Understanding System Initialization

1
Understanding System Initialization

• Chapter 9

2
Hardware Initialization

• Basic process for bootstrapping
• CPU begins executing commands in BIOS
• BIOS passes control to boot loader
• Boot loader passes control to Linux kernel

3
Hardware Initialization (continued)

• After PC-type computer started
• CPU initializes itself
• Sends signals to control other parts of computer
• BIOS performs power-on self test (POST)
• Checks memory and other hardware
• BIOS sends signal to SCSI controller card telling
  it to initialize itself
• SCSI controllers usually contain own BIOS

4
Hardware Initialization (continued)

• After PC-type computer started (continued)
• Other hardware components instructed to
  initialize themselves
• BIOS determines which devices to search for
  operating system
• BIOS loads boot loader into memory and passes
  control it

5
Hardware Initialization (continued)
6
The Boot Loader

• Boot loader or boot manager
• Small program loaded by BIOS
• Commonly used
• LILO (Linux loader)
• GRUB (grand unified boot loader)
• Default on most systems
• BIOS looks for executable program in first sector
  of 3.5-inch disk or CD-ROM

7
The Boot Loader (continued)
8
The Boot Loader (continued)

• Chained boot loader
• One boot loader passes control to another boot
  loader instead of operating system
• Use boot loader to start different versions of
  Linux installed on separate partitions

9
The Boot Loader (continued)

• uname command
• Display information about kernel
• Full version number of current kernel
• uname -a
• Timestamp of kernel
• uname -v

10
Starting the Kernel

• Initial piece of kernel
• Loads rest of kernel into memory
• Uncompresses it
• dmesg command
• View messages stored by kernel during boot process

11
Starting the Kernel (continued)
12
Starting the Kernel (continued)

• Kernel ring buffer
• Area of memory
• Stores messages from kernel
• more /var/log/dmesg
• 4294667.296000 Linux version 2.6.15-23-386
  (buildd_at_rothera) (gcc version 4.0.3 (Ubuntu
  4.0.3-1ubuntu5)) 1 PREEMPT Tue May 23 134940
  UTC 2006
• 4294667.296000 BIOS-provided physical RAM map
• 4294667.296000 BIOS-e820 0000000000000000 -
  00000000000a0000 (usable)
• 94667.296000 896MB LOWMEM available.

13
Using Boot Parameters

• Boot parameters
• Instruct Linux how to operate or access parts of
  computer systems hardware
• Use to activate features of Linux
• To enter
• GRUB
• Press Esc or a
• LILO
• Enter after operating system name

14
Configuring Boot Loaders

• Configuration files
• Created by installation program
• Examine how boot loader operates
• Usually no need to change

15
Using LILO

• Simpler than GRUB
• To use
• Create configuration file describing operating
  systems LILO should start
• Run lilo command
• Examines lilo.conf file
• Stores appropriate information in either
• MBR
• Boot record you have indicated

16
Using LILO (continued)

• Boot image
• Each version of Linux
• Parameters
• boot
• delay
• image
• other

17
Using GRUB

• Installed by default for many versions of Linux
• /boot/grub/grub.conf
• Configuration file for GRUB
• RedHat
• Similar in layout and syntax to lilo.conf
• /boot/grub/menu.lst
• Ubuntu

18
Using GRUB (continued)
19
init and its Scripts

• init program
• Master control program
• Starts many other programs
• Runs scripts to initialize system services
• Controlled by /etc/inittab configuration file
• Three main configuration files located in
  /etc/rc.d
• rc.sysinit
• rc
• rc.local

20
init and its Scripts

• cat /etc/inittab edit to set default run level
  settings
• /etc/inittab init(8) configuration.
• Id inittab,v 1.91 2002/01/25 133521 miquels
  Exp
• The default runlevel.
• id2initdefault
• Boot-time system configuration/initialization
  script.
• This is run first except when booting in
  emergency (-b) mode.
• sisysinit/etc/init.d/rcS

21
Understanding Run Levels

• Run level
• Numbered mode of operation
• Preconfigured set of services is activated
• Defined in /etc/inittab file
• Default run level 5
• init or telinit command
• Change to different run level
• Commonly done only to restart or halt system

