What are the stages of Linux OS booting and which bootloaders are used?

1. BIOS/UEFI Initiator: Hardware initialization, selection of the boot device.
2. Bootloader: Loading the operating system kernel. Main bootloaders:
   • GRUB (Grand Unified Bootloader): The most common, supports multiboot, kernel parameter configuration.
   • LILO (Linux Loader): Older than GRUB, less flexible.
3. Kernel Initialization: Loading the kernel into RAM, unpacking, searching and mounting the root filesystem.
4. initramfs/initrd: Temporary root filesystem in RAM, contains minimal drivers and utilities for mounting the real root filesystem.
5. Root Filesystem Mount: Mounting the main root filesystem.
6. Init Process (systemd/SysVinit): Launching the initialization process, which manages service startup, daemons, and system preparation.

Most often, GRUB2 is used.

# Example GRUB2 configuration
# /etc/default/grub

GRUB_DEFAULT=0
GRUB_TIMEOUT=5
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
GRUB_CMDLINE_LINUX=""