Get started with GRUB2#
https://www.certdepot.net/rhel7-get-started-grub2/
Note: This is an RHCSA 7 exam objective.
Presentation of GRUB2#
GRUB2 is the new Linux bootloader. GRUB2 stands for GRand Unified Bootloader version 2.
As GRUB was not maintained for some time and lacked some critical features like GPT management needed to handle disks bigger than 2.4TB, it was decided to start a new version from scratch with modularity in mind.
GRUB2 provides the following new features:
- ability to boot on various file systems (xfs, ext4, ntfs, hfs+, raid, etc),
- gzip files decompression on the fly,
- management of all disk geometries,
- support for GPT (GUID Partition Tables) and MBR (Master Boot Record),
- portability with different architectures (BIOS, EFI, Coreboot, etc),
- ability to load modules at execution time.
GRUB2 Organization#
The GRUB2 configuration is spread over several files:
- /boot/grub2/grub.cfg: this file contains the final GRUB2 configuration (do not edit it directly!),
- /etc/grub2.cfg: this is a symbolic link to the /boot/grub2/grub.cfg file,
- /etc/default/grub: this file contains the list of the GRUB2 variables (the values of the environment variables can be edited),
- /etc/sysconfig/grub: this is a symbolic link to the /etc/default/grub file,
- /etc/grub.d: this directory contains all the individual files internally used by GRUB2.
This tutorial will only explore knowledge required for the RHCSA exam. Refer to the Additional Resources section for more details.
Basic Management#
To get the details about the current active kernel, type:
1# grub2-editenv list
2saved_entry=CentOS Linux (3.10.0-229.11.1.el7.x86_64) 7 (Core)Note: This information is stored in the /boot/grub2/grubenv file.
To get the list of the kernels displayed at boot time, type:
1# grep ^menuentry /boot/grub2/grub.cfg
2menuentry 'CentOS Linux (3.10.0-229.20.1.el7.x86_64) 7 (Core)' ...
3menuentry 'CentOS Linux (3.10.0-229.14.1.el7.x86_64) 7 (Core)' ...
4menuentry 'CentOS Linux 7 (Core), with Linux 0-rescue-f19b719117b44bf3a3fb777bd4127' ...cafTo permanently define the kernel to execute at boot time (here 0 for the first entry), type:
1# grub2-set-default 0To display the GRUB2 variables, type:
1# cat /etc/default/grub
2GRUB_TIMEOUT=5
3GRUB_DISTRIBUTOR="$(sed 's, release .*$,,g' /etc/system-release)"
4GRUB_DEFAULT=saved
5GRUB_DISABLE_SUBMENU=true
6GRUB_TERMINAL="serial console"
7GRUB_SERIAL_COMMAND="serial --speed=115200"
8GRUB_CMDLINE_LINUX="rd.lvm.lv=rhel/swap crashkernel=auto rd.lvm.lv=rhel/root console=ttyS0,115200"
9GRUB_DISABLE_RECOVERY="true"Where
- GRUB_TIMEOUT defines the boot waiting delay (here 5 seconds),
- GRUB_DISTRIBUTOR contains the distribution name (here CentOS Linux),
- GRUB_DEFAULT specifies the default menu entry; it can be a number, an entry name or the string saved which means the entry saved during the last reboot or the execution of the grub2-set-default command,
- GRUB_DISABLE_SUBMENU allows (false) or not (true) the display of a submenu (see below),
- GRUB_TERMINAL defines the terminal input & output device (here console and serial),
- GRUB_SERIAL_COMMAND configures the serial port,
- GRUB_CMDLINE_LINUX specifies the command-line arguments added to the menu entries for the Linux kernel,
- GRUB_DISABLE_RECOVERY defines if all entries can be selected in recovery mode through a separate line (false) or only the default entry (true).
If you want to change the content of any variables in the previous file, you will need to type:
1# grub2-mkconfig -o /boot/grub2/grub.cfgNote: This is the main command to memorize for the exam. You can also replace /boot/grub2/grub.cfg with /etc/grub2.cfg
To better understand some of the environment variables, here are the standard display with GRUB_DISABLE_RECOVERY=”true” and GRUB_DISABLE_SUBMENU=true:
1 CentOS Linux 7 (Core), with Linux 3.10.0-229.20.1.el7.x86_64
2 CentOS Linux 7 (Core), with Linux 3.10.0-229.14.1.el7.x86_64
3 CentOS Linux 7 (Core), with Linux 0-rescue-f19b719117b44bf3a3fb777bd4127>
4
5 Use the ^ and v keys to change the selection.
6 Press 'e' to edit the selected item, or 'c' for a command prompt. If GRUB_DISABLE_RECOVERY is set to “false”, here is the new display:
1 CentOS Linux 7 (Core), with Linux 3.10.0-229.20.1.el7.x86_64
2 CentOS Linux 7 (Core), with Linux 3.10.0-229.20.1.el7.x86_64 (recovery m>
3 CentOS Linux 7 (Core), with Linux 3.10.0-229.14.1.el7.x86_64
4 CentOS Linux 7 (Core), with Linux 3.10.0-229.14.1.el7.x86_64 (recovery m>
5 CentOS Linux 7 (Core), with Linux 0-rescue-f19b719117b44bf3a3fb777bd4127>
6 CentOS Linux 7 (Core), with Linux 0-rescue-f19b719117b44bf3a3fb777bd4127>
7
8 Use the ^ and v keys to change the selection.
9 Press 'e' to edit the selected item, or 'c' for a command prompt. Each kernel line gets an associated line in recovery mode.