22
Understanding Run Levels (continued)
23
Understanding rc Scripts

• rc script
• Located in /etc/rc.d directory
• Started with parameter that includes run level
• Starts appropriate system services based on
  selected run level
• Each run level is associated with subdirectory
• Located in /etc/rc.d directory
• /etc/rd2.d for instance for the default multiuser
  run level with graphics on Ubuntu

24
Understanding rc Scripts (continued)

• Each file begins with K or S followed by
  two-digit number
• Begin with K? (killed) stopped
• Begin with S?started
• Files symbolic links to scripts that stop and
  start services
• Rename files in rc.d with K or S to create
  custom run time systems
• Files are symbolic links to executable scripts
• lrwxrwxrwx 1 root root 13 2007-03-13 1001
  /etc/rc2.d/S20ssh -gt ../init.d/ssh

25
Understanding rc Scripts (continued)

• Run level 2 example
• /etc/rc2.d
• S05vbesave S20avgd S20rwhod
  S89cron
• S10acpid S20dbus S20ssh
  S91apache2
• S10powernowd.early S20festival S20ssh-krb5
  S98usplash
• S10sysklogd S20hotkey-setup S21quotarpc
  S99acpi-support
• S11klogd S20inetd S21sendmail
  S99rc.local
• S13gdm S20laptop-mode
  S25bluez-utils S99rmnologin
• S14ppp S20makedev S25mdadm
  S99stop-readahead
• S18hplip S20nvidia-kernel S50proftpd
• S19cupsys S20powernowd S89anacron
• S20apmd S20rsync S89atd

26
Understanding rc Scripts (continued)
27
Controlling Services

• Many services listed in run-level directories
  relate to networking
• Scripts allow user to change almost anything on
  Linux system without restarting computer

28
Managing Services

• service command
• Control service
• Must know service name
• Examples
• RedHat
• service lpd restart
• service named start
• service named status
• Ubuntu
• /etc/init.d/ssh restart
• restart, start, and stop are options

29
Managing Services

• more ../init.d/ssh
• ! /bin/sh
• set -e
• /etc/init.d/ssh start and stop the OpenBSD
  "secure shell(tm)" daemon
• test -x /usr/sbin/sshd exit 0
• ( /usr/sbin/sshd -\? 2gt1 grep -q OpenSSH )
  2gt/dev/null exit 0
• if test -f /etc/default/ssh then
• . /etc/default/ssh
• fi

30
Managing Services (continued)

• chkconfig command on RedHat
• Alter files in run-level directories
• Modify services started or stopped each time
  system started
• By default alters run levels 2 through 5
• Example chkconfig named on

31
Managing Services Graphically

• Service configuration utility
• Main window lists services available on system
• Determined by contents of /etc/rc.d/init.d
• Active services
• Shown with check mark next to them

32
Managing Services Graphically (continued)
33
Managing Services Graphically (continued)
34
Configuring System Services

• Initialization scripts in /etc/rc.d/init.d
• Typically use configuration information located
  in /etc/sysconfig directory and subdirectories
• Files in /etc/sysconfig
• Named for services
• Each file contains name-value pairs
• Define configuration options for service

35
Configuring System Services (continued)

• Use system-config-network utility to configure
• /etc/sysconfig/ network file
• Files within /etc/sysconfig/network-scripts
• system-config-securitylevel utility
• Alters /etc/sysconfig/system-config-securitylevel
  file
• Based on firewall selections
• Service itself often has another set of
  configuration files in /etc directory

36
Shutting Down Linux

• Turning off Linux computer without shutting it
  down in orderly way can cause data loss
• Journaling file system
• Tracks each disk operation to ensure it is either
  completed or not done at all
• Journal permits operating system to back out of
  disk operation if not completed successfully

37
Shutting Down Linux (continued)

• Journaling file system
• Means file systems are unlikely to ever be
  corrupted by power loss
• Graceful shutdown
• Orderly or methodical shutdown of Linux
• All Linux services are stopped
• All data is written to disk
• Kernel changes to run level 0 or 6

38
Shutting Down Linux (continued)

• Shut down Linux gracefully
• Use reboot command
• Use halt command
• Use shutdown command
• shutdown -h now
• Pressing CtrlAltDel
• Entering the command telinit 0 or init 0
• Halt system
• Entering command telinit 6 or init 1
• Restart system

39
Shutting Down Linux (continued)

• Graphical desktop
• Use log out item on main menu of GNOME or KDE
• On servers or multiuser Linux systems
• Only system administrator should be allowed to
  shut down system