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GRUB

Commands

  • update-grub

Configuration

  • /etc/default/grub
  • /etc/grub.d/
  • /boot/grub/

Further Reading

Introduction to GRUB

GRUB is short for the GRand Unified Bootloader. It's the program responsible for bringing Linux off of the hard disk or other boot medium and into RAM. If you're familiar with Linux you've seen GRUB many times but perhaps never interacted with it. Usually you don't have to unless it's an emergency. GRUB has a BASH-like command shell that can be activated by pressing any key when you see the GRUB boot menu. 

GRUB Configuration 

On Ubuntu GRUB's configuration is spread out. This is /etc/default/grub

GRUB_DEFAULT=0
#GRUB_HIDDEN_TIMEOUT=0
GRUB_HIDDEN_TIMEOUT_QUIET=true
GRUB_TIMEOUT=10
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash nomdmonddf nomdmonisw nomdmonddf nomdmonisw nomdmonddf nomdmonisw"
GRUB_CMDLINE_LINUX=""

# Uncomment to enable BadRAM filtering, modify to suit your needs
# This works with Linux (no patch required) and with any kernel that obtains
# the memory map information from GRUB (GNU Mach, kernel of FreeBSD ...)
#GRUB_BADRAM="0x01234567,0xfefefefe,0x89abcdef,0xefefefef"

# Uncomment to disable graphical terminal (grub-pc only)
#GRUB_TERMINAL=console

# The resolution used on graphical terminal
# note that you can use only modes which your graphic card supports via VBE
# you can see them in real GRUB with the command `vbeinfo'
#GRUB_GFXMODE=640x480

# Uncomment if you don't want GRUB to pass "root=UUID=xxx" parameter to Linux
#GRUB_DISABLE_LINUX_UUID=true

# Uncomment to disable generation of recovery mode menu entries
#GRUB_DISABLE_RECOVERY="true"

# Uncomment to get a beep at grub start
#GRUB_INIT_TUNE="480 440 1"

Most of the things that are easy to tune are located here. The GRUB boot menu is generated when you run the command:

# sudo update-grub 

That command uses the scripts located in /etc/grub.d 

# ls -l /etc/grub.d 
total 76
-rwxr-xr-x 1 root root  9424 Jun 26 04:16 00_header
-rwxr-xr-x 1 root root  6058 May  8  2014 05_debian_theme
-rwxr-xr-x 1 root root 11608 May 15  2014 10_linux
-rwxr-xr-x 1 root root 10412 May 15  2014 20_linux_xen
-rwxr-xr-x 1 root root  1992 Mar 12  2014 20_memtest86+
-rwxr-xr-x 1 root root 11692 May 15  2014 30_os-prober
-rwxr-xr-x 1 root root  1416 May 15  2014 30_uefi-firmware
-rwxr-xr-x 1 root root   214 Apr  9  2013 40_custom
-rwxr-xr-x 1 root root   216 Apr  9  2013 41_custom
-rw-r--r-- 1 root root   483 Apr  9  2013 README

Notice that the file names have numbers. This is because they're run in order: 00 first and 99 last. GRUB cannot see into complex Linux partitions (like LVM) so the /boot partition is usually formatted with the ext2 filesystem. When you run update-grub GRUB's generated configuration is placed into /boot. Here's an example of grub.cfg

#
# DO NOT EDIT THIS FILE
#
# It is automatically generated by grub-mkconfig using templates
# from /etc/grub.d and settings from /etc/default/grub
#

### BEGIN /etc/grub.d/00_header ###
if [ -s $prefix/grubenv ]; then
  set have_grubenv=true
  load_env
fi
if [ "${next_entry}" ] ; then
   set default="${next_entry}"
   set next_entry=
   save_env next_entry
   set boot_once=true
else
   set default="0"
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 recordfail {
  set recordfail=1
  if [ -n "${have_grubenv}" ]; then if [ -z "${boot_once}" ]; then save_env recordfail; fi; 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
}

if [ x$feature_default_font_path = xy ] ; then
   font=unicode
else
insmod part_msdos
insmod part_msdos
insmod lvm
insmod ext2
set root='lvmid/SE3nYs-PV4x-Xlmo-poMR-N4nm-9jNb-yB0Twf/6R9aFn-EHrQ-MQsJ-QIhk-cSMx-IqfT-IeGePa'
if [ x$feature_platform_search_hint = xy ]; then
  search --no-floppy --fs-uuid --set=root --hint='lvmid/SE3nYs-PV4x-Xlmo-poMR-N4nm-9jNb-yB0Twf/6R9aFn-EHrQ-MQsJ-QIhk-cSMx-IqfT-IeGePa'  3a51e625-1863-4a2c-8e01-dedcc778d7c2
else
  search --no-floppy --fs-uuid --set=root 3a51e625-1863-4a2c-8e01-dedcc778d7c2
fi
    font="/usr/share/grub/unicode.pf2"
fi

if loadfont $font ; then
  set gfxmode=auto
  load_video
  insmod gfxterm
  set locale_dir=$prefix/locale
  set lang=en_US
  insmod gettext
fi
terminal_output gfxterm
if [ "${recordfail}" = 1 ] ; then
  set timeout=30
else
  if [ x$feature_timeout_style = xy ] ; then
    set timeout_style=menu
    set timeout=10
  # Fallback normal timeout code in case the timeout_style feature is
  # unavailable.
  else
    set timeout=10
  fi
fi
### END /etc/grub.d/00_header ###

