Class Note:

1
Class Note

• The Syllabus is available on-line at
  http//www.wildbill.org/rose

2
System Profiles
3
System Profiles

• When a user logs in a system profile is
  established
• Sets environment variables
• I.e. PATH/bin/usr/bin
• Sets aliass
• I.e. alias rm rm i
• alias cp cp I
• alias cls clear

4
System Profiles

• Various system-wide profiles
• /etc/profile (bourne shell)
• /etc/bashrc (bash shell)
• /etc/tcshrc (c-shell)
• User specific profiles (located in home dir)
• /.profile
• /.bashrc
• /.tcshrc

5
Shell Scripts

• A shell script is a group of commands, functions,
  variables, etc., that can be run from the shell
  prompt (command line)
• Chapter 12 gives an overview of how shell scripts
  work and can be used
• Automate system chores
• Application startup
• Even generate web pages
• http//www.wildbill.org/rose/spring07.cgi
• Those who attend Intro to UNIX should have a
  reasonable understanding of shell scripts.

6
Shell Scripts (/etc/init.d/)

• Many of the startup scripts are located in the
  /etc/init.d directory
• Know and understand symbolic links
• A symbolic Link is where a file has one main
  name, but there's an extra entry in the file name
  table that refers any accesses back to the main
  name

7
Symbolic Links

• Symbolic links are set up using the ln command
  with the -s option - so for exampleln -s
  filename1.txt filename2.txt

8
System Initialization
9
Overview

• Sequence of booting Linux
• Component description
• Sequence of shutting down Linux
• Conclusion

10
Booting Sequence
11
POST

• The principal duties of the main BIOS during POST
  are as follows
• verify the integrity of the BIOS code itself
• determine the reason POST is being executed
• find, size, and verify system main memory
• discover, initialize, and catalog all system
  buses and devices
• pass control to other specialized BIOSes (if and
  when required)
• provide a user interface for systems
  configuration
• identify, organize, and select which devices are
  available for booting
• construct whatever system environment that is
  required by the target OS

12
Bootstrap loader
13
Bootstrap loader

• In computing, booting is a bootstrapping process
  that starts operating systems when the user turns
  on a computer system.
• A boot sequence is the set of operations the
  computer performs when it is switched on that
  load an operating system.
• Locate and start boot loader program
• ROM BIOS
• Look for a boot sector
• Boot sector
• First sector of the disk
• 0xAA55 at byte 0x1FE (510)
• byte 510 (last two bytes of the sector)
• Load boot sector into memory

14
Boot loader program
15
Boot loader

• Load the operating system into its computer
  memory from its hard disk
• Master Boot Record
• Special reserved area
• Beginning of the system disk
• First stage boot loader
• Load and run second stage boot loader
• Second stage boot loader
• Prompt to choose OS
• Loads the kernel of the chosen OS

16
Boot loader

• Two primary Boot loader programs for Linux
• LILO (Linux Loader)
• GRUB (Grand Unified Boot loader)

17
LILO

• /etc/lilo.conf
• Location of kernel
• Disk partition to mount as root file system
• Map installer
• Read configuration file
• Write boot loaders, OS info to hard disk

18
GRUB

• Boot time shell
• GRUB interactive command prompt
• Run new configuration on the fly
• Dynamic default configuration
• Can use to boot other OS

19
GRUB

• /boot/grub/grub.conf
• general section
• splashimage (hd0,0)/grub/splash.xpm.gz
• default 0
• timeout 30
• password -md5 encoded-password
• title Linux
• root (hd0,0) kernel /vmlinuz ro root/dev/hda2

20
Compare LILO and GRUB

• LILO
• Traditional
• Need to be reinstalled in the master boot record
  after replacing the kernel or changing the boot
  configuration

• GRUB
• Newer
• Flexible
• Interactive command prompt

21
Kernel Initialization
22
Kernel initialization

• Checks system hardware
• Identifies devices
• Kernel
• Makes hardware do what the programs want

23
Kernel Initialization

• Probe essential devices
• CPU, Console, Memory
• Probe other hardware Subsystem
• I/O buses, Network interfaces, Hard disks, CD-ROM
  drives, Floppy drives, Storage devices

24
Kernel Initialization

• File System initialization
• Logical volume manager subsystem
• RAID
• SCSI Devices
• Hard disk partitions
• Change configuration of kernel
• /usr/src/linux/make menuconfig or xconfig
• rdev
• Boot loader parameter

25
Init program
26
Init

• Location /sbin/init
• Uses functions from libraries written in C
• Checks and mounts file system
• Starts up daemons to log system messages
• Starts the getty processes that put the login
  prompts on your virtual terminals
• Networking
• Serves web page
• Listen to the mouse

27
Inittab file

• Location /etc/inittab
• xxlevelsactionprocess
• Xx
• Label for the entry
• Levels
• Specific system operating mode
• Predefined set of system processes

28
Run levels in init

• 0 Halted system (ready for powering off)
• 1 Conversion to/preparation for single user mode
  
• 2 Non-networked multiuser mode
• 3 Networked multiuser mode
• 4 Networked multiuser mode with graphical login
• 6 Reboot mode

29
Run levels in init

• S,s Single user mode
• U,u Init process re-execution (pseudo run level)
  
• Q,q Force reread of configuration file (pseudo
  run level)
• a,b,c On-demand process initialization pseudo
  run levels

30
Inittab file

• Actions
• wait Start the process and wait for it to finish
  before going onto the next entry.
• once Start the process only if it is not already
  running (don't wait).
• respawn Start the process (don't wait) and
  automatically re-start it if the process later
  dies.

31
Inittab file

• bootwait Execute the process only at boot time,
  waiting for it to finish.
• initdefault Specifies default run level.
• ctrlaltdel Execute the action when the
  Ctrl-Alt-Del key sequence is detected.
• power Several keywords are defined for various
  power failure-related events (see examples below)

32
Inittab

• Program to interpret the initialization script
  bash
• Script link to other scripts
• Several scripts
• Initialization script
• Run-level script

33
Initialization script

• Initialization script
• (Red hat) /etc/rc.d/rc.sysinit
• File system get checked and mounted
• /etc/fstab
• Clock set
• Swap space enabled
• Hostname

34
Shutting down sequence

• shutdown -h/r (warning message)
• Logins are blocked
• All process are notified that the system is going
  down by signal SIGTERM
• Process to exit cleanly
• Signal the init process to change the run level
• Default 1, -h flag 0, -r flag 6

35
Shutting down

• unmount -a unmounts all the partitions listed in
  fstab
• shutdown -F Force fsck on reboot
• Journaling file system
• saves a transaction log of file system changes
• replay when the system is restarted

36
Acknowledge

• Two new links at wildbill.org/rose
• http//www.linuxforum.com/linux-introduction/ch04s
  02.html
• http//linux.about.com/library/cmd/blcmdl8_shutdow
  n.htm