This page provides the user with information on options available for repairing GRUB 2 boot issues and specific instructions on how to use the GRUB 2 terminal. The instructions are written for GRUB 1.99, which is the version of GRUB 2 which is included on Ubuntu 11.04, Natty Narwhal, and later. Differences for version 1.98 (Ubuntu 10.4, Lucid Lynx) are noted when the procedures differ.

  • g2_grub_rescue1.png

GRUB 2's ability to fix boot problems is greatly improved over the original GRUB bootloader. In addition to an automatic fallback mode if booting from a menuentry in a submenu, GRUB 2 allows the user to edit its menu before the operating system is loaded. The rescue mode GRUB 2 terminal can help boot an operating system so that permanent repairs to system files can be made.

The instructions on this page are written for a fully-installed Ubuntu operating system. WUBI boot problems are not covered in detail although they are addressed in several sections.

Overview

GRUB 2 boot problems can leave the system in several states. The user may see one of the following displays on the monitor when a boot fails. The display provides the first indication of what might be causing the failure to boot. Here are the failure prompts and displays, and the possible cause of each:

  • grub> prompt: GRUB 2 loaded modules but was unable to find the grub.cfg file.

  • grub rescue> prompt: GRUB 2 failed to find its grub folder, or failed to load the normal module.

  • grub>: - The grub prompt on a blank screen.

    • GRUB 2 has found the boot information but has been either unable to locate or unable to use an existing GRUB 2 configuration file (usually grub.cfg).

  • grub rescue>: - The rescue mode.

    • GRUB 2 is unable to find the grub folder or its contents are missing/corrupted. The grub folder contains the GRUB 2 menu, modules and stored environmental data.

  • GRUB - a single word at the top left of the screen, with no prompt and no cursor.

    • GRUB has failed to find even the most basic information, usually contained in the MBR or boot sector.
  • Busybox or Initramfs: GRUB 2 began the boot process but there was a problem passing control to the operating system. Possible causes include an incorrect UUID or root= designation in the 'linux' line or a corrupted kernel.

  • Frozen splash screen, blinking cursor with no grub> or grub rescue prompt. Possible video issues with the kernel. While these failures are not of GRUB 2's making, it may still be able to help. GRUB 2 allows pre-boot editing of its menu and the user may restore functionality by adding and/or removing kernel options in a menuentry before booting.

Each of the GRUB 2 failure modes can normally be corrected either from the GRUB 2 terminal or by using an Ubuntu LiveCD. Additionally, there are compatible 3rd party bootable "rescue" CD/USB options which may also work. If using an Ubuntu LiveCD, it is recommended, but not always necessary, to use the same version CD as the system you are trying to repair. This ensures compatability of any modules and configuration files that may be loaded while attempting to repair the system.

GRUB 2 Terminal Commands

info.png Here are some useful tips and features for use with the GRUB 2 terminal:

  • Turning off the splash image. This may make viewing the terminal easier. Press c at the GRUB 2 menu to get to the command line and then type: set color_normal=white/blue or the color combination you wish to use. "black" as the second entry retains the menu's transparency and should be avoided as a selection if the user wants to work with a solid background color.

  • TAB completion. This feature is very handy. At any point, pressing the TAB key may complete an entry, if the element is unique, or display available options. Typing a few additional characters and pressing TAB again may allow tab completion to finish the entry.

  • set pager=1 To prevent text from scrolling off the screen, type set pager=1.

  • Help. Type help to view a list of all the commands. Type help x to view help commands beginning with the letter "x". Tab completion and using the up arrow to repeat commands work the same as in a normal terminal.

Normal

When GRUB 2 is fully functional, the GRUB 2 terminal is accessed by pressing c. If the menu is not displayed during boot, hold down the SHIFT key until it appears. If it still does not appear, try pressing the ESC key repeatedly.

From a GRUB 2 terminal with the grub> prompt, a wide variety of commands are available.

