GRUB/Tips and tricks
Alternative installation methods
BIOS
Assume your USB stick's first partition is FAT32 and its partition is /dev/sdy1
# mount --mkdir /dev/sdy1 /mnt/usb # grub-install --target=i386-pc --debug --boot-directory=/mnt/usb/boot /dev/sdy # grub-mkconfig -o /mnt/usb/boot/grub/grub.cfg
Optionally backup configuration files of grub.cfg:
# mkdir -p /mnt/usb/etc/default # cp /etc/default/grub /mnt/usb/etc/default # cp -a /etc/grub.d /mnt/usb/etc
# sync; umount /mnt/usb
EFI
To have grub write its EFI image to esp/EFI/BOOT/BOOTX64.efi, which the boot firmware will be able to find without any UEFI boot entry, use --removable when you run grub-install.
Install to partition or partitionless disk
To set up grub to a partition boot sector, to a partitionless disk (also called superfloppy) or to a floppy disk, run (using for example /dev/sdaX as the /boot partition):
# chattr -i /boot/grub/i386-pc/core.img # grub-install --target=i386-pc --debug --force /dev/sdaX # chattr +i /boot/grub/i386-pc/core.img
/dev/sdaXused for example only.--target=i386-pcinstructsgrub-installto install for BIOS systems only. It is recommended to always use this option to remove ambiguity in grub-install.
You need to use the --force option to allow usage of blocklists and should not use --grub-setup=/bin/true (which is similar to simply generating core.img).
grub-install will give out warnings like which should give you the idea of what might go wrong with this approach:
/sbin/grub-setup: warn: Attempting to install GRUB to a partitionless disk or to a partition. This is a BAD idea.
/sbin/grub-setup: warn: Embedding is not possible. GRUB can only be installed in this setup by using blocklists.
However, blocklists are UNRELIABLE and their use is discouraged.
Without --force you may get the below error and will not setup its boot code in the partition boot sector:
/sbin/grub-setup: error: will not proceed with blocklists
With --force you should get:
Installation finished. No error reported.
The reason why does not by default allow this is because in case of partition or a partitionless disk is that GRUB relies on embedded blocklists in the partition bootsector to locate the file and the prefix directory . The sector locations of core.img may change whenever the file system in the partition is being altered (files copied, deleted etc.). For more info, see https://bugzilla.redhat.com/show_bug.cgi?id=728742 and https://bugzilla.redhat.com/show_bug.cgi?id=730915.
The workaround for this is to set the immutable flag on (using chattr command as mentioned above) so that the sector locations of the core.img file in the disk is not altered. The immutable flag on needs to be set only if GRUB is installed to a partition boot sector or a partitionless disk, not in case of installation to MBR or simple generation of core.img without embedding any bootsector (mentioned above).
Unfortunately, the grub.cfg file that is created will not contain the proper UUID in order to boot, even if it reports no errors. see https://bbs.archlinux.org/viewtopic.php?pid=1294604#p1294604.
In order to fix this issue the following commands:
# mount /dev/sdxY /mnt #Your root partition. # mount /dev/sdxZ /mnt/boot #Your boot partition (if you have one). # arch-chroot /mnt
Now, install linux, then:
# grub-mkconfig -o /boot/grub/grub.cfg
Generate core.img alone
To populate the directory and generate a file without embedding any GRUB bootsector code in the MBR, post-MBR region, or the partition bootsector, add --grub-setup=/bin/true to grub-install:
# grub-install --target=i386-pc --grub-setup=/bin/true --debug /dev/sda
You can then chainload GRUB's core.img from GRUB Legacy or syslinux as a Linux kernel or as a multiboot kernel (see also Syslinux#Chainloading).
GUI configuration tools
Visual configuration
In GRUB it is possible, by default, to change the look of the menu. Make sure to initialize the GRUB graphical terminal, gfxterm, in :
GRUB_TERMINAL_OUTPUT="gfxterm"
Setting the framebuffer resolution
GRUB can set the framebuffer for both GRUB itself () and the kernel (). The old way is deprecated. The preferred method is editing to set width (pixels) x height (pixels) x color depth:
GRUB_GFXMODE=1024x768x32 GRUB_GFXPAYLOAD_LINUX=keep
Multiple resolutions can be specified, including the default , so it is recommended that you edit the line to resemble . For more information, refer to the GRUB gfxmode documentation. The gfxpayload property will make sure the kernel keeps the resolution.
