Booting the System in Linux

Here’s a detailed overview of Booting the System in Linux, covering all the topics you’ve listed:

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🔧 1. BIOS Boot Sequence

• BIOS (Basic Input/Output System):

  • Initializes hardware (CPU, RAM, keyboard, disk, etc.)
  • Looks for a bootable device (like HDD, SSD, USB)
  • Loads the first sector (MBR or GPT) into memory
  • Passes control to the bootloader (GRUB/LILO)

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🚀 2. Linux Boot Events Overview

• Boot Process Stages:

  1. BIOS/UEFI initializes hardware
  2. Bootloader (e.g., GRUB) loads the kernel
  3. Kernel initializes hardware and mounts root filesystem
  4. init/systemd runs as PID 1 and starts user-space services

• Important Utilities:

  • dmesg – Displays kernel ring buffer messages (hardware detection, drivers, errors)

    dmesg | less
    

  • journalctl -k – Shows kernel messages (systemd)
  • lsinitrd or lsinitramfs – Lists content of initramfs/initrd (temporary root before real rootfs)
  • cat /proc/cmdline – View current kernel boot options

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💻 3. Common Commands to the Boot Loader (GRUB)

At the GRUB menu:

• Press e to edit boot entry

• You can modify:

  • Kernel path
  • Initrd path
  • Kernel parameters

Common GRUB commands:

• set root=(hd0,1) – Sets the root partition
• linux /vmlinuz root=/dev/sda1 ro – Loads kernel
• initrd /initrd.img – Loads initramfs
• boot – Boots with the current setup

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⚙️ 4. Kernel Options at Boot Time

Kernel boot parameters affect system behavior:

• Examples:

  • single or 1 – Boots into single-user mode
  • init=/bin/bash – Skips init system and gives a root shell
  • nomodeset – Disables kernel mode setting (graphics)
  • quiet – Reduces boot messages
  • root=/dev/sda1 – Specifies root filesystem

To apply:

1. At GRUB menu, press e
2. Append options to the linux line
3. Press Ctrl + X or F10 to boot

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🔄 5. Init Systems: SysVinit, SystemD, Upstart

Feature	SysVinit	Upstart	SystemD (modern default)
PID 1 process	/sbin/init	/sbin/init	/lib/systemd/systemd
Init files	/etc/init.d/*	/etc/init/*.conf	Unit files in /etc/systemd/system/
Parallel	No	Yes	Yes
Active distro	Legacy (Debian, Slackware)	Older Ubuntu (9.10–14.10)	Most modern distros (Ubuntu, Arch, RHEL)

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🔁 6. Changing Runlevels / Targets / Single User Mode

SysVinit (legacy)

• Runlevels:

  • 0: Halt
  • 1: Single user
  • 3: Multi-user
  • 5: Multi-user + GUI
  • 6: Reboot

• Commands:

  init 1     # Switch to single-user mode
  runlevel   # Shows previous and current runlevel
  telinit 3  # Switch to runlevel 3
  

SystemD (modern)

• Targets:

  • rescue.target – Single-user mode
  • multi-user.target – Non-GUI
  • graphical.target – GUI

• Commands:

  systemctl isolate rescue.target     # Switch to single-user
  systemctl get-default               # Show default target
  systemctl set-default graphical.target
  

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🔚 7. Reboot & Shutdown (with User Alerts)

Commands:

shutdown -r now           # Reboot immediately
shutdown -h +10 "System will shut down in 10 minutes."  # Graceful halt with warning
halt                      # Immediate halt
poweroff                  # Turn off the system
reboot                    # Reboot the system

Alerting Logged-in Users:

• wall sends messages to all terminals:

  wall "System will reboot in 5 minutes. Please save your work."
  

• shutdown does this automatically when scheduled:

  shutdown -r +5 "System rebooting for maintenance."
  

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