Boot and runlevels

Course's Plan:

• Boot sequence. The x86 family bootstrapping. 
• Runlevel implementation. Init and rc. 

Boot sequence

Description

Linux bootup steps:

1. BIOS : Tests and MBR chain-up
2. Bootloader: Kernel selection
3. Kernel startup
4. /sbin/init: Userland loading

BIOS (Basic Input Output System)

• POST (Power On Self Test): 
• Detect hardware 
• Accessing bootable devices 
•  Read MBR (Master Boot Record) 
•  512 first bytes 
• Hosts the partition table 
• Hosts the starting procedure (to launch the boot manager) 
• Start Boot manager (GRUB/LILO)   

System Boot

From BIOS to Kernel:

And the kernel makes its way to init

Kernel

/boot/vmlinuz

• Uncompress it self.
• organizes and takes control of the continued booting of system
• Checks and sets the console, reads BIOS settings
• initializes basic hardware interfaces.
• Drivers (included into the kernel) probe existing hardware and initialize it accordingly.

The kernel controls the entire system hardware.

About the initrd

Early userland  and pivot_root.

• initial ram disk
• Filesystem image
• CPIO archive
• Compressed
• Early userland
• linuxrc script
• “fake” init
• Load drivers
• Pivot_root to real root fs.
• Distros provide creation tools

/sbin/init

Userland entry point.

The parent of all other processus (PID number 1). Allocate VT’s and run rc to switch to default runlevel.

relies on the /etc/inittab file → Read the default runlevel

              ↓

/etc/init.d/boot script → Main services

              ↓

starts the /etc/init.d/rc  script → Runlevel specific services

Runlevels implementation

About runlevels

System “profiles”.

• Identifiers for system state
• Numeric (most distros)
• 0 -> 6
• Used by init and rc
• Start/Stop services
• Enable/Disable multiuser
• /etc/inittab
• /etc/rcX.d/

Runlevels

The seven runlevels:

0    : System Halted.

1    : Singler User Mode.

2-5 : Custom levels used by distributions. Run services.

6    : System reboot.

/etc/inittab 

Init configuration file syntax:

id : runlevels : action : command

Some actions:

Action	Definitions
once	Run the command and continue to next entry
wait	Run the command and wait for completion before continuing to the next entry.
respawn	Run the command, continue to the next entry and restart the process if it dies.

Working with runlevels

Common Commands

[root@linux ~]# runlevel

N 3

[root@linux ~]# who -r

    run-level 3 2011-09-21 08:35 last=S

[root@linux ~]# init 2

[root@linux ~]# telinit 1

[root@linux ~]# halt

[root@linux ~]# reboot