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Linux Boot Loaders

2021-11-13 JGTechLinux

A Linux boot loader’s core functionality includes the ability to do the follow:

  • Select from multiple kernels
  • Switch between sets of kernel parameters
  • Allow the user to manually override and edit kernel image names and parameters
  • Provide support for booting other operating systems

Overview

  • GRUB

A near-universal standard on Linux systems, with BIOS/MBR and UEFI versions

  • LILO

One of the first Linux boot loaders. ELILO is a UEFI version.

  • SYSLINUX

Can be configured to run from many different kinds of filesystems.

  • LOADLIN

Boots a kernel from MS-DOS.

  • systemd-boot

A simple UEFI boot manager.

  • coreboot

A high-performance replacement for the PC BIOS that can include a kernel.

  • Linux Kernel EFISTUB

A kernel plug-in for loading hte kernel directly from EFI/UEFI System Partition (ESP).

  • efilinux

A UEFI boot loader intended to serve as a model and reference for other UEFI boot loaders.

How GRUB Works

  1. The PC BIOS or firmware initializes the hardware and searches its boot-order storage devices for boot code
  2. Upon finding the boot code, the BIOS/firmware loads and executes it. GRUB begins.
  3. The GRUB core loads.
  4. The core initializes. At this point, GRUB can now access disks and filesystems.
  5. GRUB identifies its boot partition and loads a configuration there.
  6. GRUB gives the user a chance to change the configuration.
  7. After a timeout or user action, GRUB executes the sequence of commands in the grub.cfg file.
  8. In the course of executing the configuration, GRUB may load additional modules in the boot partitions. Some of these modules may be preloaded.
  9. GRUB executes a boot command to load and execute the kernel as specified by the configuration’s linux command.

Where is the GRUB core?

  • Partially stuffed between MBR and the beginning of the first partition
  • In a regular partition
  • In a special boot partition: a GPT boot partition, ESP, or elsewhere
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