Linux CLI HowTo 0 🐧 Linux Boot Process

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:

1. Open a terminal (press Ctrl+Alt+T)
2. 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:

1. Look for the /boot/efi folder:

ls -la /boot/efi/

2. If it exists, that’s your EFI System Partition.

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📝 How to See What Operating Systems Are Installed

Why: To see all your boot options and understand what’s on your computer.

Steps:

1. At startup, when GRUB menu appears:

• Don’t press anything yet
• You’ll see a list like “Ubuntu”, “Windows”, etc.

1. 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:

1. Edit GRUB settings:

sudo nano /etc/default/grub

2. Find line GRUB_DEFAULT= and change the number or name (e.g., GRUB_DEFAULT="Ubuntu").
3. Update GRUB:

sudo update-grub

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📦 How to Check What Kernel Version You’re Running

Why: Shows which Linux version you have installed.

Steps:

1. 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:

1. List contents:

sudo lsinitrd | head -20

2. Or view full content:

sudo lsinitrd | less

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🔍 How to See What Services Are Starting at Boot

Why: Understand what programs run when your system starts.

Steps:

1. 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:

1. Check fstab file:

cat /etc/fstab

Look for lines with /dev/sda1 or similar device names.

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❗ How to Boot from a Different Kernel if Current One Fails

Why: If your current kernel breaks, you can try an older one.

Steps:

1. At GRUB menu, press e (edit)
2. Find the line starting with linux and add recovery at the end:

linux ... recovery

3. Press Ctrl+X to boot

🔄 How to Fix Bootloader Issues

Why: If you can’t boot into your OS anymore.

Steps:

1. Boot from a live USB
2. Mount your root partition:

sudo mount /dev/sda1 /mnt

3. Reinstall GRUB:

sudo grub-install --root-directory=/mnt /dev/sda

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📁 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

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🧪 How to Make a Boot Menu Change

1. Open terminal
2. Edit GRUB config:

sudo nano /etc/default/grub

3. Change GRUB_TIMEOUT=5 to GRUB_TIMEOUT=10
4. Save and exit (Ctrl+O, then Ctrl+X)
5. Update GRUB:

sudo update-grub

Now when you restart, the boot menu will wait 10 seconds instead of 5

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