Post on 15-Dec-2015
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.