Booting And Shutting Down

Bootstrapping

Bootstrapping is standard term for “starting up a computer”

During bootstrapping the kernel is loaded into memory and begin its execution

Overview of Linux Booting

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Overview Linux booting

Turn on CPU jump to address of BIOS BIOS runs POST (Power-On Self Test) and it also

performs system integrity check BIOS searches, loads, and executes boot loader

program BIOS finds bootable devices BIOS gives control to MBR reads from the memory BIOS loads and execute boot sector form MBR

Booting Process

Recovery Boot to a shell

Basic minimal shell is loaded in case of system failed to load.

This shell can be used as a recovery tool by system admins.

Traditionally known as booting in ‘single user mode’ or ‘maintenance mode’

On most system ,you request a boot to ‘single user mode’ by passing an argument to kernel at boot time.

If system is alredy up and running, you can bring it down to single user mode with shutdown or telinit command.

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Steps in the boot process

1. Reading of the boot loader from the MBR (Master Boot Record)

2. Loading and initialization of the kernel3. Device detection and configuration4. Creation of kernel processes5. Administrator intervention (single-user-mode

only)6. Execution of system startup scripts

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MBR (Master Boot Record)

OS is booted from a hard disk, where the Master Boot Record (MBR) contains the primary boot loader

The MBR is a 512-byte sector, located in the first sector of the bootable disk (sector 1 of cylinder 0, head 0)

After the MBR is loaded into RAM, the BIOS yields control to it.

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MBR (Master Boot Record)

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MBR (Master Boot Record)

The first 446 bytes are the primary boot loader, which contains both executable code and error message text

The next sixty-four bytes are the partition table, which contains a record for each of four partitions

The MBR ends with two bytes that are defined as the magic number (0xAA55). The magic number serves as a validation check of the MBR

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Extracting the MBR

To see the contents of MBR, use this command:

# dd if=/dev/hda of=mbr.bin bs=512 count=1

# od -xa mbr.bin**The dd command, which needs to be run from

root, reads the first 512 bytes from /dev/hda (the first Integrated Drive Electronics, or IDE drive) and writes them to the mbr.bin file.

**The od command prints the binary file in hex and ASCII formats.

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Boot loader Boot loader could be more aptly called the

kernel loader. The task at this stage is to load the Linux kernel

MBR contains information about boot loaders

MBR loads and executes GRUB (Linux boot loader)

GRUB(Grand Unified Bootloader) contains images of the kernel i.e. loads and execute kernel, it also loads initial RAM disk (init rd)

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2. Loading and initialization of the kernel

Kernel is itself a program, and the first bootstrapping task is to get this program into memory so that it can be executed

Linux system kernel - /boot/vmlinuz Kernel mounts root filesystem i.e. loads

filesystems Probes the system to determine how much

RAM is available Kernel executes init program located in

/sbin/init

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3. Device detection and configuration

Kernel loads device drivers

The kernel prints out a line of cryptic information about each device it finds

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4. Creation of kernel processes

Kernel creates several “spontaneous” processes in user space

They are called spontaneous processes because they are not created through the normal system call fork

Processes sched have PID 0 and init have PID 1

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5. Administrator intervention (single user mode – recovery mode)

During ‘single-user-boot’ user is prompted to enter the root password.

A root shell is spawned by Init process which is aware of the fact that system is booted in recovery mode. (because of the extra argument passed by kernel)

Only root portion is mounted

Fsck cmd is run during a normal boot to check and repair filesystems.

When the system in single user mode, need to run fsck by hand

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6. Execution of startup scripts

Startup scripts are just normal shell scripts, and they are selected and run by init

Startup scripts serves in setting up the user environment

Finally boot process completed

But Init is not done with its job…. Continues to play an important role

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Booting PCs

When a machine boots, it begins by executing boot code stored in ROMs.

The exact location and nature of this boot code varies, depending on the type of machine you have.

The boot code is typically firmware that knows how to use the devices connected to the machine, how to talk to the network on a basic level, and how to understand disk-based filesystems.

On PCs, the initial boot code is generally called a BIOS.

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Built-in BIOS knows about some of the devices that live on the motherboard, IDE and disks, network interface, power and temperature meters and system hardware.