If GRUB_DISABLE_RECOVERY is now set to “true” (like in the initial standard display) and GRUB_DISABLE_SUBMENU is set to false, here is the new display:
1 CentOS Linux 7 (Core)
2 Advanced options for CentOS Linux 7 (Core)
3 Use the ^ and v keys to change the selection.
4 Press 'e' to edit the selected item, or 'c' for a command prompt. If the first entry is selected (“CentOS Linux 7 (Core)”), the system boots. If the second option is chosen, the standard menu is shown with an additional line at the bottom to go back to the first menu with the Esc key:
1 CentOS Linux 7 (Core), with Linux 3.10.0-229.20.1.el7.x86_64
2 CentOS Linux 7 (Core), with Linux 3.10.0-229.14.1.el7.x86_64
3 CentOS Linux 7 (Core), with Linux 0-rescue-f19b719117b44bf3a3fb777bd4127>
4
5 Use the ^ and v keys to change the selection.
6 Press 'e' to edit the selected item, or 'c' for a command prompt.
7 Press Escape to return to the previous menu. Additional Resources#
To better explore GRUB2, you can:
- read Brendan Swigart’s article,
- loot at a GRUB2 CentOS article,
- explore the GRUB2 configuration manual,
- read a Tecmint article about configuring and troubleshooting GRUB2,
- read a LinuxSecrets tutorial about changing boot order with GRUB2,
- read this article describing the RHEL 7 booting process.
Customizing the GRUB 2 Configuration File#
GRUB 2 scripts search the user's computer and build a boot menu based on what operating systems the scripts find. To reflect the latest system boot options, the boot menu is rebuilt automatically when the kernel is updated or a new kernel is added.
However, users may want to build a menu containing specific entries or to have the entries in a specific order. GRUB 2 allows basic customization of the boot menu to give users control of what actually appears on the screen.
GRUB 2 uses a series of scripts to build the menu; these are located in the /etc/grub.d/ directory. The following files are included:
01_users, which is created only when a boot loader password is assigned in a kickstart file.10_linux, which locates kernels in the default partition of Fedora.30_os-prober, which builds entries for operating systems found on other partitions.40_custom, a template, which can be used to create additional menu entries.
Scripts from the /etc/grub.d/ directory are read in alphabetical order and can be therefore renamed to change the boot order of specific menu entries.
With the GRUB_TIMEOUT key set to 0 in the /etc/default/grub file, GRUB 2 does not display the list of bootable kernels when the system starts up. In order to display this list when booting, press and hold any alphanumeric key when the BIOS information is displayed; GRUB 2 will present you with the GRUB menu.
By default, the key for the GRUB_DEFAULT directive in the /etc/default/grub file is the word saved. This instructs GRUB 2 to load the kernel specified by the saved_entry directive in the GRUB 2 environment file, located at /boot/grub2/grubenv. You can set another GRUB record to be the default, using the grub2-set-default command, which will update the GRUB 2 environment file.
By default, the saved_entry value is set to the name of latest installed kernel of package type kernel. This is defined in /etc/sysconfig/kernel by the UPDATEDEFAULT and DEFAULTKERNEL directives. The file can be viewed by the root user as follows:
1~]# cat /etc/sysconfig/kernel
2# UPDATEDEFAULT specifies if new-kernel-pkg should make
3# new kernels the default
4UPDATEDEFAULT=yes
5
6# DEFAULTKERNEL specifies the default kernel package type
7DEFAULTKERNEL=kernel-coreThe DEFAULTKERNEL directive specifies what package type will be used as the default. Installing a package of type kernel-debug will not change the default kernel while the DEFAULTKERNEL is set to package type kernel.
GRUB 2 supports using a numeric value as the key for the saved_entry directive to change the default order in which the operating systems are loaded. To specify which operating system should be loaded first, pass its number to the grub2-set-default command. For example:
1~]# grub2-set-default 2Note that the position of a menu entry in the list is denoted by a number starting with zero; therefore, in the example above, the third entry will be loaded. This value will be overwritten by the name of the next kernel to be installed.
To force a system to always use a particular menu entry, use the menu entry name as the key to the GRUB_DEFAULT directive in the /etc/default/grub file. To list the available menu entries, run the following command as root:
1~]# awk -F\' '$1=="menuentry " {print $2}' /etc/grub2.cfgThe file name /etc/grub2.cfg is a symlink to the grub.cfg file, whose location is architecture dependent. For reliability reasons, the symlink is not used in other examples in this chapter. It is better to use absolute paths when writing to a file, especially when repairing a system.
Changes to /etc/default/grub require rebuilding the grub.cfg file as follows:
GRUB 2 - Fedora Project Wiki#
https://fedoraproject.org/wiki/GRUB_2
GRUB 2 is the latest version of GNU GRUB, the GRand Unified Bootloader. A bootloader is the first software program that runs when a computer starts. It is responsible for loading and transferring control to the operating system kernel, (Linux, in the case of Fedora). The kernel, in turn, initializes the rest of the operating system.
GRUB 2 has replaced what was formerly known as GRUB (i.e. version 0.9x), which has, in turn, become GRUB Legacy.
Starting with Fedora 16, GRUB 2 is the default bootloader on x86 BIOS systems. For upgrades of BIOS systems the default is also to install GRUB 2, but you can opt to skip bootloader configuration entirely.