- Only the modes supported by the graphics card via VESA BIOS Extensions can be used. To view the list of supported modes, install hwinfo and run
hwinfo --framebufferas root. Alternatively, enter the GRUB command line and run the commandvideoinfo. - Earlier versions of the NVIDIA proprietary driver (tested with GeForce GTX 970, driver: nvidia 370) accepts
GRUB_GFXMODEin formatwidthxheight-depth(e.g.1920x1200-24, but not1920x1200x24). This does not appear to apply to newer cards and drivers. Pascal cards with more recent drivers (tested with GeForce GTX 1060 and nvidia 381.22) will not work with the suggested format and attempting to use it results in serious issues, including but not limited to system crashes and hard locks. The current driver and cards are best configured withGRUB_GFXMODEin the standardwidthxheightxdepthformat. - Make sure to run
grub-mkconfig -o /boot/grub/grub.cfgafter making changes.
If this method does not work for you, the deprecated method will still work. Just add it next to the "GRUB_CMDLINE_LINUX_DEFAULT=" line in for example: will give you a resolution.
915resolution hack
Sometimes for Intel graphic adapters neither nor will show you the desired resolution. In this case you can use the hack. This hack will temporarily modify video BIOS and add needed resolution. See 915resolution's home page archive. The package can be found here:
First you need to find a video mode which will be modified later. For that we need the GRUB command shell:
Next, we overwrite the with 1440x900 resolution:
/etc/grub.d/00_header
[...]
'''915resolution 30 1440 900'''
set gfxmode=${GRUB_GFXMODE}
[...]
Lastly we need to set as described earlier, regenerate grub.cfg and reboot to test changes.
Background image and bitmap fonts
GRUB comes with support for background images and bitmap fonts in format. The unifont font is included in the package under the filename , or, as only ASCII characters under the name . Run to get the file paths.
Image formats supported include tga, png and jpeg, providing the correct modules are loaded. The maximum supported resolution depends on your hardware.
Make sure you have set up the proper framebuffer resolution.
Edit like this:
GRUB_BACKGROUND="/boot/grub/myimage" #GRUB_THEME="/path/to/gfxtheme" GRUB_FONT="/path/to/font.pf2"
Re-generate grub.cfg to apply the changes. If adding the splash image was successful, the user will see in the terminal as the command is executed. If this phrase is not seen, the image information was probably not incorporated into the grub.cfg file.
If the image is not displayed, check:
- The path and the filename in are correct
- The image is of the proper size and format (tga, png, 8-bit jpg)
- The image was saved in the RGB mode, and is not indexed
- The console mode is not enabled in
- The command must be executed to place the background image information into the
/boot/grub/grub.cfgfile - The scripts will not quote the file name in
grub.cfgso make sure it does not contain spaces
Theme
Here is an example for configuring Starfield theme which was included in GRUB package.
Edit :
GRUB_THEME="/usr/share/grub/themes/starfield/theme.txt"
Re-generate grub.cfg to apply the changes. If configuring the theme was successful, you will see in the terminal.
Your splash image will usually not be displayed when using a theme.
Menu colors
You can set the menu colors in GRUB. The available colors for GRUB can be found in the GRUB Manual. Here is an example:
Edit :
GRUB_COLOR_NORMAL="light-blue/black" GRUB_COLOR_HIGHLIGHT="light-cyan/blue"
Hidden menu
One of the unique features of GRUB is hiding/skipping the menu and showing it by holding when needed. You can also adjust whether you want to see the timeout counter.
Edit as you wish. Here are the lines you need to add to enable this feature, the timeout has been set to five seconds and to be shown to the user:
GRUB_TIMEOUT=5 GRUB_TIMEOUT_STYLE='countdown'
GRUB_TIMEOUT is how many seconds before displaying menu.
Disable framebuffer
Users who use NVIDIA proprietary driver might wish to disable GRUB's framebuffer as it can cause problems with the binary driver.
To disable framebuffer, edit and uncomment the following line:
GRUB_TERMINAL_OUTPUT=console
Another option if you want to keep the framebuffer in GRUB is to revert to text mode just before starting the kernel. To do that modify the variable in :
GRUB_GFXPAYLOAD_LINUX=text
Booting ISO9660 image file directly via GRUB
GRUB supports booting from ISO images directly via loopback devices, see Multiboot USB drive#Using GRUB and loopback devices for examples.
Password protection of GRUB menu
If you want to secure GRUB so it is not possible for anyone to change boot parameters or use the command line, you can add an username and password to GRUB's configuration files. To do this, run the command , then enter a password and confirm it:
Then, adjust permissions on such that only root can read it by running . Then modify the file as following:
where password is the string starting with grub.pbkdf2 generated by .
Regenerate your configuration file with . Accessing the GRUB command line, boot parameters and also booting an entry now require the specified username and password. Note, that the latter can prevent by following the next topic.
