Booting & shut down,

Booting and Shut Downof

Operating System

Prepared ByProf.Bhushan Pawar

MESCOE,Pune(Wadia Campus)bhushan.pawar@mescoepune.org

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BootstrappingStandard 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, whichis always process number 1.

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Recovery boot to a shell

In normal operation, systems boot themselvesindependently and are then accessed remotely byadministrators.

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 systemconsole known as booting to single-user mode, recoverymode, or maintenance mode.

Single-user mode does not allow network operation weneed physical access to the system console to use it.

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Recovery boot to a shell (Contin…)

On most systems, you request a boot to single-user mode by passing an argument to thekernel at boot time. If the system is already upand running, you can bring it down to single-user mode with the shutdown or telinitcommand.

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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

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Kernel initialization

• First bootstrapping task is to get kernel intomemory 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 asboot loader.

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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.

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Operator intervention (recovery mode only)

• If the system is to be brought in recoverymode, a command-line flag passed by thekernel.

• During a single-user boot on same systems,you are prompted to enter the root password.If you enter the right password, the systemspawns a root shell.

• You can execute commands in same way aswhen logged in on a fully booted system.

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Booting PC

• When a machine boots, it begins by executingcode stored in ROMs.

• The initial boot code is generally called a BIOS(Basic Input/Output System)

• Actually, a PC has several levels of BIOS: onefor the machine itself, one for the video card,one for the SCSI card if the system has one,and sometimes components for otherperipherals such as network cards.

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Booting PC (Continue…)

• Built-in BIOS knows about some of the devicesthat live on the motherboard. E.g. IDE(integrateddevelopment environment), SATA Controller,network interfaces.

• The BIOS normally lets you select which devicesyou want the system to try to boot from. You canusually specify an ordered list of preferences suchas “Try to boot from a DVD, then a USB drive,then the hard disk.” Network booting withPXE(Preboot eXecution Environment) is also acommon option.

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Booting PC (Continue…)

• Once the BIOS has figured device to boot fromit tries to read the first block of the device.This 512-byte segment is known as the masterboot record or MBR. The MBR contains aprogram that tells the computer from whichpartition to load a secondary boot program,the “boot loader.”

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Booting PC (Continue…)

• The default MBR contains a simple programthat tells the computer to get its boot loaderfrom the first partition on the disk.

• Once the MBR has chosen a partition to bootfrom, it tries to load the boot loader specificto that partition. This loader is thenresponsible for loading the kernel.

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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 thatkernel with options specified by theadministrator.

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GRUB Loader

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GRUB Loader (Continue..)

• By default, GRUB reads its default bootconfiguration from /boot/grub/menu.lst or/boot/grub/grub.conf

• GRUB allows dynamic changes at each systemboot.

• The menu.lst and grub.conf files are slightlydifferent but have a similar syntax. Red Hatsystems use grub.conf, and Solaris, SUSE, andUbuntu still use menu.lst

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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

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BOOTING TO SINGLE-USER MODE• The beginnings of the boot process are system

dependent. Systems with non-Intel processors havecustom boot loader software.

• Single-user mode with GRUB– don’t need to use the command line.– boot options should be easily modifiable anddecided on the ‘a’ key as the appropriate tool.– At the GRUB splash screen, highlight the desired

kernel and press ‘a’ to append to the bootoptions.

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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 existingkernel options.

• E.g grub append> ro root=LABEL=/ rhgb quiet single

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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.

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Single-user mode on SPARC(scalable processor architecture)

• To interrupt the boot procedure and enter theOpenBoot PROM on Sun hardware, press the L1 and‘a’ keys simultaneously. L1 is sometimes labeledSTOP on modern Sun keyboards.

• Type boot -s to boot to single-user mode.

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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

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WORKING WITH STARTUP SCRIPTS (Continue…)

• Tasks that are often performed in the startup scriptsSetting the name of the computerSetting the time zoneChecking the disks with fsck (i.e file system check)Mounting the system’s disksRemoving old files from the /tmp directoryConfiguring network interfacesStarting up daemons and network services

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Rebooting & Shutting Down

• On consumer-oriented operating systems,rebooting the operating system is anappropriate first course of treatment for manyproblems.

• Whenever we modify a startup script or makesignificant system changes, we should rebootjust to make sure that the system comes upsuccessfully.

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shutdown: the proper way to halt the system

• It is the safest, most considerate, and most thoroughcommand to initiate a halt or reboot or to return thesystem to single-user mode.

• You can ask shutdown to wait awhile before shuttingdown the system. During the waiting period,shutdown sends messages to logged-in users atprogressively shorter intervals, warning them of theimpending downtime.

• Most versions of shutdown, specify whether themachine should halt, go to single-user mode, orreboot. (Path is /sbin/shutdown)

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halt and reboot: simpler ways to shut down

• The halt command performs the essential dutiesrequired to shut the system down. It is called byshutdown -h but can also be used by itself.

• Halt logs the shutdown, kills nonessential processes,executes the sync system call (called by andequivalent to the sync command), waits for filesystem writes to complete, and then halts the kernel.

• halt -n prevents the sync call.

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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.

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