  • A few of the more important commands:

    Command

    Result / Example

    boot

    Initiate the boot sequence, also F10 or CTRL-x

    cat

    Display the contents of readable files; cat (hd0,1)/boot/grub/grub.cfg

    configfile

    Load a GRUB 2 configuration file such as grub.cfg; configfile (hd0,5)/boot/grub/grub.cfg

    initrd

    Loads the initrd.img, necessary for booting; initrd (hd0,5)/initrd.img

    insmod

    Loads a module; insmod (hd0,5)/boot/grub/normal.mod, or insmod normal

    linux

    Loads the kernel; insmod /vmlinuz root=(hd0,5) ro

    loop

    Mount a file as a device; loopback loop (hd0,2)/iso/my.iso

    ls

    Lists the contents of a partition/folder; ls, ls /boot/grub, ls (hd0,5)/, ls (hd0,5)/boot

    lsmod

    List loaded modules

    normal

    Activate the normal module, if loaded

    search

    Search for a device. Type help search for the available options.

    set

    Review current settings, or set XXX to set a variable such as colors, prefix, root

    vbeinfo

    Display GRUB 2 available resolutions

    To view the complete command list, type help. For details on a particular command, type help [command]

grub>

When GRUB 2 is unable to boot or display the GRUB 2 menu the system will be left at a GRUB 2 terminal if possible. If the system boots to the grub> prompt, the usual modules and commands are normally available. The user may need to manually load modules using the insmod [module] command before some commands will work.

grub rescue>

In the grub rescue mode, only a limited set of commands are available. These commands are sufficient to investigate the contents of the drives, set prefix (path to the grub folder) and root (partition), load modules and boot.

The rescue mode provides fewer commands than the normal GRUB prompt line, but also provides these additional commands:

  • Command

    Result / Example

    dump

    Clears memory

    exit

    Exit GRUB 2

    normal

    Return to the standard "grub>" mode if possible.

Among the commands which can be used in the grub rescue mode:

  • boot

    cat

    chain

    help

    insmod

    linux

    ls

    multiboot

    normal

    search

    set

    unset

General Troubleshooting Preparation

Boot-Repair

The majority of this guide is devoted to working with the GRUB 2 terminal, which is used to enter commands to attempt to repair a broken bootloader. info.png Boot-Repair is a GUI application which can automatically analyze boot problems and select the proper course of action to repair them. Its simple interface provides beginners as well as experienced Linux users an easy method of repairing the majority of GRUB 2 problems.

Additionally, Boot-Repair contains an automated method to run a boot info script which performs a series of tests and provides a file containing much valuable information regarding the status of the computer's operating and boot systems. This file can be inspected by community helpers on forums or IRC channels to help diagnose boot problems.

Boot-Repair can be run from a LiveCD or an operating Linux system. To download the package run the following commands:

  • sudo add-apt-repository ppa:yannubuntu/boot-repair && sudo apt-get update
    sudo apt-get install -y boot-repair && boot-repair

Visit the Boot-Repair community document for more information.

Boot-Repair is available from the Launchpad repositories and can be run from a CD or a working Linux OS.

Search & Set

A great many boot problems are due to incorrect paths to required files. The GRUB 2 terminal, in either 'failure' mode, provides a robust ability to search hard drive(s) and partitions and to inspect their contents.

In order to boot successfully, the root, prefix, linux and initrd variables must be correct. The user must verify the paths and names of these items. If they are incorrect, use the commands below to find and fix them. GRUB 2 variable settings can be viewed with the set command.

In the following examples, X is a hard drive number; Y is a partition number. If a command example includes either of these replace them with the appropriate value.

In the graphic below, the text in red are commands to be entered by the user, and text in green is the output of the command on an operating system (i.e. what you would like to see if your Ubuntu installation is on sda1).

g2_grub_set_color.png

  • The first hard drive is 0. The first partition is 1. Thus sda1 becomes (hd0,1), sdb5 is (hd1,5).

  • Use the ls command in the follow manner

  • "Tab completion" may work - enter part of the filename and press the TAB key.

Command

Purpose

ls

Search the entire computer for devices and partitions: (hd0) (hd1) (hd0,1) (hd0,5) (hd1,1)

ls /

Search the root directory of the device designated as root (use the set command to check root).

ls (hdX,Y)

View information about a partition - format, size, UUID, etc.

ls (hdX,Y)/

View the root contents of a partition. Use this command to look for the presence of vmlinuz and initrd.img symlinks

ls (hdX,Y)/boot/

View the contents of a folder

ls (hdX,Y)/boot/

Inspect the /boot folder. It should contain the actual kernel (linux-3.2...) and initrd image (initrd.img-3.2....)

ls (hdX,Y)/boot/grub/

Inspect the /boot/grub folder. It should contain grub.cfg and many *.mod files. If looking for a specific file, include the name in the search to limit the number of returns. If available, the command set pager=1 will also limit returns to a single screen.

