GRUB can not only load Linux but other Operating Systems as well.
Because GRUB understands filesystems and kernel image formats, it can load a kernel by just specifying its filename, drive and partition (as opposed to specifying the physical address).
Except an odd specialised exception, GRUB has only code to read from filesystems in order to to assure users that GRUB cannot be responsible for filesystem corruption.
GRUB Legacy vs GRUB 2
There are two versions of GRUB:
- GRUB Legacy (also referred to as GRUB 1)
- GRUB 2
Both of these versions can be referred to as GRUB, making it impossible to distinguish between them by name only.
Development on GRUB Legacy has halted. GRUB 2 should be used.
Naming conventions
A device is recognized by a name in paranthes, for example (hd1).
The name of first hard disk is hd0, the name of the the second is hd1 etc.
Partitions are separated from the drive name by a comma, for example (hd1,msdos3) or (hd0,gpt4).
A file is specfied like so: (hd0,gpt4)/boot/vmlinuz.
Grub does not distinguish SATA and IDE when counting the drive numbers.
Two boot methods
GRUB boots either
- the OS directly, or
- it uses chain-load which loads another boot loader.
Chain loading is only supported on PC BIOS and EFI platforms.
Autogenerating config files for multi-boot environments depends on os-prober and has several shortcomings.
TODO
Partition numbers start with 1.
save_env and load_env (in grub.cfg?) combined with the grub-editenv command make it possible to store a small amount of data accross reboots.
Stanza
/usr/local/lib/grub/i386-pc/modinfo.sh
EFI systems
On EFI systems with «fixed disk install», the EFI system partition must be mounted to /efi before executing grub-install.
If the EFI system partition is mounted to a different directory (for example /mnt/efi), it needs to be specfied:
grub-install --efi-directory=/mnt/efi
Booting an OS
Directly
An OS can be booted directly with the boot command.
Chain loading an OS
An OS such as Windows must be chain loaded.
menuentry "Windows" {
insmod chain
insmod ntfs
set root=(hd0,1)
chainloader +1
}
Reserving a partition for GRUB (BIOS boot partition)
It's possible to reserve a partition (the BIOS boot partition) for GRUB.
Putting GRUB on this partition reduces the risk for it to be overwritten.
This partition requires the correct partition type (bios_grub):
parted /dev/… set partition-number bios_grub on
GRUB shell
The GRUB shell is caused to start by pressing c when GRUB is started.
I also had it started with executing grub-install but without grub-mkconfig.
Minimal BAHS-like editing is supported. For the first word, TAB lists possible command completions. Anywhere else TAB lists possible device or file completions.
grub> ls
(proc) (hd0) (hd0,gpt6) (hd0,gpt5) (hd0,gpt4) (hd0,gpt3) (hd0,gpt2) (hd0,gpt1)
grub> ls (hd0,gpt1)
Partition hd0,gpt1: Filesystem type fat, UUID BC6a-B14F - Partition start at 1024KiB - Total Size 102400KiB
grub> ls (hd0,gpt1)/
efi/ System Volumen Information
grub> ls -l (hd0,gpt1)
DIR 20230810202914 efi/
DIR 20230515092630 System Volume Information/
grub> ls -l (hd0,gpt6)/boot
12046688 20230810202558 vmlinuz
…
grub> set root=(hd0,gpt6)
grub> linux /boot/vmlinuz
grub> boot
error: no suitable video mode found.
Booting in blind mode
In order to get around the no suitable video mode found. error, execute the same sequence of command, but add insmod all_video before calling boot.
Source files, development etc.
GRUB uses Autoconf and Automake. Input for Automake is mostly generated by a Python script.
Some interesting directories and files include:
util/ | Sources for command line utilities such as grub-install etc. ? | |
grub-core/ | | Contains code that is run at boot time (as opposed to code being run within an operating system located in the top level directory) |
grub-core/boot/ | Low level code | For example the MBR implementation for PC BIOS systems. |
grub-core/kern/ | The grub kernel | Functionality handling devices, disks, files, environment variables etc. |
grub-core/term/ | Terminal functionality | |
grub-core/disk/ | Disk access | |
grub-core/partmap/ | Reading partition tables | |
grub-core/fs/ | Access filesystems | |
grub-core/bus/ | PCI and USB handling | |
grub-core/video/ | Video handling | |
grub-core/gfxmenu/ | Graphical menu | |
grub-core/commands/ | Most commands | There are exceptions and some commands are implemented elsewhere. |
grub-core/osdep/ | | |
include/grub/dl.h | | Defines the macros GRUB_MOD_INIT and GRUB_MOD_FINI. |
Often, the _start symbol is found in assembly files named startup.S:
$ grep -rlP '\b_start\b'