Updating GRUB 2 configuration on BIOS systems#
The grub2 packages contain commands for installing a bootloader and for creating a bootloader configuration file.
grub2-install will install the bootloader - usually in the MBR, in free unpartioned space, and as files in /boot. The bootloader is installed with something like:
1grub2-install /dev/sdagrub2-mkconfig will create a new configuration based on the currently running system, what is found in /boot, what is set in /etc/default/grub, and the customizable scripts in /etc/grub.d/ . A new configuration file is created with:
1grub2-mkconfig -o /boot/grub2/grub.cfgThe configuration format has evolved over time, and a new configuration file might be slightly incompatible with the old bootloader. It is therefore a good idea to first run grub2-install whenever you would need to run grub2-mkconfig.
The Fedora installer, anaconda, will run these grub2 commands and there is usually no reason to run them manually.
It is generally safe to directly edit /boot/grub2/grub.cfg in Fedora. Grubby in Fedora patches the configuration when a kernel update is performed and will try to not make any other changes than what is necessary. (Other distributions, in particular Debian and Debian-derived distributions provide a software patch that adds an update-grub command which is neither included nor needed in Fedora.) Manual changes might however be overwritten with grub2-mkconfig next time the system is upgraded with anaconda. Some customizations can be placed in /etc/grub.d/40_custom or /boot/grub2/custom.cfg and will survive running grub2-mkconfig.
Updating GRUB 2 configuration on UEFI systems#
To install or fix GRUB 2 on a UEFI system on Fedora 18 or newer, you need to do four things:
UEFI firmware, in general, likes to boot from an EFI System Partition on a disk with a GPT label. In gdisk, it looks something like this:
1Number Start (sector) End (sector) Size Code Name
2 1 2048 264191 128.0 MiB EF00 EFI SystemThat partition should be formatted as FAT. If in doubt, FAT32 is a good dialect of FAT to choose.
Fedora expects this partition to be mounted at /boot/efi.
Install the bootloader files#
If you don't already have the relevant packages installed, do for Fedora 22 and later versions with DNF or with YUM for older Fedora releases:
1dnf install grub2-efi shim
2yum install grub2-efi shimIf you do, then try:
1dnf reinstall grub2-efi shim
2yum reinstall grub2-efi shiminstead.
Make sure that /boot/efi is mounted when you do this.
This installs the signed shim and the GRUB 2 binary.
Create a GRUB 2 configuration#
Under EFI, GRUB 2 looks for its configuration in /boot/efi/EFI/fedora/grub.cfg. For newly installed kernels to work, grubby expects /etc/grub2-efi.cfg to be a symlink to the real grub.cfg (i.e. /boot/efi/EFI/fedora/grub.cfg).
If you already have a grub 2 EFI config file, you should be okay. If not, grub2-mkconfig can help, but your mileage may vary.
1grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfggrub2-install shouldn't be used on EFI systems. The grub2-efi package installs a prebaked grubx64.efi on the EFI System partition, which looks for grub.cfg on the ESP in /EFI/fedora/ whereas the grub2-install command creates a custom grubx64.efi, deletes the original installed one, and looks for grub.cfg in /boot/grub2/.
Create a boot menu entry#
TL;DR: This should happen automatically. If it doesn't, read on.
When you power on your system, your firmware will look for EFI variables that tell it how to boot. If you're already booted in EFI mode and EFI runtime services are working correctly, you can configure your boot menu with efibootmgr. If not, you'll have to bootstrap the process.
Fortunately, shim can help you bootstrap. The EFI program /boot/efi/EFI/BOOT/fallback.efi will look for files called BOOT.CSV in your ESP and will add boot entries corresponding to them, if such entries do not already appear to exist. shim provides a BOOT.CSV file that will add an entry for grub2-efi for you. So just using the EFI Shell to invoke fallback.efi should do the trick. You can do this with commands like:
1> fs0:
2> cd EFI\BOOT
3> fallback.efiIf you have no boot entries at all, then just booting off your disk in UEFI mode should automatically invoke /boot/efi/EFI/BOOT/BOOTX64.EFI, which will, in turn, invoke fallback.efi.
If you already have incorrect boot entries, you'll either need to delete them or to modify BOOT.CSV to create new entries with different names.
Adding Other operating systems to the GRUB 2 menu#
grub2-mkconfig will add entries for other operating systems it can find. That will be done based on the output of the os-prober tool.
That might however not work so well, especially not for booting other Linux operating systems, and especially not on UEFI systems. See http://www.gnu.org/software/grub/manual/grub.html#Multi_002dboot-manual-config .
Setting default entry#
Please look to (default) kernel sysconfig options.
if file /etc/sysconfig/kernel have
1UPDATEDEFAULT=yesin every kernel update the grub entry is update to last entry, if you don't want that please set:
1UPDATEDEFAULT=no(write "no" in lower case)
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.
