This document discusses the boot and shutdown processes of an operating system. It describes the six phases of the boot process: reading the boot loader, loading and initializing the kernel, detecting and configuring devices, creating kernel processes, optional administrator intervention in single-user mode, and executing startup scripts. It also discusses booting to single-user mode for recovery purposes and explains how to properly shut down a system using the shutdown command.

1. Booting and Shut Down
of
Operating System
Prepared By
Prof.Bhushan Pawar
MESCOE,Pune(Wadia Campus)
bhushan.pawar@mescoepune.org
1Prof.Bhushan Pawar

2. Bootstrapping
Standard term for starting up a computer.
During bootstrapping, the kernel is loaded into
memory and begins to execute.
When a computer is turned on, it first executes
boot code that is stored in ROM.
The kernel probes the system’s hardware and
then spawns the system’s init process, which
is always process number 1.
2Prof.Bhushan Pawar

3. Recovery boot to a shell
 In normal operation, systems boot themselves
independently and are then accessed remotely by
administrators.
 If a disk crashes, administrators need a recovery tool.
 Then instead of shooting for full system operation,
UNIX systems can boot just run a shell on the system
console known as booting to single-user mode, recovery
mode, or maintenance mode.
 Single-user mode does not allow network operation we
need physical access to the system console to use it.
3Prof.Bhushan Pawar

4. Recovery boot to a shell (Contin…)
On most systems, you request a boot to single-
user mode by passing an argument to the
kernel at boot time. If the system is already up
and running, you can bring it down to single-
user mode with the shutdown or telinit
command.
4Prof.Bhushan Pawar

5. Steps in the boot process
• consists of 6 distinct phases
– Reading of the boot loader from the master boot
record
– Loading and initialization of the kernel
– Device detection and configuration
– Creation of kernel processes
– Administrator intervention (single-user mode
only)
– Execution of system startup scripts
5Prof.Bhushan Pawar

6. Kernel initialization
• First bootstrapping task is to get kernel into
memory so that it can be executed.
• The pathname of the kernel /boot/vmlinuz
• System ROM loads a small boot program into
memory from disk. This program is called as
boot loader.
6Prof.Bhushan Pawar

7. Creation of kernel processes
Kernel process created through the normal
system fork mechanism
init is always pid 1. Most UNIX systems have
sched as process 0.
7Prof.Bhushan Pawar

8. Operator intervention
(recovery mode only)
• If the system is to be brought in recovery
mode, a command-line flag passed by the
kernel.
• During a single-user boot on same systems,
you are prompted to enter the root password.
If you enter the right password, the system
spawns a root shell.
• You can execute commands in same way as
when logged in on a fully booted system.
8Prof.Bhushan Pawar

9. Booting PC
• When a machine boots, it begins by executing
code stored in ROMs.
• The initial boot code is generally called a BIOS
(Basic Input/Output System)
• Actually, a PC has several levels of BIOS: one
for the machine itself, one for the video card,
one for the SCSI card if the system has one,
and sometimes components for other
peripherals such as network cards.
9Prof.Bhushan Pawar

10. Booting PC (Continue…)
• Built-in BIOS knows about some of the devices
that live on the motherboard. E.g. IDE(integrated
development environment), SATA Controller,
network interfaces.
• The BIOS normally lets you select which devices
you want the system to try to boot from. You can
usually specify an ordered list of preferences such
as “Try to boot from a DVD, then a USB drive,
then the hard disk.” Network booting with
PXE(Preboot eXecution Environment) is also a
common option.
10Prof.Bhushan Pawar

11. Booting PC (Continue…)
• Once the BIOS has figured device to boot from
it tries to read the first block of the device.
This 512-byte segment is known as the master
boot record or MBR. The MBR contains a
program that tells the computer from which
partition to load a secondary boot program,
the “boot loader.”
11Prof.Bhushan Pawar

12. Booting PC (Continue…)
• The default MBR contains a simple program
that tells the computer to get its boot loader
from the first partition on the disk.
• Once the MBR has chosen a partition to boot
from, it tries to load the boot loader specific
to that partition. This loader is then
responsible for loading the kernel.
12Prof.Bhushan Pawar

13. GRUB: GRAND UNIFIED BOOT LOADER
• GRUB, developed by the GNU project
• It is the default boot loader for UNIX & LINUX
systems with Intel Processors.
• GRUB’s job is to choose a kernel from a
previously assembled list and to load that
kernel with options specified by the
administrator.
13Prof.Bhushan Pawar