What to Look For

Where It Should Be (Default Installation)

Specific / General Search Example

grub.cfg

(hdX,Y)/boot/grub/ or /boot/grub/

ls (hdX,Y)/boot/grub/grub.cfg or ls /boot/grub/

vmlinuz

(hdX,Y)/ or /

ls (hdX,Y)/vmlinuz or ls /vmlinuz or ls /

linux-3.2.0-14*

(hdX,Y)/boot/ or /boot/

ls (hdX,Y)/boot/vmlinuz-3.2.0-14

initrd

(hdX,Y)/ or /

ls (hdX,Y)/ or ls /initrd

initrd.img-3.20-14

(hdX,Y)/ or /boot/

ls (hdX,Y)/boot/initrd.img-3.20-14 or ls (hdX,Y)/boot/

* Note: Use the full kernel name, including -generic, when searching or setting a kernel variable. Using the "TAB completion" technique may eliminate some typing and be more accurate if available.

Use the following commands to set these parameters (if incorrect). Substitute the correct value for X and Y. (Example: set root=(hdX,Y) becomes set root=(hd0,5) )

Task

Command

Notes

Set the prefix

set prefix=(hdX,Y)/boot/grub

Use the actual location of the grub folder

Set root

set root=(hdX,Y)

Set the kernel

linux /vmlinuz root=/dev/sda1 ro

Set the kernel if the symlink vmlinuz exists in /

Set the kernel

linux (hdX,Y)/boot/vmlinuz-3.0.2-14 root=/dev/sda1 ro

Set the kernel itself

Set the initrd image

initrd /initrd.img

Set the initrd image if the symlink exists in /

Set the initrd image

initrd (hdX,Y)/boot/initrd.img-3.0.2-14

Set the initrd image itself

Specific Troubleshooting

Use the section below based on the type of GRUB 2 terminal prompt displayed on the monitor.

important.png The commands which follow assume you have determined the proper parameters for prefix, root, linux, and initrd. Review the Search & Set section of this page for guidance .

grub>

Terminal Display: The GRUB 2 header/version information and a grub> prompt.

  • g2_grub>.png

If GRUB 2 leaves you at the grub> prompt, it has normally found the grub folder and loaded at least some basic modules. The configuration file (grub.cfg) may be missing, misnamed, or corrupted.

Quick Fix:

  • Use the Search & Set section to confirm the correct paths are set. Inspect the contents of the /boot/grub folder. Look for the grub.cfg file. It could be misnamed or missing. If not located in /boot/grub, use the ls command look for another .cfg file or look in /boot or other locations. If you locate the correct .cfg file:

  1. configfile /boot/grub/grub.cfg or configfile (hdX,Y)/boot/grub/grub.cfg

    If a .cfg with another name is located, substitute its path/filename in the command.

    If the configuration file is loaded and not corrupt, when the above command is executed the GRUB 2 menu should appear and the user can make a selection to boot. Once booted into the system, correct the filename or move the configuration file to its proper location. Run update-grub. If the configuration file is not found, a message will be generated and the user must enter the boot commands manually.

Extended Fix:

The following commands should set the root and prefix paths and load the kernel and initrd image. With this information, GRUB 2 does not need a configuration file and should be able to boot the system if the only problem was a corrupted or missing menu.

  • Press ENTER after completing each line. Some entries will not provide feedback. This is normal.

  • If a "file not found" or similar error message is displayed while running these commands, ensure you are using the correct X,Y values and the correct paths are set.

1. set root=(hdX,Y)

Confirm the correct X,Y values and press ENTER.

Example: If the Ubuntu system is on sda5, enter: set root=(hd0,5)

2. linux /vmlinuz root=/dev/sdXY ro

Example: linux /vmlinuz root=/dev/sda5 ro

If the vmlinuz symlink does not exist, use the full path to the kernel in /boot

Example: linux /boot/vmlinuz-3.2.0-14-generic root=/dev/sda1 ro

If successful, after ENTER there will be a slight delay and no messages.

