Master Linux Boot Process: From BIOS to Init with Practical Commands

1. nohup command

Run commands without hanging up; output is appended to nohup.out in the current directory or the home directory if unwritable. Use nohup COMMAND & to run in background and manage with jobs.

2. Setting and adjusting nice value

Change process priority with nice and renice. Example: # nice -n -5 ping 127.0.0.1 Result shows increased priority. Ordinary users can only increase the nice value (lower priority).

3. rc#.d run‑level scripts

Services prefixed with K (kill) or S (start) are executed in numeric order; lower numbers run first.

4. ramdisk (initramfs) handling

Use mkinitrd initramfs-`uname -r`.img `uname -r` to generate an initramfs image. The image can be inspected with file and cpio -vt < initramfs‑…img to view its contents.

5. Viewing runlevel

# who -r

Displays the current runlevel (e.g., 3). runlevel shows the previous and current levels.

6. Inspecting initramfs files

# file initramfs-2.6.32-642.el6.x86_64.img

Shows compression type and confirms it is a CPIO archive.

7. /etc/inittab and ctrl‑alt‑del

In CentOS 5, /etc/inittab defines the Ctrl‑Alt‑Del reboot; in CentOS 6 this is moved to /etc/init/control‑alt‑delete.conf. The mingetty respawn setting is now in /etc/init/serial.conf.

System boot process details

Step 1: Power‑On Self‑Test (POST) and BIOS – BIOS checks CPU, memory, disks, and loads configuration from CMOS.

Step 2: Read MBR – The first 512‑byte sector contains the boot loader and partition table.

Step 3: Boot Loader – Loads kernel and initramfs; legacy GRUB uses stage1, stage1.5, and stage2.

Step 4: Load Kernel – Kernel image is decompressed; initramfs provides necessary drivers for the real root filesystem.

Step 5: Init process – /sbin/init reads /etc/inittab to determine the default runlevel.

Step 6: System initialization – Scripts in /etc/rc.d/rc.sysinit set hostname, load udev, SELinux, mount filesystems, configure swap, set kernel parameters, and start LVM/RAID.

Step 7: Load kernel modules – Modules listed in /etc/modules.conf or /etc/modules.d are loaded.

Step 8: Execute run‑level scripts – Scripts in /etc/rc?.d run according to the current runlevel.

Step 9: rc.local – Final user‑custom script executed after all other init steps.

Step 10: Login – mingetty spawns /bin/login for user authentication.

2. chkconfig command (graphical interface)

Use ntsysv for a GUI view. Command‑line examples:

# chkconfig --list name

# chkconfig --add name

# chkconfig --del name

# chkconfig --level 2345 name on|off|reset

3. GRUB command‑line interface

If grub.conf is corrupted, the system drops to the GRUB prompt. Commands such as kernel and initrd can be entered manually; help lists available commands.

4. grub.conf configuration options

default=#

– default menu entry (starting from 0) timeout=# – wait time before auto‑boot splashimage=(hd#,#)/path/to/image – background image hiddenmenu – hide menu unless a key is pressed password [--md5] STRING – protect menu editing title, root, kernel, initrd – define boot entries

5. Installing or repairing GRUB

Two methods:

Use grub-install --root-directory=/PATH /dev/sdX from a rescue shell.

Enter GRUB, set root (hd#,#) and run setup (hd#).

# grub-install --root-directory=/ /dev/sda

3. Post‑lab exercises

Exercise 1: Add a system service

Create a script in /etc/init.d/ with header lines:

#!/bin/bash
# chkconfig: 2345 10 90
# description: A

Then run chkconfig --add myservice.sh and manage it with service myservice.sh start|stop|status.

Exercise 2: Build a custom Linux system

Steps include creating a separate disk image, partitioning it for /boot and root, mounting, copying kernel and initramfs, installing GRUB to the new disk, creating /etc/fstab, and using a script ( copycmd0.sh) to copy required binaries and libraries into the new root filesystem.

#!/bin/bash
ch_root="/mnt/sysroot"
[ ! -d $ch_root ] && mkdir $ch_root
# functions bincopy and libcopy defined …
read -p "Please input a command or quit: " command
while [ "$command" != "quit" ]; do
  if bincopy $command; then
    libcopy $command
  fi
  read -p "Please input a command or quit: " command
done

After copying, chroot sysroot/ can be used to test the new system.

Images illustrating key steps