BIOS 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 DVD, USB drive, hard disk”.

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Once the BIOS has figured out what 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.

MBR contains a program that tells the computer from which partition to load a secondary boot program, the “boot loader”.

Default MBR contains a simple program that tells the computer to get its boot loader from the first partition on the disk.

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MBR that knows how to deal with multiple operating systems and kernels.

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.

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GRUB : The Grand Unified Boot Loader

GRUB, developed by the GNU (Gnu’s Not Unix)

project.

It is the default boot loader for most UNIX and

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.

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There are two branches of the GRUB lineage:

original GRUB called GRUB Legacy, and the

newer GRUB 2.

Grub 2 is similar in concept but varies in its

config file syntax.

By default, GRUB reads its default boot

configuration from /boot/grub/menu.lst or

/boot/grub/grub.conf.

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GRUB reads the configuration file at startup

time.

menu.lst and grub.conf files are slightly different

but have a similar syntax.

Red hat system use grub.conf and solaris, suse

and ubuntu still use menu.lst

GRUB 2 : New Layout

GRUB 2 places its files in three core locations: /boot/grub/grub.cfg :

Should not be edited by hand /etc/grub.d/

Contains grub scripts , these scripts generate grub.cfg

/etc/default/grub This file contains the GRUB menu settings that are read by the GRUB scripts and written into grub.cfg

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Red Hat system - grub.conf file

default=0

timeout=10

splashimage=(hd0,0)/boot/grub/splash.xpm.gz # displays

splash image

title Red Hat Enterprise Linux Server (2.6.18-92.1.10.el5)

root (hd0,0) # root file system

kernel /vmlinuz-2.6.18-92.1.10.el5ro root=LABEL=/

#Grub loads this kernel

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Ubuntu system - menu.lst file

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GRUB is an operating system independant boot loader

Flexible command line interface File system access

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Multibootingdefault=0

timeout=5

splashimage=(hd0,2)/boot/grub/splash.xpm.gz

hiddenmenu

title Windows XP

rootnoverify (hd0,0)

chainloader +1

title Red Hat

root (hd0,1)

kernel /vmlinuz

Grub Command Line

Press ‘c’ from grub boot screen to enter in command line mode

Press <TAB> to obtain quick list of possible commands.

We can boot OS that is not in grub.cong file

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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

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Booting To Single-User Mode

Boot options should be easily modifiable and

decided on the ‘a’ key as the appropriate tool.

To boot into single-user mode, add the single flag

to the end of the existing kernel options.

ro root=LABEL=/rhgb quiet single

Init and start-up scripts

Bootloader loads the kernel, kernel spawns the init process.

Init executes start up scripts

Start up scripts are kept in /etc/init.d ( rc0.d , rc1.d and so on)

Tasks often performed by start-up scripts

Setting the name of computer Setting the time zone Checking the disk with fsck Mounting the system disk Removing old files from the temp

directory Configuring network settings Starting up daemon and netwwork

services

Init and its run level

Init controls the system run levels and decides (based on /etc/inittab) which set of processes to run at each run level

Each run level is normally a single digit (0 to 6) or S

When system is booted init first runs at level 1 or S.

Run levels

At level 0, the system is completely shut down.

Levels 1 and S represent single-user mode.

Levels 2 through 5 include support for networking.

Level 6 is a “reboot” level.

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Rebooting and Shutting Down

Always a good idea to shut down the machine

when possible.

Improper shutdown can result problem in the

system.

To modify or make significant system changes,

you should reboot just to make sure that the

system comes up successfully.

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Shutdown : the gentle way to halt the system Shutdown sends messages to logged-in users at

progressively shorter intervals, warning them of the impending downtime.

Warning simply say that the system is being shut down and give the time remaining until the event.

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$ sudo shutdown –h +15 "system is going down for system halt after 15 minutes! “

$ sudo shutdown –r +1 “system is going down for system halt after 1 minutes!”

Halt cmd performs the essential duties required to shut the system down.

Halt logs the shutdown, kills nonessential processes, executes the sync system call, waits for filesystem writes to complete, and then halts the kernel.

Reboot is almost identical to halt, but it reboots instead of halting.