  • Wubi users only - substitute these commands in Steps 1 and 2:

  • 1.set root=(loop0)

    2. linux /vmlinuz root=/dev/sdXY loop=/ubuntu/disks/root.disk ro

3. initrd /initrd.img

Selects the latest initrd image.

If the vmlinuz symlink does not exist, use the full path to the initrd image in /boot

If successful, after ENTER there will be a slight delay and no messages.

4. boot

Boot to the latest kernel on the selected partition.

If the system fails to boot:

  1. Attempt to find the GRUB 2 configuration file. The normal name is grub.cfg file. If not found, look for a misnamed .cfg file or in alternate locations. The menu configuration file may contain settings required for a successful boot (such as non-standard kernel options) even if the paths/filenames are correct.

    1. Use the set command to confirm the correct root and prefix settngs. Also inspect the folders' actual contents with the ls command. The grub prompt normally means the GRUB 2 folder is intact but doesn't guarantee the integrity of other system files.

    2. root= should point to the drive and partition on which Ubuntu is installed. If you use the ls / command you should see the main Ubuntu system folders.

If the system boots, please refer to the [[#Post-Boot Follow Up|Post Boot Follow Up] section

If the system fails to boot, proceed to the [#grub rescue|grub rescue]] section for more detailed troubleshooting options.

grub rescue>

The GRUB 2 rescue mode is a major enhancement to the GRUB bootloader. The presence of the grub rescue> prompt signifies that GRUB 2 has failed to find the grub folder, the grub.cfg file, and/or the associated modules. The rescue prompt is presented so the user can provide the path to the grub folder, load the necessary modules, and provide the proper boot commands.

  • g2_grub_rescue.png

A common reason for the grub rescue> prompt is an incorrect path to the grub folder. Reasons for the prompt also include a failure to update GRUB 2 after certain system or partition operations, improper designation of the grub folder location, missing linux or initrd.img symlinks in /, or a failed installation.

To successfully boot from the grub rescue> prompt:

  • The grub folder must exist and contain the necessary GRUB 2 files and modules.

  • The proper paths must be set via the set prefix command.

    • Many GRUB 2 commands will not work until the correct path is set.

      If the path to the grub folder (normally /boot/grub) is not correct, an unknown command or file not found message is likely.

  • The necessary modules must be loaded.
    • The kernel cannot be loaded until the 'linux' module is loaded.
  • A Linux kernel and initrd.img must be located and loaded.

Use the General Troubleshooting Preparation section to locate the correct partitions and file locations. Once the user has confirmed the paths and existence of the proper folders using the Search & Set section, run the following commands:

1. set prefix=(hdX,Y)/boot/grub

Use the values determined earlier.

Example: If the Ubuntu system is on sda5, enter: set prefix=(hd0,5)/boot/grub

2.* set root=(hdX,Y)

Confirm the correct X,Y values and press ENTER.

Example: If the Ubuntu system is on sda5, enter: set root=(hd0,5)

3. insmod normal

Load the normal module.

If the module loads there will be no message.

If the module fails to load, try the full path: insmod (hdX,Y)/boot/grub/normal.mod

4. normal

Transition to the normal GRUB 2 mode with increased functionality.

If the module loads there will be no message.

If the module loads, HELP, TAB completion and command recall using the UP/DN keys should be available.

5. set

(Optional) Review the current settings.

6. insmod linux

Load the linux module. An error message usually means the path is incorrect.

7.* linux /vmlinuz root=/dev/sdXY ro

Selects the latest kernel.

Example: linux /vmlinuz root=/dev/sda5 ro

If the vmlinuz symlink does not exist in /, use the full path to the kernel in /boot

Example: linux /boot/vmlinuz-3.2.0-14-generic root=/dev/sda1 ro

8. initrd /initrd.img

Selects the latest initrd image.

If the initrd symlink does not exist in /, use the full path to the initrd image in /boot

If successful, after ENTER there will be a slight delay and no messages.

9. boot

Boot to the latest kernel on the selected partition.

  • * Wubi users only - substitute these commands in Steps 2 and 7:

  • 1.set root=(loop0)

    2. linux /vmlinuz root=/dev/sdXY loop=/ubuntu/disks/root.disk ro

Some additional considerations:

  • The current prefix and root settings may be checked at any time with the set command. To remove a setting, use the unset command.

