Understanding Linux Boot Process: From BIOS to systemd

System Directory Structure

The /boot directory stores bootloader files, the kernel image ( vmlinuz-*.x86_64) and the initramfs image ( initramfs-*.img) that provides early drivers.

[root@web01 boot]# ls
config-4.18.0-305.3.1.el8.x86_64                         initramfs-4.18.0-305.3.1.el8.x86_64kdump.img
efi                                                      loader
grub2                                                    System.map-4.18.0-305.3.1.el8.x86_64
initramfs-0-rescue-4396f3518626409aa9770c4df8d21cb2.img  vmlinuz-0-rescue-4396f3518626409aa9770c4df8d21cb2
initramfs-4.18.0-305.3.1.el8.x86_64.img                  vmlinuz-4.18.0-305.3.1.el8.x86_64

initramfs-4.18.0-305.3.1.el8.x86_64.img

: early‑stage driver image. vmlinuz-4.18.0-305.3.1.el8.x86_64: the Linux kernel binary.

Kernel Version and CPU Information

Use uname -r to display the running kernel version and cat /proc/cpuinfo to list CPU flags and capabilities.

# uname -r
4.18.0-305.3.1.el8.x86_64
# cat /proc/cpuinfo
flags : fpu vme de pse tsc msr pae mce cx8 apic ... (many flags omitted for brevity)

Network Configuration and Kernel Parameters

Check and modify networking sysctl settings such as IP forwarding and ICMP echo ignore.

# cat /proc/sys/net/ipv4/ip_forward
1   # 1 = enabled, 0 = disabled
# cat /proc/sys/net/ipv4/icmp_echo_ignore_all
1   # 1 = ignore ping requests
# cat /etc/sysctl.conf
vm.dirty_expire_centisecs = 4500
net.ipv4.ip_forward = 0
net.ipv4.icmp_echo_ignore_all = 1
# sysctl -p   # apply changes immediately

Linux Boot Sequence

1. BIOS / UEFI Stage

Hardware detection : Power‑on self‑test (POST) validates hardware.

Select boot device : Firmware chooses the device (disk, USB, network) to load the next stage.

2. GRUB2 Bootloader

Load /boot partition and read configuration.

Example commands used by GRUB:

set root='hd0,msdos1'

search --no-floppy --fs-uuid --set=root ...

Load kernel : GRUB loads the kernel image and passes control to it.

3. Kernel Loading and Initialization

Kernel start : Initializes hardware, memory, and core drivers.

Mounts the initial RAM filesystem ( initramfs) which provides early drivers and tools.

Mounts the real root filesystem according to /etc/fstab.

4. systemd Startup

systemd

becomes the first userspace process (PID 1) after the kernel hands over control.

It mounts all declared filesystems, determines the default target unit (e.g., graphical.target or multi-user.target), and starts dependent services.

5. Target Units

multi-user.target

: Non‑graphical multi‑user mode (runlevel 3). graphical.target: Graphical mode, depends on multi-user.target. basic.target: Minimal environment, started before most other targets. reboot.target and halt.target: Used for rebooting or shutting down.

6. Service Startup

systemd launches services according to unit dependencies. Common services include: dbus.service – D‑Bus message bus. sshd.service – SSH daemon for remote login. firewalld.service – Firewall management. NetworkManager.service – Network configuration. gnome-shell.service or lightdm.service – Graphical login managers.

7. User Login

Login manager : If a graphical target is active, GDM, LightDM, etc., present a login screen.

User session : After authentication, the user's desktop session starts and applications launch.

8. System Ready

When all target units and services have started without errors, the system reaches a stable operational state and is ready for user interaction.

CentOS 8 Boot Process Example

GRUB Configuration File

GRUB_TIMEOUT=5
GRUB_DISTRIBUTOR="$(sed 's, release .*$,,g' /etc/system-release)"
GRUB_DEFAULT=saved
GRUB_DISABLE_SUBMENU=true
GRUB_TERMINAL_OUTPUT="console"
GRUB_CMDLINE_LINUX="crashkernel=auto resume=/dev/mapper/cl-swap rd.lvm.lv=cl/root rd.lvm.lv=cl/swap rhgb quiet"
GRUB_DISABLE_RECOVERY="true"
GRUB_ENABLE_BLSCFG=true

Regenerate the GRUB configuration with grub2-mkconfig -o /boot/grub2/grub.cfg and install the bootloader to the disk using grub2-install /dev/sda.

Loading Kernel and Modules

# lsmod          # list currently loaded kernel modules
# modinfo kvm    # display detailed information about the kvm module