram don't update the `boot-device'/`Boot*' NVRAM
variables. This option is only available on EFI
and IEEE1275 targets.
--no-rs-codes Do not apply any reed-solomon codes when
embedding core.img. This option is only available
on x86 BIOS targets.
--product-version=STRING use STRING as product version
--recheck delete device map if it already exists
--removable the installation device is removable. This option
is only available on EFI.
-s, --skip-fs-probe do not probe for filesystems in DEVICE
--target=TARGET install GRUB for TARGET platform
[default=i386-pc]; available targets: arm-efi,
arm-uboot, arm64-efi, i386-coreboot, i386-efi,
i386-ieee1275, i386-multiboot, i386-pc,
i386-qemu, i386-xen, ia64-efi, mips-arc,
mips-qemu_mips, mipsel-arc, mipsel-loongson,
mipsel-qemu_mips, powerpc-ieee1275,
sparc64-ieee1275, x86_64-efi, x86_64-xen
-?, --help give this help list
--usage give a short usage message
-V, --version print program version
Mandatory or optional arguments to long options are also mandatory or optional
for any corresponding short options.
INSTALL_DEVICE must be system device filename.
grub2-install copies GRUB images into boot/grub2. On some platforms, it may
also install GRUB into the boot sector.
Report bugs to <bug-grub@gnu.org>.
as an example grub2-install /dev/sdb will install grub on sdb device, to the master boot record of my hard disk.
How ever instaling GRUB a from running system seems cool, but most of the time there is a problem during boot process and we can not get into our system any more, so we need to reinstall GRUB from GRUB shell. Unfortunately the setup command has been removed from GRUB2 shell and it would need more efforts. We need to bring up the system in rescue mode using a live cd and then install GRUB on our hard disk using current temporary root system file:
it is just an example and do not run it because you would mess up your current system!