Open /etc/default/grub and ensure this line exists:
1GRUB_DEFAULT=savedand ensure this line not exists:
1GRUB_SAVEDEFAULT=trueor ensure this line exists:
1GRUB_SAVEDEFAULT=falseApply the change to grub.cfg by running:
Now list all possible menu entries
1grep -P "submenu|^menuentry" /boot/grub2/grub.cfg | cut -d "'" -f2Now set the desired default menu entry
1grub2-set-default "<submenu title><menu entry title>"Verify the default menu entry
1grub2-editenv listIf 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
1set default="0" to
1set default="5"Encountering the dreaded GRUB 2 boot prompt#
If improperly configured, GRUB 2 may fail to load and subsequently drop to a boot prompt. To address this issue, proceed as follows:
- Load the XFS and LVM modules
1insmod xfs
2insmod lvm- List the drives which GRUB 2 sees:
1grub2> ls- The output for a dos partition table /dev/sda with three partitons will look something like this:
1(hd0) (hd0,msdos3) (hd0,msdos2) (hd0,msdos1)- While the output for a gpt partition table /dev/sda with four partitions will look something like this:
1(hd0) (hd0,gpt4) (hd0,gpt3) (hd0,gpt2) (hd0,gpt1)- With this information you can now probe each partition of the drive and locate your vmlinuz and initramfs files:
1ls (hd0,1)/ Will list the files on /dev/sda1. If this partition contains /boot, the output will show the full name of vmlinuz and initramfs.
- Armed with the location and full name of vmlinuz and initramfs you can now boot your system.
5a. Declare your root partition:
1grub> set root=(hd0,3)5b. Declare the kernel you wish to use:
1grub> linux (hd0,1)/vmlinuz-3.0.0-1.fc16.i686 root=/dev/sda3 rhgb quiet selinux=0
2# NOTE : add other kernel args if you have need of them
3# NOTE : change the numbers to match your system5c. Declare the initrd to use:
1
2grub> initrd (hd0,1)/initramfs-3.0.0-1.fc16.i686.img
3# NOTE : change the numbers to match your system5d. Instruct GRUB 2 to boot the chosen files:
1grub> bootAfter boot, open a terminal.
Issue the grub2-mkconfig command to re-create the grub.cfg file grub2 needed to boot your system:
1grub2-mkconfig -o /boot/grub2/grub.cfg- Issue the grub2-install command to install grub2 to your hard drive and make use of your config:
1grub2-install --boot-directory=/boot /dev/sda
2# Note: your drive may have another device name. Check for it with mount command output.Additional Scenario#
It's also possible to boot into a configfile that's located on another partition. If the user is faced with such a scenario, as is often the case with multi-boot systems containing Ubuntu and Fedora, the following steps in the grub rescue shell might become useful to know:
1insmod part_msdos
2insmod xfs
3insmod lvm
4set root='hd0,msdos1'
5configfile /grub2/grub.cfgWhere, hd0,msdos1 is the pertinent boot partition, which holds the grub.cfg file.
Other GRUB 2 issues#
Absent Floppy Disk : It has been reported by some users that GRUB 2 may fail to install on a partition's boot sector if the computer floppy controller is activated in BIOS without an actual floppy disk drive being present. A possible workaround is to run (post OS install) from rescue mode:
1grub2-install <target device> --no-floppySetting a password for interactive edit mode#
If you wish to password-protect GRUB2's interactive edit mode but you do not want to require users to enter a password to do a plain, simple, ordinary boot, create /etc/grub.d/01_users with the following lines:
1cat << EOF
2set superusers="root"
3export superusers
4password root secret
5EOFTo apply your changes run:
1grub2-mkconfig -o /boot/grub2/grub.cfgYou can encrypt the password by using pbkdf2. Use grub2-mkpasswd-pbkdf2 to encrypt the password, then replace the password line with:
1password_pbkdf2 root grub.pbkdf2.sha512.10000.1B4BD9B60DE889A4C50AA9458C4044CBE129C9607B6231783F7E4E7191D8254C0732F4255178E2677BBE27D03186E44815EEFBAD82737D81C87F5D24313DDDE7.E9AEB53A46A16F30735E2558100D8340049A719474AEEE7E3F44C9C5201E2CA82221DCF2A12C39112A701292BF4AA071EB13E5EC8C8C84CC4B1A83304EA10F74Starting from atleast Fedora 21, the --md5pass kickstart option must be set using output from grub2-mkpasswd-pbkdf2.
Using old graphics modes in bootloader#
Terminal device is chosen with GRUB_TERMINAL; additional quote from http://www.gnu.org/software/grub/manual/grub.html#Simple-configuration
Valid terminal output names depend on the platform, but may include ‘console’ (PC BIOS and EFI consoles), ‘serial’ (serial terminal), ‘gfxterm’ (graphics-mode output), ‘ofconsole’ (Open Firmware console), or ‘vga_text’ (VGA text output, mainly useful with Coreboot).
The default is to use the platform's native terminal output.
The default in Fedora is gfxterm and to get the legacy graphics modes you need to set GRUB_TERMINAL to right variable from the description above in /etc/default/grub
Enable Serial Console in Grub#
To enable Serial console in grub add the following entry's to /etc/default/grub
( Adjust baudrate/parity/bits/flow control to fit your environment and cables)
1GRUB_CMDLINE_LINUX='console=tty0 console=ttyS0,115200n8'
2GRUB_TERMINAL=serial
3GRUB_SERIAL_COMMAND="serial --speed=115200 --unit=0 --word=8 --parity=no --stop=1"And re-generate grub
grub2-mkconfig -o /boot/grub2/grub.cfg
NOTE: in UEFI boot environment, use efi0 instead of --unit=0. If that doesn't work, check that your serial port is visible in your UEFI environment, e.g. by running devtree or dh -p SerialIO in EFI Shell. See this discussion for more information.
