Linux CLI HowTo 0 🐧 Linux Boot Process
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0 - HowTos Linux Boot Process
✅ How to Check if Your Computer Uses BIOS or UEFI
Why: Knowing this helps you understand how your computer boots and what files are needed.
Steps:
Open a terminal (press Ctrl+Alt+T)
Run:
ls -la /sys/firmware/efi/
If you see output, you have UEFI. If not, you likely have BIOS.
Alternative:
sudo dmesg | grep -i efi
📂 How to Find Your Boot Partition (UEFI Only)
Why: This is where the boot files live in modern computers.
Steps:
Look for the /boot/efi folder:
ls -la /boot/efi/
If it exists, that's your EFI System Partition.
📝 How to See What Operating Systems Are Installed
Why: To see all your boot options and understand what's on your computer.
Steps:
At startup, when GRUB menu appears:
Don't press anything yet
You'll see a list like "Ubuntu", "Windows", etc.
If you want to see it in terminal:
cat /boot/grub/grub.cfg | grep -i menuentry
🛠️ How to Change Default Boot Option
Why: To make your preferred OS boot automatically.
Steps:
Edit GRUB settings:
sudo nano /etc/default/grub
Find line GRUB_DEFAULT= and change the number or name (e.g., GRUB_DEFAULT="Ubuntu").
Update GRUB:
sudo update-grub
📦 How to Check What Kernel Version You're Running
Why: Shows which Linux version you have installed.
Steps:
In terminal:
uname -r
Example output might be: 5.4.0-72-generic
🔧 How to See What Files Are in Your Initramfs
Why: To understand what drivers and tools are loaded early in the boot.
Steps:
List contents:
sudo lsinitrd | head -20
Or view full content:
sudo lsinitrd | less
🔍 How to See What Services Are Starting at Boot
Why: Understand what programs run when your system starts.
Steps:
Check services that start at boot:
systemctl list-unit-files --state=enabled | head -20
📋 How to Find Your Root Filesystem Location
Why: This tells where the main Linux folder lives on your hard drive.
Steps:
Check fstab file:
cat /etc/fstab
Look for lines with /dev/sda1 or similar device names.
❗ How to Boot from a Different Kernel if Current One Fails
Why: If your current kernel breaks, you can try an older one.
Steps:
At GRUB menu, press e (edit)
Find the line starting with linux and add recovery at the end:
linux ... recovery
Press Ctrl+X to boot
🔄 How to Fix Bootloader Issues
Why: If you can't boot into your OS anymore.
Steps:
Boot from a live USB
Mount your root partition:
sudo mount /dev/sda1 /mnt
Reinstall GRUB:
sudo grub-install --root-directory=/mnt /dev/sda
📁 Boot - Key Files to Remember
File/Directory Purpose
/boot/grub/grub.cfg Where GRUB finds all OS options
/boot/vmlinuz The Linux kernel image
/boot/initrd.img or /boot/initramfs.img Temporary root filesystem
/etc/fstab Tells where the real hard drive folders are
/boot/efi EFI partition needed for UEFI systems
🧪 How to Make a Boot Menu Change
Open terminal
Edit GRUB config:
sudo nano /etc/default/grub
Change GRUB_TIMEOUT=5 to GRUB_TIMEOUT=10
Save and exit (Ctrl+O, then Ctrl+X)
Update GRUB:
sudo update-grub
Now when you restart, the boot menu will wait 10 seconds instead of 5
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