This can be relaxed and further customized with more users as described in the "Security" part of the GRUB manual.
Password protection of GRUB edit and console options only
Adding to a menu entry will allow any user to boot the OS while preventing the user from editing the entry and preventing access to the grub command console. Only a superuser or users specified with the switch will be able to edit the menu entry.
/boot/grub/grub.cfg
menuentry 'Arch Linux' --unrestricted --class arch --class gnu-linux --class os ...
In order to make Linux entries , the variable in the beginning of can be modified.
Hide GRUB unless the Shift key is held down
In order to achieve the fastest possible boot, instead of having GRUB wait for a timeout, it is possible for GRUB to hide the menu, unless the key is held down during GRUB's start-up.
In order to achieve this, you should add the following line to :
GRUB_FORCE_HIDDEN_MENU="true"
Then create the file /etc/grub.d/31_hold_shift containing , make it executable, and regenerate the grub configuration:
# grub-mkconfig -o /boot/grub/grub.cfg
Combining the use of UUIDs and basic scripting
If you like the idea of using UUIDs to avoid unreliable BIOS mappings or are struggling with GRUB's syntax, here is an example boot menu item that uses UUIDs and a small script to direct GRUB to the proper disk partitions for your system. All you need to do is replace the UUIDs in the sample with the correct UUIDs for your system. The example applies to a system with a boot and root partition. You will obviously need to modify the GRUB configuration if you have additional partitions:
Multiple entries
Disable submenu
If you have multiple kernels installed, say linux and linux-lts, by default groups them in a submenu. If you do not like this behaviour you can go back to one single menu by adding the following line to :
GRUB_DISABLE_SUBMENU=y
Recall previous entry
GRUB can remember the last entry you booted from and use this as the default entry to boot from next time. This is useful if you have multiple kernels (i.e., the current Arch one and the LTS kernel as a fallback option) or operating systems. To do this, edit and change the value of :
GRUB_DEFAULT=saved
This ensures that GRUB will default to the saved entry. To enable saving the selected entry, add the following line to :
GRUB_SAVEDEFAULT=true
This will only work if /boot is not a btrfs, because grub cannot write to btrfs. But it will generate a misleading error message: "sparse file not allowed. Press any key to continue.".
Changing the default menu entry
To change the default selected entry, edit and change the value of :
Using menu titles :
GRUB_DEFAULT='Advanced options for Arch Linux>Arch Linux, with Linux linux'
Using numbers :
GRUB_DEFAULT="1>2"
Grub identifies entries in the generated menu (i.e. /boot/grub/grub.cfg) counted from zero. That means for the first entry which is the default value, 1 for the second and so on. Main and submenu entries are separated
by a .
The example above boots the third entry from the main menu 'Advanced options for Arch Linux'.
Boot non-default entry only once
The command is very helpful to boot another entry than the default only once. GRUB loads the entry passed in the first command line argument, when the system is rebooted the next time. Most importantly GRUB returns to loading the default entry for all future booting. Changing the configuration file or selecting an entry in the GRUB menu is not necessary.
GRUB_DEFAULT=saved in /etc/default/grub (and then regenerating grub.cfg) or, in case of hand-made grub.cfg, the line set default="${saved_entry}".Play a tune
You can play a tune through the PC-speaker while booting (right before the menu appears) by modifying the variable :
GRUB_INIT_TUNE="tempo [note_pitch note_duration] [second_note_pitch second_note_duration] ..."
You can add a menu entry to play each of these common samples by creating the linked and then re-running .
For information on this, you can look at , while some collections exist.
Manual configuration of core image for early boot
If you require a special keymap or other complex steps that GRUB is not able to configure automatically in order to make /boot available to the GRUB environment, you can generate a core image yourself. On UEFI systems, the core image is the file that is loaded by the firmware on boot. Building your own core image will allow you to embed any modules required for very early boot, as well as a configuration script to bootstrap GRUB.
Firstly, taking as an example a requirement for the keymap embedded in early-boot in order to enter a password for an encrypted /boot on a UEFI system:
Determine from the generated /boot/grub/grub.cfg file what modules are required in order to mount the crypted /boot. For instance, under your you should see lines similar to:
Take note of all of those modules: they will need to be included in the core image. Now, create a tarball containing your keymap. This will be bundled in the core image as a memdisk:
# grub-kbdcomp -o dvorak.gkb dvorak # tar cf memdisk.tar dvorak.gkb
Now create a configuration file to be used in the GRUB core image. This is in the same format as your regular grub config, but need contain only a few lines to find and load the main configuration file on the /boot partition:
early-grub.cfg
set root=(memdisk) set prefix=($root)/ terminal_input at_keyboard keymap /dvorak.gkb cryptomount -u 1234abcdef1234abcdef1234abcdef set root='cryptouuid/1234abcdef1234abcdef1234abcdef' set prefix=($root)/grub configfile grub.cfg
Finally, generate the core image, listing all of the modules determined to be required in the generated grub.cfg, along with any modules used in the script. The example above needs memdisk, , , and .