    • Example: unset prefix

  • Modules must be loaded before they can be used. If a module has not been loaded a unknown command error is displayed. If an incorrect path is specified, a file not found error message may be displayed.

  • The linux module must be loaded to be able to load both the kernel and the initrd image unless the normal module is loaded first.

  • If the modules cannot be found in the /boot/grub folder, the user may be able to load them from the /usr/lib/grub/i386-pc folder. The address if Ubuntu was installed on sda1 would be (hd0,1)/usr/lib/grub/i386-pc and the command would be:

    • insmod (hd0,1)/usr/lib/grub/i386-pc/normal.mod

Refer to the Post Boot Follow Up section if the system successfully boots.

GRUB

Presence of the word GRUB at the top left of the monitor with no blinking cursor indicates that GRUB 2 can not even find the Master Boot Record (or equivalent) information. Thus, the core.img file, the /boot and /grub folder locations and contents are completely unknown to GRUB 2.

  • g2_grub_fail.png

The seriousness of the problem cannot be assessed without the use of another operating system or an Ubuntu LiveCD or equivalent. The Ubuntu partition should be mounted for inspection and the user can then check to see if the system files are intact. If so, the user can use the ''ChRoot'' procedure from the LiveCD to reinstall GRUB 2 and rewrite the information to the MBR.

Details on this procedure are contained in the Grub2/Installing community documentation.

Editing the GRUB 2 Menu During Boot

Following a failed boot, GRUB 2 is designed to display the GRUB 2 menu on the next boot and await user input. This will happen even if the user has set the system to boot without displaying the GRUB 2 menu. This provides the user the opportunity to select a different menu option or edit a menuentry to change boot parameters. While this can cause problems on a server, for most users it is a benefit which will prevent constant rebooting during unmonitored operations.

  • g2_grub_edit_menu.png

In addition to its troubleshooting benefits, pre-boot editing of the GRUB 2 menu also allows users to add or remove kernel options, change operating modes, and accomplish other tasks such as testing fonts and splash images. For users who like to experiment, the settings altered via the GRUB 2 terminal are non-persistent and won't affect future boots.

Key Points About Terminal Menu Editing:

  • If the menu is displayed, the automatic countdown may be stopped by pressing any key other than the ENTER key.
  • If the menu is not normally displayed during boot, hold down the SHIFT key as the computer attempts to boot to display the GRUB 2 menu.
    • In certain circumstances, if holding the SHIFT key method does not display the menu pressing the ESC key repeatedly may display the menu.
  • Press the 'e' key to reveal the selection's settings.
  • Use the UP/DN/Left/Right cursor keys to navigate to the desired point for editing.
  • Make a single or numerous changes at any spot in the menu. Do not use ENTER to move between lines.

  • TAB completion is available, which is useful in entering kernel and initrd entries.

    • After starting to type the kernel or initrd name, press TAB.
    • If additional characters are not added, look at the bottom of the menu as there may be multiple choices. Add characters until only one entry is visible at the bottom, then TAB again.
  • When editing is complete:
    • CTRL-x or F10 - boot with the changed settings (highlighted for emphasis).

    • CTRL-c or F2 - jump to the command line to perform diagnostics, load modules, change settings, etc.

      • If an edit is incomplete and GRUB 2 detects an error in the kernel or initrd line it will return to the line when exiting this mode.

    • ESC - Discard all changes and return to the main menu.

    • The choices are listed at the bottom of the screen as a reminder.
  • Edits made to the menu in this manner are non-persistent. They remain in effect only for the current boot.
    • Once successfully booted, the changes can be made permanent by editing the appropriate file, saving the file, and running update-grub as 'root'.

  • Change a kernel version if one is available but not listed on the GRUB 2 menu.
    • Use the TAB key after entering part of the kernel or initrd version to see which ones are available.

  • Add or remove kernel options from the linux line.

    • Remove quiet to display system messages during boot.

    • Remove existing options and add nomodeset to avoid module loading, especially if having video issues.

  • Boot to the recovery mode even if it is not listed on the menu.
    • Remove existing kernel options from the linux line and add single

  • Remove references to UUIDs
    • Remove the entire search line

    • On the linux line, replace "root=UUID=<some alphanumeric>" with "root=/dev/sdXY"

Post Boot Follow Up

Any changes made from the GRUB 2 terminal are not permanent. After successfully booting into the system the user must take several steps to ensure the problem is permanently fixed.