Normally, GRUB2 will be installed and set up by the installer, Anaconda, during the installation process. You will probably never have to deal with manual installation of GRUB2. However, in certain situations , you will want to install GRUB2 manually, especially if you need to repair the existing GRUB2 installation or you want to change its configuration.
This procedure shows the steps to install GRUB2 on a UEFI system on Fedora 18 or newer. The procedure consists of four parts.
Creating an EFI System Partition#
The UEFI firmware requires to boot from an EFI System Partition on a disk with a GPT label. To create such a partition:
List available block devices to find a place to create your ESP.
Create at least a 128 MiB disk partition using a GPT label on the primary hard disk.
For the sake of this procedure, we assume that the created partition is recognized as
/dev/sda1.Format the partition with the FAT32 file system.
Create the
/boot/efidirectory as a mount point for the new partition.Mount the partition to the
/boot/efimount point.1# mount /dev/sda1 /boot/efiProceed to the next part.
Install the bootloader files#
In order to use GRUB2 with on the UEFI systems, you need to install or re-install appropriate packages:
Re-install the necessary packages.
1# dnf reinstall grub2-efi grub2-efi-modules shimIf the above command ends with an error, install the packages.
1# dnf install grub2-efi grub2-efi-modules shim
More information
- This installs the signed shim and the GRUB2 binary.
Create a GRUB2 configuration#
If you already have a working GRUB2 EFI configuration file, you do not need to do anything else.
Otherwise, create the configuration file using the grub2-mkconfig command.
1# grub2-mkconfig -o /boot/grub2/grub.cfgMore information
Under EFI, GRUB2 looks for its configuration in
/boot/efi/EFI/fedora/grub.cfg, however the postinstall script ofgrub2-commoninstalls a small shim which chains to the standard configuration at/boot/grub2/grub.cfgwhich is generated above. To reset this shim to defaults, delete the existing/boot/efi/EFI/fedora/grub.cfgand thendnf reinstall grub2-common.For newly installed kernels to work,
grubbyexpects/etc/grub2-efi.cfgto be a symlink to the realgrub.cfg(for example/boot/grub2/grub.cfg).
Solving problems with UEFI bootloader#
When you power on your system, your firmware will look for EFI variables that tell it how to boot. On running systems, which have booted into the EFI mode and their EFI runtime services are working correctly, you can configure your boot menu with efibootmgr.
If not, shim can help you bootstrap. The EFI program /boot/efi/EFI/BOOT/fallback.efi will look for files called BOOT.CSV in your ESP and will add boot entries corresponding to them. The shim command provides its own BOOT.CSV file that will add an entry for grub2-efi.
During the boot process, you can use the EFI Shell to invoke the fallback.efi profile to boot the system:
Enter the boot partition.
Navigate into the
EFI\BOOTdirectory.Invoke the
fallback.efiprofile.
More information
If you have no boot entries at all, then just booting off your disk in UEFI mode should automatically invoke
/boot/efi/EFI/BOOT/BOOTX64.EFI, which will, in turn, invokefallback.efi.If you already have incorrect boot entries, you’ll either need to delete them or to modify
BOOT.CSVto create new entries with different names.
The Grub2 Rescue shell#
https://aty.sdsu.edu/bibliog/latex/debian/grub2rescue.html
Introduction#
Nothing is more frustrating than making some small change to your system, and then discovering that it won't boot. This must have happened to me dozens of times over the years; and the problem is always something different and unexpected.
The most frustrating experience you can have with GRUB is to be dropped into its “Rescue shell”. You might get a cryptic error message, followed by
Entering rescue mode . . .
grub rescue>
No help; no advice on how to proceed; and even the Grub2 Manual tells you nothing useful. Yuck.
The rescue shell is an exceedingly minimal, Spartan environment. It offers only a tiny subset of the regular Grub shell's commands:
set unset ls insmod
These commands offer no options, and it is impossible even to learn which ones are available. Even common commands like cat and cd are not available. At first glance, it would seem impossible to do anything useful in this limited environment.
And yet, it's actually possible to go through the booting process manually — provided that you know something about the booting process, and how Grub handles it. In fact, the operations that Grub executes automatically can be done by hand, just with the limited means provided by the rescue shell.
Understanding Disaster#
Actually, the fact that you see the grub rescue> prompt is good news in disguise. It means you haven't wiped out Grub's booting system. The hardware has gone through the POST process, and loaded the primary stage of the boot loader; but the information available to that stage (supposedly, the addresses of the disk blocks that hold the next stage) was wrong. Grub is alive, but it needs a little help.
The common causes of this situation are things that changed the locations of the next pieces of Grub's boot-time subsystem. Maybe you re-formatted the filesystem that holds those files; maybe you updated a kernel, but didn't re-generate Grub's configuration files; maybe you ran update-grub with incorrect partition identifiers in the /etc/grub.d/40_custom shell script. The result was that Grub's early stages that depend on absolute block addresses couldn't find the following pieces. You can tell the part of Grub that's already working how to proceed.
With luck, the error message that precedes the rescue prompt will tell you what Grub looked for but couldn't find. Write it down, and take appropriate action after you've re-booted your box.
Diagnosing disaster#
The first thing to do is to find out what Grub knows already. At the
grub rescue>
prompt, enter the command:
set
and Grub will tell you what little it knows. Again, take note of this information, as it will provide clues to what you need to do next — as well as clues to fixing the problem before the next reboot.
