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Boot Modes & Recovery

New Boot Process (CentOS/RHEL 7 and above):

  • systemd manages boot sequence
  • backward compatible with SysV init scripts from previous versions

 

BIOS - Basic Input/Output System (firmware interface)

POST - Power-On-Self-Test started

  • MBR - Master Boot Record
    • information saved in first sector of hard disk that indicates where the GRUB2 is located so it can be loaded in RAM
      • GRUB2 - Grant Unified Boot Loader v2 - loads linux kernel

/boot/grub2/grub.cfg

  • Kernel - Core of the operating system - loads required drivers from initrd.img starts first OS process

systemd

  • Systemd = System Daemon (PID #1) then starts all required processes

read /etc/systemd/system/default.target to bring system to run level (7 run levels 0-6)

 

How to Reboot / Shutdown

systemctl poweroff

stops all services, unmounts file systems, powers off system

poweroff

systemctl reboot

all of the above plus reboot

reboot

 

Select systemd target

  • 0-6 run levels now referred to as targets
  • important targets:
    • graphical.target - system supports multiple users, graphical and text based logins
    • multi-user.target - system supports multiple users, text based logins only
    • rescue.target - sulogin prompt, basic system initialisation completed
    • emergency.target - sulogin prompt, initramfs pivot complete and system root mounted on / read only
  • a target can be part of another target (graphical.target includes mult-user.target)

 

systemctl get-default

check current target

who -r

run level

systemctl list-dependencies graphical.target | grep target

list target dependencies

ls -l /lib/systemd/system/runlevel*

list run levels

systemctl set-default graphical.target

set default target

 

Recover Root Password (RHEL 9)

Reboot

 

Press esc to get to grub

 

Find line beginning with Linux, add " rd.break" to the end

RHEL 9 boot into emergency recovery mode

Find line beginning with Linux, add "init=/bin/bash" to the end

RHEL 10 boot into emergency recovery mode

Press Ctrl X

start boot using the modified settings

mount -o remount,rw /sysroot

RHEL 9 remount the root filesystem as read-write to allow changes

mount -o remount,rw /

RHEL 10 remount the root filesystem as read-write to allow changes

chroot /sysroot

change root to /sysroot (RHEL OS)

passwd

 

Enter your new password

 

touch /.autorelabel

Force SELinux to relabel files on next boot

exit

RHEL 9 exit back to emergency shell

mount -o remount,ro /

RHEL 10 reboot cleanly

exit

exit to boot

 

Repair Filesystem Corruption

Common errors

Corrupt file system

systemd attempts to repair file system
if it cannot be repaired, go to emergency shell

nonexistent device/UUID in /etc/fstab

if device remains unavailable, go to emergency shell

nonexistent mount point in /etc/fstab

go to emergency shell

incorrect mount option in /etc/fstab

go to emergency shell

 

Emergency target can diagnose and fix the issue, because no file systems are mounted before emergency shell is displayed

systemctl daemon-reload

when using emergency shell, remember to run this after editing /etc/fstab
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