Startup operation:
BIOS: BIOS stands for Basic Input/Output System. Performs some system integrity checks Searches, loads, and executes the boot loader program. It looks for boot loader in floppy, cd-rom, or hard drive. You can press a key (typically F12 of F2, but it depends on your system) during the BIOS startup to change the boot sequence. Once the boot loader program is detected and loaded into the memory, BIOS gives the control to it. So, in simple terms BIOS loads and executes the MBR boot loader.
MBR stands for Master Boot Record.: It is located in the 1st sector of the bootable disk. Typically /dev/hda, or /dev/sda. MBR is less than 512 bytes in size. This has three components 1) primary boot loader info in 1st 446 bytes 2) partition table info in next 64 bytes 3) mbr validation check in last 2 bytes. It contains information about GRUB (or LILO in old systems). So, in simple terms MBR loads and executes the GRUB boot loader.
GRUB stands for Grand Unified Bootloader: If you have multiple kernel images installed on your system, you can choose which one to be executed. GRUB displays a splash screen, waits for few seconds, if you don’t enter anything, it loads the default kernel image as specified in the grub configuration file. GRUB has the knowledge of the filesystem.Grub configuration file is /boot/grub/grub.conf. GRUB just loads and executes Kernel and initrd images.
Kernel: Mounts the root file system as specified in the “root=” in grub.conf. Kernel executes the /sbin/init program. Since init was the 1st program to be executed by Linux Kernel, it has the process id (PID) of 1. Do a ‘ps -ef | grep init’ and check the pid. initrd stands for Initial RAM Disk. initrd is used by kernel as temporary root file system until kernel is booted and the real root file system is mounted. It also contains necessary drivers compiled inside, which helps it to access the hard drive partitions, and other hardware.
Init: Following are the available run levels
0 – halt
1 – Single user mode
2 – Multiuser, without NFS
3 – Full multiuser mode
4 – unused
5 – X11
6 – reboot
Init identifies the default initlevel from /etc/inittab and uses that to load all appropriate program. Execute ‘grep initdefault /etc/inittab’ on your system to identify the default run level. If you want to get into trouble, you can set the default run level to 0 or 6. Since you know what 0 and 6 means, probably you might not do that. Typically you would set the default run level to either 3 or 5.
Runlevel programs: When the Linux system is booting up, you might see various services getting started.
Shutdown operation:
The shutdown command brings down system in a secure way. All the logged-in users are notified about the system shutdown.
Signal SIGTERM notifies all the processes that the system is going down, so that processes can be saved and exit properly.
Command shutdown signals the init process to change the runlevel.
Runlevel 0 halts the system
Runlevel 6 reboots the system
Runlevel 1 is default state.
You can shutdown a system by passing a definite time (in minutes).
System will automatically shutdown after specified minute giving a message and time to save all work
Syntax: shutdown <time>