14. 14
GRUB Loader
Prof.Bhushan Pawar

15. GRUB Loader (Continue..)
• By default, GRUB reads its default boot
configuration from /boot/grub/menu.lst or
/boot/grub/grub.conf
• GRUB allows dynamic changes at each system
boot.
• The menu.lst and grub.conf files are slightly
different but have a similar syntax. Red Hat
systems use grub.conf, and Solaris, SUSE, and
Ubuntu still use menu.lst
15Prof.Bhushan Pawar

16. GRUB Loader (Continue..)
• To enter command-line mode, type c from the GRUB boot
screen.
GRUB command-line options
Command Meaning
reboot Reboots the system
find Finds files on all mountable partitions
root Specifies the root device (a partition)
kernel Loads a kernel from the root device
help Gets interactive help for a command
boot Boots the system from the specified kernel image
16Prof.Bhushan Pawar

17. BOOTING TO SINGLE-USER MODE
• The beginnings of the boot process are system
dependent. Systems with non-Intel processors have
custom boot loader software.
• Single-user mode with GRUB
– don’t need to use the command line.
– boot options should be easily modifiable and
decided on the ‘a’ key as the appropriate tool.
– At the GRUB splash screen, highlight the desired
kernel and press ‘a’ to append to the boot
options.
17Prof.Bhushan Pawar

18. Single-user mode with GRUB
(Continue..)
• To boot into single-user mode, add the single
(or -s on Solaris) flag to the end of the existing
kernel options.
• E.g
grub append> ro root=LABEL=/ rhgb quiet single
18Prof.Bhushan Pawar

19. Single-user mode with GRUB
(Continue..)
• rhgb = redhat graphical boot - This is a GUI mode booting
screen with most of the information hidden while the user
sees a rotating activity icon spining and brief information as to
what the computer is doing.
quiet = hides the majority of boot messages before rhgb
starts. These are supposed to make the common user more
comfortable. They get alarmed about seeing the kernel and
initializing messages, so they hide them for their comfort.
19Prof.Bhushan Pawar

20. Single-user mode on SPARC
(scalable processor architecture)
• To interrupt the boot procedure and enter the
OpenBoot PROM on Sun hardware, press the L1 and
‘a’ keys simultaneously. L1 is sometimes labeled
STOP on modern Sun keyboards.
• Type boot -s to boot to single-user mode.
20Prof.Bhushan Pawar

21. WORKING WITH STARTUP SCRIPTS
• After exiting from single-user mode init executes the
system startup scripts & that are interpreted by sh or
bash.
• Systems use approach in which scripts are numbered
and executed.
• Scripts are kept in /etc/init.d, and links to them are
made in the directories /etc/rc0.d, /etc/rc1.d
21Prof.Bhushan Pawar

22. WORKING WITH STARTUP SCRIPTS
(Continue…)
• Tasks that are often performed in the startup
scripts
Setting the name of the computer
Setting the time zone
Checking the disks with fsck (i.e file system check)
Mounting the system’s disks
Removing old files from the /tmp directory
Configuring network interfaces
Starting up daemons and network services
Prof.Bhushan Pawar 22

23. Rebooting & Shutting Down
• On consumer-oriented operating systems,
rebooting the operating system is an
appropriate first course of treatment for many
problems.
• Whenever we modify a startup script or make
significant system changes, we should reboot
just to make sure that the system comes up
successfully.
Prof.Bhushan Pawar 23

24. shutdown: the proper way to halt
the system
• It is the safest, most considerate, and most thorough
command to initiate a halt or reboot or to return the
system to single-user mode.
• You can ask shutdown to wait awhile before shutting
down the system. During the waiting period,
shutdown sends messages to logged-in users at
progressively shorter intervals, warning them of the
impending downtime.
• Most versions of shutdown, specify whether the
machine should halt, go to single-user mode, or
reboot. (Path is /sbin/shutdown)
Prof.Bhushan Pawar 24

25. halt and reboot: simpler ways to
shut down
• The halt command performs the essential duties
required to shut the system down. It is called by
shutdown -h but can also be used by itself.
• Halt logs the shutdown, kills nonessential processes,
executes the sync system call (called by and
equivalent to the sync command), waits for file
system writes to complete, and then halts the kernel.
• halt -n prevents the sync call.
Prof.Bhushan Pawar 25

26. halt and reboot: simpler ways to
shut down (Continue…)
• reboot is almost identical to halt, but it causes
the machine to reboot instead of halting.
• Reboot is called by shutdown -r.
Prof.Bhushan Pawar 26