### BEGIN /etc/grub.d/05_debian_theme ###
set menu_color_normal=white/black
set menu_color_highlight=black/light-gray
if background_color 44,0,30,0; then
  clear
fi
### END /etc/grub.d/05_debian_theme ###

### BEGIN /etc/grub.d/10_linux ###
function gfxmode {
set gfxpayload="${1}"
if [ "${1}" = "keep" ]; then
set vt_handoff=vt.handoff=7
else
set vt_handoff=
fi
}
if [ "${recordfail}" != 1 ]; then
  if [ -e ${prefix}/gfxblacklist.txt ]; then
    if hwmatch ${prefix}/gfxblacklist.txt 3; then
      if [ ${match} = 0 ]; then
        set linux_gfx_mode=keep
      else
        set linux_gfx_mode=text
      fi
    else
      set linux_gfx_mode=text
    fi
  else
    set linux_gfx_mode=keep
  fi
else
  set linux_gfx_mode=text
fi
export linux_gfx_mode
menuentry 'Ubuntu' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-simple-3a51e625-1863-4a2c-8e01-dedcc778d7c2' {
recordfail
load_video
gfxmode $linux_gfx_mode
insmod gzio
insmod part_msdos
insmod ext2
set root='hd0,msdos1'
if [ x$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  75f5ce48-1b75-4e4f-b36f-8028834acbe9
else
 search --no-floppy --fs-uuid --set=root 75f5ce48-1b75-4e4f-b36f-8028834acbe9
fi
linux /vmlinuz-3.13.0-63-generic root=/dev/mapper/ubuntu--vg-root ro  quiet splash nomdmonddf nomdmonisw nomdmonddf nomdmonisw nomdmonddf nomdmonisw $vt_handoff
initrd /initrd.img-3.13.0-63-generic
}
submenu 'Advanced options for Ubuntu' $menuentry_id_option 'gnulinux-advanced-3a51e625-1863-4a2c-8e01-dedcc778d7c2' {
menuentry 'Ubuntu, with Linux 3.13.0-63-generic' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-3.13.0-63-generic-advanced-3a51e625-1863-4a2c-8e01-dedcc778d7c2' {
recordfail
load_video
gfxmode $linux_gfx_mode
insmod gzio
insmod part_msdos
insmod ext2
set root='hd0,msdos1'
if [ x$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  75f5ce48-1b75-4e4f-b36f-8028834acbe9
else
 search --no-floppy --fs-uuid --set=root 75f5ce48-1b75-4e4f-b36f-8028834acbe9
fi
echo 'Loading Linux 3.13.0-63-generic ...'
linux /vmlinuz-3.13.0-63-generic root=/dev/mapper/ubuntu--vg-root ro  quiet splash nomdmonddf nomdmonisw nomdmonddf nomdmonisw nomdmonddf nomdmonisw $vt_handoff
echo 'Loading initial ramdisk ...'
initrd /initrd.img-3.13.0-63-generic
}
menuentry 'Ubuntu, with Linux 3.13.0-63-generic (recovery mode)' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-3.13.0-63-generic-recovery-3a51e625-1863-4a2c-8e01-dedcc778d7c2' {
recordfail
load_video
insmod gzio
insmod part_msdos
insmod ext2
set root='hd0,msdos1'
if [ x$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  75f5ce48-1b75-4e4f-b36f-8028834acbe9
else
 search --no-floppy --fs-uuid --set=root 75f5ce48-1b75-4e4f-b36f-8028834acbe9
fi
echo 'Loading Linux 3.13.0-63-generic ...'
linux /vmlinuz-3.13.0-63-generic root=/dev/mapper/ubuntu--vg-root ro recovery nomodeset 
echo 'Loading initial ramdisk ...'
initrd /initrd.img-3.13.0-63-generic
}
}

### 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_memtest86+ ###
menuentry 'Memory test (memtest86+)' {
insmod part_msdos
insmod ext2
set root='hd0,msdos1'
if [ x$feature_platform_search_hint = xy ]; then
 search --no-floppy --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1  75f5ce48-1b75-4e4f-b36f-8028834acbe9
else
 search --no-floppy --fs-uuid --set=root 75f5ce48-1b75-4e4f-b36f-8028834acbe9
fi
knetbsd /memtest86+.elf
}
menuentry 'Memory test (memtest86+, serial console 115200)' {
insmod part_msdos
insmod ext2
set root='hd0,msdos1'
if [ x$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  75f5ce48-1b75-4e4f-b36f-8028834acbe9
else
 search --no-floppy --fs-uuid --set=root 75f5ce48-1b75-4e4f-b36f-8028834acbe9
fi
linux16 /memtest86+.bin console=ttyS0,115200n8
}
### END /etc/grub.d/20_memtest86+ ###

### BEGIN /etc/grub.d/30_os-prober ###
### END /etc/grub.d/30_os-prober ###

### BEGIN /etc/grub.d/30_uefi-firmware ###
### END /etc/grub.d/30_uefi-firmware ###

### 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 ###


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