Linux - Linux Boot Process

linux boot process

A Linux system goes through several stages when booting.

the firmware stage

• the computer runs code in BIOS or UEFI during power on self test or post.
• Older computers have a BIOS and newer computers have UEFI.

the boot loader stage

• After the firmware stage, the BIOS or UEFI executes the boot loader stage.
• the boot loader is grub2, or grand unified boot loader two.
• Grub’s job is to read configuration file and boot the Linux kernel
  • For BIOS machine, grub reads in /boot/grub2/grub.cfg.
  • For UEFI systems, it loads /boot/efi/EFI/redhat/grub.efi.
• The boot loader then executes the kernel.

the kernel stage

• the kernel loads a ramdisk into memory.
• This ramdisk serves as a temporary root file system, includes kernel modules, drivers, and possibly even kickstart files.
• Later, the kernel unmounts the ramdisk and mounts the root file system on the hard drive.
• And then, starts the initialization stage by executing the first process.

the initialization stage

• In the initialization stage, the grandfather process runs.
• In older versions of Red Hat this was the Init process. Init was replaced by Upstart, which has now been replaced by systemd.
• Systemd then starts all system services, a login shell or a graphical interface.
• When it’s finished, the OS is ready to be used.
• Systemd has the concept of Targets that similar to the old Init run levels. You can think of a target as a system configuration.
  • default, boots up into the graphical.target.
• A system can be booted into different Targets for different purposes, such as rescuing the system after a crash.