# grub-mkimage -c early-grub.cfg -o grubx64.efi -O x86_64-efi -m memdisk.tar diskfilter cryptodisk luks gcry_rijndael gcry_sha256 ext2 memdisk tar at_keyboard keylayouts configfile
The generated EFI core image can now be used in the same way as the image that is generated automatically by grub-install: place it in your EFI partition and enable it with , or configure as appropriate for your system firmware.
See also Debian cryptsetup docs.
UEFI further reading
Below is other relevant information regarding installing Arch via UEFI.
Alternative install method
Usually, GRUB keeps all files, including configuration files, in /boot, regardless of where the EFI System Partition is mounted.
If you want to keep these files inside the EFI System Partition itself, add --boot-directory=esp to the grub-install command:
# grub-install --target=x86_64-efi --efi-directory=esp --bootloader-id=grub --boot-directory=esp --debug
This puts all GRUB files in , instead of in . When using this method, make sure you have grub-mkconfig put the configuration file in the same place:
# grub-mkconfig -o esp/grub/grub.cfg
Configuration is otherwise the same.
UEFI firmware workaround
GRUB standalone
This section assumes you are creating a standalone GRUB for x86_64 systems (x86_64-efi). For 32-bit (IA32) EFI systems, replace with where appropriate.
It is possible to create a grubx64_standalone.efi application which has all the modules embedded in a tar archive within the UEFI application, thus removing the need to have a separate directory populated with all of the GRUB UEFI modules and other related files. This is done using the command (included in ) as follows:
# echo 'configfile ${cmdpath}/grub.cfg' > /tmp/grub.cfg
# grub-mkstandalone -d /usr/lib/grub/x86_64-efi/ -O x86_64-efi --modules="part_gpt part_msdos" --locales="en@quot" --themes="" -o "esp/EFI/grub/grubx64_standalone.efi" "boot/grub/grub.cfg=/tmp/grub.cfg" -v
Then copy the GRUB configuration file to and create a UEFI Boot Manager entry for using efibootmgr.
grub.cfg file is not loaded due to ${cmdpath} missing a slash (i.e. (hd1,msdos2)EFI/Boot instead of (hd1,msdos2)/EFI/Boot) and so you are dropped into a GRUB shell. If this happens determine what ${cmdpath} is set to (echo ${cmdpath} ) and then load the configuration file manually (e.g. configfile (hd1,msdos2)/EFI/Boot/grub.cfg).Technical information
The GRUB EFI file always expects its configuration file to be at . However in the standalone GRUB EFI file, the is located inside a tar archive and embedded inside the standalone GRUB EFI file itself (inside the GRUB environment, it is denoted by , without quotes). This tar archive contains all the files that would be stored normally at in case of a normal GRUB EFI install.
Due to this embedding of contents inside the standalone image itself, it does not rely on actual (external) for anything. Thus in case of standalone GRUB EFI file and the standalone GRUB EFI file reads expects the configuration file to be at .
Hence to make sure the standalone GRUB EFI file reads the external grub.cfg located in the same directory as the EFI file (inside the GRUB environment, it is denoted by ), we create a simple which instructs GRUB to use ${cmdpath}/grub.cfg as its configuration ( command in ). We then instruct grub-mkstandalone to copy this file to (which is actually ) using the option "boot/grub/grub.cfg=/tmp/grub.cfg".
This way, the standalone GRUB EFI file and actual grub.cfg can be stored in any directory inside the EFI System Partition (as long as they are in the same directory), thus making them portable.
Speeding up LUKS decryption in GRUB
Upon boot GRUB may in some cases take a long time to verify the password. This can be due to a high PBKDF iteration count, which you can check as follows:
# cryptsetup luksDump /dev/sda3
The problem is that the iteration count for a given keyslot is generated when the key is added to ensure a balance between being high enough to protect against brute force attacks and low enough to allow for fast key derivation by estimating the capabilities of your computer. However, when GRUB is started, it might not have the same computational resources at hand, thus being vastly slower.
If your password provides enough entropy to counter common attacks by itself, you can lower this number:
# cryptsetup luksChangeKey --pbkdf-force-iterations 1000 /dev/sda3
A minimum of 1000 iterations is recommended as per RFC 2898, but you should aim for higher values if you can (The cost for an attacker as well as the time for key derivation scale linearly).