The main Grub variables available at this point are usually the prefix and root parameters. What Grub calls prefix is the location (in Grub's peculiar notation) of the directory that holds Grub's pieces. In normal *IX terminology, this would usually be /boot/grub. But Grub doesn't have mounted filesystems yet; it only knows about disk partitions, which it calls things like (hd0,msdos1).
So, if you have a separate boot partition, which is normally mounted on /boot, Grub will need to have prefix set to an address of the form (hd0,msdos1)/grub. On the other hand, if your /boot directory is in your root-filesystem's partition, Grub will need to have prefix=(hd0,msdos1)/boot/grub.
Similarly, what Grub calls root is NOT the partition that contains the root filesystem, but the partition that contains the kernel and initial RAM filesystem (i.e., initramfs) files. That usually means that Grub thinks root=(hd0,msdos1).
Fixing the problem#
First of all, make sure Grub can see the partitions that contain these vital parts. Tell the rescue shell:
ls
and it will show you the partitions it knows about. This will be a list of things like
(hd0,msdos1) (hd0,msdos2) . . .
which mean /dev/sda1, /dev/sda2, and so on. Remember that Grub's name for the first disk is hd0; the second disk is hd1, etc. — but the partitions themselves are numbered normally, starting at 1 instead of zero.
Don't be misled by a superficial resemblance between the rescue-shell ls command and the normal ls that you use at a bash prompt. The normal command has scads of options; the rescue-shell command is a stripped-down version with no options, and a different output format. Only the names are similar.
If the rescue shell had a cat command, you could list the contents of /boot/grub/device.map to learn Grub's correspondence between disks and names; but it doesn't, so you can't. Keep a copy of the device.map file printed out beforehand, because the devices might not map out the way you expect. (For example, on my machine, Grub thinks sda is hd0, as you'd expect; but it also thinks sdb is hd2, and sdc is hd1, which is screwy.)
Then, if Grub has the wrong values for either prefix or root, you can fix its error by telling the rescue shell something like
set prefix=(hd0,msdos1)/boot/grub
or whatever the correct value is in your box.
Remember that you can verify that Grub has the correct values for these parameters with a simple
set
command. And you can double-check by telling the rescue shell to list the contents of those directories:
ls $prefix/
Proceed to boot manually#
Now you need to lead Grub through its normal booting sequence. The first step is to make sure it has its module available:
insmod normal
This module is the “guts” of Grub2: it contains the apparatus for reading configuration files, and displaying the usual Grub boot menu. The correct prefix and root parameter values are needed, if Grub is to find any modules.
Once the module is inserted, you can execute it as a new rescue-shell command:
normal
Another vital module that must be loaded is the linux.mod file:
insmod linux
Now you can set up the linux kernel's command line:
linux /boot/vmlinuz-3.16.0-4-686-pae root=/dev/sda2 ro
or whatever is the correct name for your kernel. Note that the “root” in this line points (in normal terms) to your root filesystem's disk partition; it is different from Grub's root parameter! You can specify the root partition here in perfectly normal terms — either by specifying --fs-uuid and the UUID of the partition, or by using a filesystem label, as in root=LABEL=ROOT (if your root filesystem is named ROOT). If you need additional command-line parameters, like video=640x480, be sure to add them here as well.
Similarly, you need to tell Grub where the initial RAMdisk filesystem is:
initrd /boot/initrd.img-3.16.0-4-686-pae
Make sure it matches the kernel version!
Now, with both the kernel and the initrd modules installed, and their arguments supplied, Grub should be able to boot. Tell the rescue shell to do so:
boot
Once the linux and normal modules are inserted, most of Grub's apparatus will be working, and the larger set of normal GrubScript commands will be accessible. Or, if you get everything messed up and want to start over, the usual invocation will re-boot the system.
Back to normal#
When you are back up, be sure to fix the errors that caused the booting problem. If the configuration parameters are set in the file and any special scripts (like ), you should be able to get a correct just by executing (as root):
update-grub
and
grub-install /dev/sda
(or whatever disk is marked as the boot device in your BIOS).
Sometimes, the error that brought up the rescue shell was
error: no such device:
followed by a UUID for some partition that has disappeared — usually, due to a re-made partition. In this case, you may need to invoke
update-initramfs -u -v
to get the correct partition named in the initrd.img file.
Copyright © 2015, 2017 Andrew T. Young
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GrubEFIReinstall#
https://wiki.debian.org/GrubEFIReinstall
Starting with Windows 8 most Desktop PC have EFI as firmware instead of the legacy BIOS. If your EFI based PC is not booting debian, here is an easy way to reinstall grub-efi, the bootloader used by debian on these PCs.
To reinstall grub, you need either a live CD/USB to access your current system , or you can use the rEFInd boot manager on a live CD/USB to boot your current system.
Using the rEFInd rescue media#
At the author's web page http://www.rodsbooks.com/refind/getting.html, you will find updated direct links to all sorts of packaging. To boot from a rescue media, select either the CD iso or the image for USB stick, most firmware offers the choice nowadays. If choosing the latter make sure to follow the instructions in the readme. It is recommended to read the author's web pages to get a better understanding of what you are doing.
Boot your computer with the Refind media#
Refind will parse your hard drive for installed kernels, and provide you a graphic menu to boot them. Choose your Linux Kernel and boot it.
