Case Study: Linux Boot Process — From BIOS to systemd
It is a case study that explains the Linux boot process from BIOS to sytemd.
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Objectives
Understand each step of the Linux Boot process
Understand the role of GRUB, initramfs, and systemd in the boot process
Getting some experience in solving boot-related issues in Linux
System Environment
OS: Ubuntu 22.04
Boot Mode: UEFI
Init System: systemd
Filesystem: ext4
Kernel: 5.x
UEFI/BIOS
It has some specific functions:
It initiates three hardware components, namely RAM, CPU, and disks.
It also locates the bootloader on the disk.
Observation:
On UEFI systems, the bootloader is loaded from
/boot/efi/EFI/ubuntu/grubx64.efi
Bootloader == GRUB
It serves the following purposes:
Provides a boot menu to select an OS.
It loads the Linux kernel and initramfs. Then, passes kernel parameters.
Important Files are:
/boot/grub/grub.cfg
/etc/default/grub
Linux kernel initialization
The kernel has some steps to fulfil, like:
Hardware detection
It mounts the temporary root file system
Starts the init process
Command to check the kernel version:
uname -r
Command to see kernel logs:
dmesg | less
initramfs
What is initramfs?
It is a compressed temporary filesystem.
It is loaded into RAM.
It consists of drivers and scripts for detecting the disks and mounting the real root file system.
The location of this file is :
/boot/initrd.img-*
- Now, the command to inspect the content of the initramfs file is:
ls initramfs /boot/initrd.img-$(uname -r) | less
- Its significance lies in the fact that if the file is missing or corrupted, then the boot process will fail. Secondly, the "root filesystem not found" error is related to this file.
systemd
systemd plays an important role as it starts all system services and manages targets and dependencies.
Command to see systemd status:
systemctl status
- Command to see boot targets:
systemclt list-units --type=target
Boot performance analysis
- Command to analyze boot time:
systemd-analyze
- Command to see a detailed breakdown
systemd-analyze blame
- Command to see a visual graph of boot performance
systemd-analyze plot > boot.svg
Troubleshooting Case Scenarios
Case: Boot hangs at GRUB
Reason: Corrupted GRUB file
Solution:
sudo update-grub
Recovery Mode
- Methods to access recovery mode:
-> Hold Shift (BIOS)
-> Press Esc (UEFI)
- Reasons to access recovery mode:
-> Filesystem repair
-> Network Repair
-> Root shell access
Conclusion
By understanding the boot process in Linux systems, we can have:
-> faster troubleshooting
-> better system optimization
-> safe kernel and system updates
In technical terms:
initramfs is critical for disk and driver support.
GRUB misconfiguration is a common reason for boot failure.
systemd-analyze is significant for performance tuning.
So, in this case study, we can see the steps involved in the Linux boot process and conclude that failure of any step can result in boot failure.
