Linux BOOT Process

Boot sequence summary

When a PC is booted it starts running a BIOS program which is a memory resident program on an EEPROM integrated circuit. The BIOS program will eventually try to read the first sector on a booting media such as a hard or floppy drive. The boot sector contains a small program that the BIOS will load and attempt to pass run control to. This program will attempt to read the operating system from the disk and run it. LILO is the program that Linux systems typically use to give users a choice of operating systems to run. It is usually installed in the boot sector which is also called the master boot record. If the user chooses to boot Linux, LILO will attempt to load the Linux kernel causing the following basic events to happen:

BIOS

Load boot sector from one of:
•Floppy
•CDROM
•Hard drive
The boot order can be changed from within the BIOS. BIOS setup can be entered by pressing a key during bootup. The exact key depends varies, but is often one of Del, F1, F2, or F10.
Loading sequence
•load menu code, typically /boot/boot.b
•prompt for (or timeout to default) partition or kernel
•for "image=" (ie Linux) option load kernel image
•for "other=" (ie DOS) option load first 512 bytes of the partition
•copy kernel image (bzImage) and modules to /boot and /lib/modules
•edit /etc/lilo.conf
•duplicate image= section, eg:
image=/bzImage-2.4.14
label=14
read-only
•man lilo.conf for details
•run /sbin/lilo
•reboot to test

•GRUB

•Understands file systems
•config lives in /boot/grub/menu.lst or /boot/boot/menu.lst

Kernel

•initialise devices
•(optionally loads initrd, see below)
•mounts root filesystem
•specified by lilo or loadin with root= parameter
•kernel prints: VFS: Mounted root (ext2 filesystem) readonly.
•runs /sbin/init which is process number 1 (PID=1)
•init prints: INIT: version 2.76 booting
•can be changed with boot= parameter to lilo, eg boot=/bin/sh can be useful to rescue a system which is having trouble booting.

initrd

Allows setup to be performed before root FS is mounted
•lilo or loadlin loads ram disk image
•kernel runs /linuxrc
•load modules
•initialise devices
•/linuxrc exits
•"real" root is mounted
•kernel runs /sbin/init
Details in /usr/src/linux/Documentation/initrd.txt (part of the kernel source).

/sbin/init

•reads /etc/inittab (see man inittab which specifies the scripts below
•Run boot scripts:
•debian: run /etc/init.d/rcS which runs:
•/etc/rcS.d/S* scripts
•/etc/rc.boot/* (depreciated)
•redhat: /etc/rc.d/rc.sysinit script which: loads modules, check root FS and mount RW, mount local FS, setup network, and mount remote FS
•switches to default runlevel eg 3.
•run scripts /etc/rc3.d/S*
•run programs specified in /etc/inittab

Run Levels

•0 halt
•1 single user
•2-4 user defined
•5 X11 only (0 or 1 text console)
•6 Reboot
•Default is defined in /etc/inittab, eg:
•id:3:initdefault:
•The current runlevel can be changed by running /sbin/telinit # where # is the new runlevel, eg typing telinit 6 will reboot.

Run Level programs

•Scripts in /etc/rc*.d/* are symlinks to /etc/init.d
•Scripts prefixed with S will be started when the runlevel is entered, eg /etc/rc5.d/S99xdm
•Scripts prefixed with K will be killed when the runlevel is entered, eg /etc/rc6.d/K20apache
•X11 login screen is typically started by one of S99xdm, S99kdm, or S99gdm.
•Run programs for specified run level
•/etc/inittab lines:
•1:2345:respawn:/sbin/getty 9600 tty1
•Always running in runlevels 2, 3, 4, or 5
•Displays login on console (tty1)
•2:234:respawn:/sbin/getty 9600 tty2
•Always running in runlevels 2, 3, or 4
•Displays login on console (tty2)
•l3:3:wait:/etc/init.d/rc 3
•Run once when switching to runlevel 3.
•Uses scripts stored in /etc/rc3.d/
•ca:12345:ctrlaltdel:/sbin/shutdown -t1 -a -r now
•Run when control-alt-delete is pressed

Boot Summary

•lilo
•/etc/lilo.conf
•debian runs
•/etc/rcS.d/S* scripts
•/etc/rc3.d/S* scripts
•redhat runs
•/etc/rc.d/rc.sysinit script
•/etc/rc.d/rc3.d/S* scripts

Comments

Post a Comment

Popular posts from this blog

Installation of Linux version of XAMPP

Image
Step 1: Download you can download Linux version of XAMPP from the give link A complete list of downloads (older versions) is available Step 2: Installation After downloading simply type in the following comm ands: Go to a Linux shell and login as the system administrator root: su Extract the downloaded archive file to /opt: tar xvfz xampp-linux-1.7.tar.gz -C /opt Note: Please use only this command to install XAMPP. DON'T use any Microsoft Windows tools to extract the archive, it won't work. Note : already installed XAMPP versions get overwritten by this command. That's all. XAMPP is now installed below the /opt/lampp directory. Step 3: Start To start XAMPP simply call this command: /opt/lampp/lampp start You should now see something like this on your screen: Starting XAMPP 1.7... LAMPP: Starting Apache... LAMPP: Starting MySQL... LAMPP started. Ready. Apache and MySQL are running. If you get any error messages please take a look at the Step 4:...
Read more

How can I prevent users from connecting to a USB storage device? in Windows OS

It is easy to lock a machine down, disable the floppy, and cdrom in the bios. There is a simple registry change that will keep the USB storage drivers from starting when the system boots. Keeps people from walking up to a PC and copying data off with a USB key, but allows you to keep your scanner, keyboard, and mouse working. As always - back your system up before messing around in the registry. Just open regedit and browse to this key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\UsbStor Notice the value 'Start' Switch this value to 4, and USB storage devices are disabled. Switch this value to 3, and USB storage devices are enabled. Hence you have Prevented users from connecting a USB device
Read more

Basic Linux Commands

command cd : Use cd change directories example: cd home or complete path must be written cd /root/home command clear : clean up your command window command ls : The ls command lists the contents of your current working directory command mkdir : makes directory command df : df provides a very quick check of your file system disk space. command du : du display usage of the disk space example du -a command finger : finger allows you to see who else is on the system or get detailed information about a person who has access to the system. command man : man followed by a command will give detailed information about that command Example: man ls command logout : logout will log your account out of the system command find : find will locate a files/directories qui...
Read more