Reinstalling grub-efi on your hard drive#
Check that the computer booted in computer in EFI mode:
1[ -d /sys/firmware/efi ] && echo "EFI boot on HDD" || echo "Legacy boot on HDD"
2should return "EFI boot on HDD".After starting a root shell ( if you boot from a live media, you should start a chroot shell instead, as explained in https://help.ubuntu.com/community/Grub2/Installing#via_ChRoot ) check that your EFI system partition (most probably /dev/sda1) is mounted on /boot/efi. If the /boot/efi directory does not exist, you will need to create it.
1mount /dev/sda1 /boot/efiReinstall the grub-efi package
1apt-get install --reinstall grub-efiPut the debian bootloader in /boot/efi and create an appropriate entry in the computer NVRAM
1grub-install /dev/sdaRe create a grub config file based on your disk partitioning schema
1update-grubYou should check afterwards that:
Check 1. the bootloader is existing in /boot/efi/EFI/debian/grubx64.efi
1file /boot/efi/EFI/debian/grubx64.efi
2
3/boot/efi/EFI/debian/grubx64.efi: PE32+ executable (EFI application) x86-64 (stripped to external PDB), for MS WindowsCheck 2. the nvram entry was properly created.
1efibootmgr --verbose | grep debianYou can now reboot, and Grub should greet you.
Troubleshooting#
If after this steps you're not booting, the EFI of your PC might have some bugs.
Problem1: Weak EFI implementation only recognizes the fallback bootloader#
The uefi firmware refuses to boot the debian/grubx64.efi bootloader, and so we have to hijack the uefi fallback boot loader. See http://mjg59.livejournal.com/138188.html for details.
Using debian installer in rescue mode, /dev/sda1 being the FAT32 ESP partition, /dev/sda2 the root partition
1mkdir /target
2mount /dev/sda2 /target
3mount /dev/sda1 /target/boot/efi
4for i in /sys /proc /dev; do mount --bind $i /target$i; done
5chroot /target1cd /boot/efi/EFI
2mkdir boot
3cp debian/grubx64.efi boot/bootx64.efi
4exit
5for i in /sys /proc /dev; do umount /target$i; done
6umount /target/boot/efi
7umount /targetOnce booted into your normal Debian, tell grub to ensure the fallback boot loader up to date. To do that, run the following:
1echo "grub-efi-amd64 grub2/force_efi_extra_removable boolean true" | sudo debconf-set-selectionsNote: The above command will permanently hijack the fallback boot loader, which might be undesirable in dual-boot setups.
Problem2: EFI boot entries disappear after reboot#
The uefi firmware did not create a proper boot entry in NVRAM. This has been seen in a Lenovo Thinkcenter M92Z. The symptom for this will be a missing HD path after the debian entry in the efibootmgr --verbose output.
1BootCurrent: 0024
2Timeout: 0 seconds
3BootOrder: 0024,0022,0023,0016,0000,0001
4Boot0000* debian Vendor(99e275e7-75a0-4b37-a2e6-c5385e6c00cb,)
5Boot0016* Generic Usb Device Vendor(99e275e7-75a0-4b37-a2e6-c5385e6c00cb,)
6Boot0022* UEFI: IPv4 Intel(R) 82579LM Gigabit Network Connection ACPI(a0341d0,0)PCI(19,0)MAC(d43d7e6d8bfc,0)IPv4(0.0.0.0:0<->0.0.0.0:0,0, 0AMBO
7Boot0023* UEFI: IPv6 Intel(R) 82579LM Gigabit Network Connection ACPI(a0341d0,0)PCI(19,0)MAC(d43d7e6d8bfc,0)030d3c000000000000000000000000000000000000000000000000000000000000000000000000000000004000000000000000000000000000000000AMBO
8Boot0024* UEFI: Generic Flash Disk 8.00 ACPI(a0341d0,0)PCI(1d,0)USB(1,0)USB(1,0)HD(1,800,2a5f,02f23208-1aa9-4b6c-b6e1-8155390eb9db)AMBOYou can then try to install Refind as your bootloader in the hard drive, following the steps at this gist: https://gist.github.com/EmmanuelKasper/9590327.
How to update GRUB2 using grub2-editenv and grubby in RHEL 8 Linux#
https://www.golinuxcloud.com/update-grub2-grubby-grub2-editenv-rhel-8/
Notes:#
- In rescue mode, in case chroot to the filesystem on-disk does not work, do the following manually:
- Check the partitions and type (fdisk -l), and identify if LVM volumes exist.
- If LVM exists, activate them using
lvm vgscanandlvm vgchange -ay - Mount /dev/<VG_name>/root onto a folder that will be used as a chroot folder later (mount /dev/<VG_name>/root /sysroot)
- Mount /dev/sda1 on /boot (Check if /dev/sda1 is the disk containing /boot)
- Mount other valid logical volumes (Check with lvm lvs) onto folders under /sysroot/
- Eg.. /dev/rhel_vg/var/ on /sysroot/var
- Bind mount /proc, /sys, /dev onto /sysroot
- mount —bind /proc /sysroot/proc
- mount —bind /dev /sysroot/dev
- mount —bind /sys /sysroot/sys
The boot loader Grand Unified Boot Loader (GRUB2) in RHEL 8 differs from the GRUB2 in RHEL 7. In this article I will share different commands to update GRUB2 and set kernel command line argument in RHEL 8 Linux.
An update to Red Hat Enterprise Linux 8 Beta results in /etc/default/grub changes no longer being included when issuing grub2-mkconfig -o /boot/grub2/grub.cfg. It seems that at least some options set there are now silently ignored.