  1. Update the GRUB 2 configuration file
    • sudo update-grub
  2. Reinstall GRUB 2 to the drive's MBR or equivalent

    • Install to the drive, not to the partition. Example: sda, not sda1

      sudo grub-install /dev/sdX  
  3. Inspect the GRUB 2 configuration file. The default is /boot/grub/grub.cfg

    • For problems with booting the main linux kernel, verify the search, linux, and initrd lines in the [### BEGIN /etc/grub.d/10_linux ###] section of the file.

      • Ensure the paths and kernel/initrd image versions are correct.
      • Confirm the UUID numbers.
      • UUIDs can be checked with the sudo blkid command.

  4. Verify the existence and contents of the system boot folders.
    • / should contain the symlinks vmlinuz and initrd.img

    • /boot/ should contain the actual kernel (vmlinuz-X.X.X-XX...) and initrd image (initrd.img-X.X.X-XX...)

    • /boot/grub should contain grub.cfg and numerous module files (*.mod)

For a corrupted GRUB 2 installation, purging and reinstalling GRUB 2 is very easy if the user has a working Internet connection. Refer to Grub2/Installing#Purging & Reinstalling GRUB 2 for guidance.

Fallback mode

It is possible to configure Grub2 to fall back to a known good menu-entry if the default menu-entry for some reason fails to boot. An example script that can be used for this is available at this webpage (in German).

GRUB 2 Errors

GRUB 2 does not report error numbers. If a number is associated with an error, it is a problem with the transition from GRUB legacy to GRUB 2.

A GRUB 2 error will leave the user at the grub> or grub rescue> prompt, the word GRUB with no cursor, or a hung boot caused by improper system path designations or a corrupted operating system. These issues are addressed earlier on this page - go to the General Troubleshooting Preparation section to start the recovery process.

Selected Problems and Bugs

External Drive Installs and ''grub-pc'' Updates

Launchpad Bug 496435 Installs of Ubuntu on external drives can cause problems as grub-install uses device names (e.g. sda, sdb) rather than UUIDs in certain circumstances. If connected to another machine when an update of grub-pc is made, the upgrade may be written to the incorrect device and make the computer unbootable.

A workaround is posted on the bug link above.

External Drive Installs and MBR Selection

Launchpad Bug 414996

When installing Ubuntu to a USB drive, the potential exists for GRUB 2 to write to the hard drive's MBR or split the installation between the hard drive and the USB drive (rather than completely on the USB device). This can render the main drive unbootable.

Workaround: During the final stages of the install there is an "Advanced" button which allows the user to select the install location. See the bug report for more details.

Boot Partition is in Logical Volume whose Volume Group contains a snapshot

Launchpad Bug 563895

When your boot partition (the one providing /boot) is a LV, make sure not to have any LVM snapshots inside the containing VG. At reboot this will render your system unbootable, dropping you in a "grub rescue>"-shell with the following message: "error: no such disk."

The grub-rescue-shell provided does not help you, because all modules (esp. lvm) are unreachable. You'll have to boot from a install medium and remove all snapshots that are in the same VG as your boot partition by hand.

This issue is extremely annoying if your server is in a remote datacenter since the origin of the problem cannot be spotted easily and repairing the system may be hard.

While this will be fixed in the upcoming Maverick Meerkat 10.10 release, the current Lucid Lynx LTS 10.04 is affected.

A workaround would be to install ubuntu using a separated non-LVM partition for /boot if you are using LVM snapshots regularly.

There's some kind of irony that if you're careful and take a LVM snapshot before upgrading (and possibly rebooting) your system, this will trigger the bug.

insmod fails with "error: no such disk"

Several grub2 modules, such as the linux-module depends on other modules, which is attempted to be auto-loaded. For this to work, the variable $prefix, must be set to where the grub2 modules can be found. Usually, this is accomplished with the command "set prefix=/boot/grub".

Links

Grub2

Grub2/Displays

Grub2/Installing

Grub2/Submenus

Grub2/Upgrading

Boot-Repair


CategoryBootAndPartition

Grub2/Troubleshooting (last edited 2013-02-27 11:10:09 by robert-meerman)