Linux Boot Process → Older Version

• BIOS → Basic Input / Output System

  • executes MBR

• MBR → Master Boot Record

  • executes GRUB

• GRUB → Grand Unified Bootloader

  • executes kernel

• KERNEL

  • kernel executes /sbin/init
  • mount the root file system

• INIT

  • init executes run level programs
  • initial RAM disk → contains drivers

• RUN LEVEL

  • run level programs executed from /etc/rc.d/rc*.d/

Linux Boot Process → CentOS 7, 8, 9

• BIOS → MBR → GRUB 2

• /boot/grub2/grub.cfg

• KERNEL

  • load drivers from initrd.img
  • start the first OS process (systemd)

• SYSTEMD = system daemon

  • read /etc/systemd/system/default.target

systemd-analyze → Optimizing Boot Performance

• Understand how long the Linux system takes to boot by:

  • kernel
  • service
  • initrd

• initrd → initialize system hardware

• systemd-analyze blame

  • sort by time all service

Message of the Day → Message at Login

• nano /etc/motd

• Customize:

  • touch /etc/profile.d/motd.sh

• In /etc/ssh/sshd_config

  • PrintMotd no

• systemctl restart sshd.service