In this article I will disable IPv6 using GRUB2 (ipv6.disable) on RHEL 8 Linux to demonstrate the steps to update GRUB2 in RHEL 8 Linux host.
Update GRUB2 using grub2-editenv#
The grub2-editenv utility is actually the recommended path to alter these variables. As a result, the following can be used. Appending an extra argument:
1[root@rhel-8 ~]# grub2-editenv - list | grep kernelopts
2kernelopts=root=/dev/mapper/rhel-root ro crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quietHere copy paste the content from above command and append the additional kernel parameter you wish to add to the GRUB2.
1[root@rhel-8 ~]# grub2-editenv - set "kernelopts=root=/dev/mapper/rhel-root ro crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet ipv6.disable=1"Alternatively you can also choose a shorter and less error prone approach as shown below
1[root@rhel-8 ~]# grub2-editenv - set "$(grub2-editenv - list | grep kernelopts) ipv6.disable=1"Here, $(grub2-editenv - list | grep kernelopts) will automatically select the existing kernelopts and will append the additional kernel command line argument.
Verify the newly added output
1[root@rhel-8 ~]# grub2-editenv - list | grep kernelopts
2kernelopts=root=/dev/mapper/rhel-root ro crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet ipv6.disable=1Next reboot the node and verify the configuration to make sure your changes are persistent
1[root@rhel-8 ~]# grub2-editenv - list | grep kernelopts
2kernelopts=root=/dev/mapper/rhel-root ro crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet ipv6.disable=1As you can see our IPv6 is disabled in the GRUB2 command line.
1[root@rhel-8 ~]# cat /proc/cmdline
2BOOT_IMAGE=(hd0,msdos1)/vmlinuz-4.18.0-80.el8.x86_64 root=/dev/mapper/rhel-root ro crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet ipv6.disable=1Similarly to remove a parameter or argument from kernel command line in GRUB2, use the below syntax
1# grub2-editenv - set "$(grub2-editenv - list | grep kernelopts | sed -e 's///')"Lastly reboot your system for the changes to take effect.
Update GRUB2 using grub2-mkconfig#
The older method of achieving this behaviour is still possible, but the existing kernelopts value will need to be unset first:
1[root@rhel-8 ~]# grep GRUB_CMDLINE_LINUX /etc/default/grub
2GRUB_CMDLINE_LINUX="crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet"Here I will update ipv6.disable=1 in the GRUB2 configuration file /etc/default/grub
1[root@rhel-8 ~]# grep GRUB_CMDLINE_LINUX /etc/default/grub
2GRUB_CMDLINE_LINUX="crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet ipv6.disable=1"But as you see the existing kernelopts reflects old GRUB2 entry, so if we reboot the node or rebuild our GRUB2 then the new changes will not reflect on the node.
1[root@rhel-8 ~]# grub2-editenv - list | grep kernelopts
2kernelopts=root=/dev/mapper/rhel-root ro crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quietSo we need to unset the kernelopts which is an additional step here:
1[root@rhel-8 ~]# grub2-editenv - unset kerneloptsnext rebuild the grub configuration
1[root@rhel-8 ~]# grub2-mkconfig -o /boot/grub2/grub.cfg
2Generating grub configuration file ...
3doneNext re-verify kernelopts
1[root@rhel-8 ~]# grub2-editenv - list | grep kernelopts
2kernelopts=root=/dev/mapper/rhel-root ro crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet ipv6.disable=1So our changes are visible as expected.
Update GRUB2 using grubby#
grubby is a utility for manipulating bootloader-specific configuration files.
You can use grubby also for changing the default boot entry, and for adding/removing arguments from a GRUB2 menu entry.
1[root@rhel-8 ~]# grubby --info DEFAULT
2index=0
3kernel="/boot/vmlinuz-4.18.0-80.el8.x86_64"
4args="ro crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet $tuned_params"
5root="/dev/mapper/rhel-root"
6initrd="/boot/initramfs-4.18.0-80.el8.x86_64.img $tuned_initrd"
7title="Red Hat Enterprise Linux (4.18.0-80.el8.x86_64) 8.0 (Ootpa)"
8id="f653c3662e81432aa484cd1639a04047-4.18.0-80.el8.x86_64"Add the argument you wish to append to the kernel command line menu
1[root@rhel-8 ~]# grubby --args ipv6.disable=1 --update-kernel DEFAULTNext verify the GRUB2 configuration
1[root@rhel-8 ~]# grubby --info DEFAULT
2index=0
3kernel="/boot/vmlinuz-4.18.0-80.el8.x86_64"
4args="ro crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet $tuned_params ipv6.disable=1"
5root="/dev/mapper/rhel-root"
6initrd="/boot/initramfs-4.18.0-80.el8.x86_64.img $tuned_initrd"
7title="Red Hat Enterprise Linux (4.18.0-80.el8.x86_64) 8.0 (Ootpa)"
8id="f653c3662e81432aa484cd1639a04047-4.18.0-80.el8.x86_64"Next reboot the node to activate the changes
1[root@rhel-8 ~]# cat /proc/cmdline
2BOOT_IMAGE=(hd0,msdos1)/vmlinuz-4.18.0-80.el8.x86_64 root=/dev/mapper/rhel-root ro crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet ipv6.disable=1Lastly I hope the steps from the article to set kernel command line argument, update GRUB2 using grub2-editenv, grub2-mkconfig and grubby in RHEL 8 Linux was helpful. So, let me know your suggestions and feedback using the comment section.
