SUSE Linux Enterprise Desktop Administration

1
SUSE Linux Enterprise Desktop Administration

• Chapter 11
• Manage System Initialization

2
Objectives

• Objective 1Describe the Linux Load Procedure
• Objective 2GRUB (Grand Unified Bootloader)
• Objective 3Manage Runlevels

3
Objective 1Describe the Linux Load Procedure

• The basic steps of booting a computer with a
  Linux system installed
• BIOS and Boot Manager
• Kernel
• initramfs (Initial RAM File System)
• init

4
Figure 11-1 The basic steps of booting a computer
with Linux
5
BIOS and Boot Manager

• BIOS (Basic Input Output System)
• Performs a power-on self test, conducts the
  initial detection and setup of hardware, and
  accesses bootable devices
• If the bootable device is a hard drive, BIOS also
  reads the MBR (Master Boot Record)
• Using the code in the MBR, the BIOS starts the
  boot manager
• The boot manager (such as GRUB) loads the kernel
  and the initrd to memory and starts the kernel

6
Kernel

• Kernel
• Uncompresses itself and then organizes and takes
  control of the continued booting of the system
• Checks and sets the console, reads BIOS settings,
  and initializes basic hardware interfaces
• Next, the drivers probe existing hardware and
  initialize it accordingly
• The kernel controls the entire system
• Managing hardware access and allocating CPU time
  and memory to programs

7
initramfs (Initial RAM File System)

• Initial RAM File System (initramfs)
• A cpio archive that the kernel can load to a RAM
  disk
• Provides a minimal Linux environment that enables
  the execution of programs before the actual root
  file system is mounted
• Provides an executable named init
• Should execute the actual init program on the
  root file system for the boot process to proceed
• Former SUSE Linux versions used an initial RAM
  disk, initrd, instead
• The kernel starts the program init

8
init

• The program init is located in initramfs starts
  /sbin/init
• Boots the system with all its programs and
  configurations
• The init process is always assigned a process ID
  number of 1
• Relies on the /etc/inittab file for configuration
  information
• After the init process starts, it begins by
  accessing the /etc/init.d/boot script
• Controls the start of services

9
init (continued)

• After the boot script has been completed, init
  starts the /etc/init.d/rc script
• Uses configured runlevels to start services and
  daemons
• Each runlevel has its own set of services that
  are initiated

10
Objective 2GRUB (Grand Unified Bootloader)

• To manage GRUB, the Grand Unified Bootloader, you
  need to know the following
• What a Boot Manager Is
• Boot Managers in SUSE Linux
• Start the GRUB Shell
• Modify the GRUB Configuration File
• Configure GRUB with YaST
• Boot a System Directly into a Shell

11
What a Boot Manager Is

• Boot loader
• A program that loads the operating system into
  memory
• Loads the operating system kernel, which then
  loads the system
• After running the power-on self test (POST), the
  PC BIOS searches for a boot loader
• If it finds one, it turns control of the boot
  process over to the boot loader
• The boot loader then locates the operating system
  files and starts the operating system

12
What a Boot Manager Is (continued)

• Boot manager
• More than a boot loader
• Can also handle several operating systems
• Linux boot managers can be used to load Linux or
  other operating systems
• GRUB is designed with the following two-stage
  architecture
• Stage 1Usually installed in the Master Boot
  Record (MBR) of the hard disk
• Stage 2Usually contains the actual boot loader

13
Boot Managers in SUSE Linux

• GRUB boot manager
• The standard boot manager in SUSE Linux
  Enterprise Desktop
• Some special features of GRUB
• File system support
• Interactive control
• LILO boot manager
• LILO configuration file is /etc/lilo.conf
• Structure is similar to that of the GRUB
  configuration file

14
Boot Managers in SUSE Linux (continued)

• Map files, GRUB, and LILO
• The kernel is usually a file within a file system
  on a partition on a disk
• These concepts are unknown to the BIOS
• Maps simply note the physical block numbers on
  the disk that comprise the logical files
• When a map is processed, the BIOS loads all the
  physical blocks in sequence as noted in the map
• Building the logical file in memory
• GRUB tries to become independent from the fixed
  maps at an early stage

15
Start the GRUB Shell

• Start the GRUB shell in the running system
• Enter the grub command as root
• As in a bash shell, you can complete GRUB shell
  commands with the Tab key
• Start the GRUB shell at the boot prompt
• From the graphical boot selection menu, press Esc
• A text-based menu appears
• Start the GRUB shell by typing c (U.S. keyboard
  layout)

16
Modify the GRUB Configuration File

• Configure GRUB by editing the /boot/grub/menu.lst
  file
• General structure of the file
• General options
• Options for the various operating systems that
  can be booted with GRUB
• kernel /boot/vmlinuz
• initrd /boot/initrd

17
Configure GRUB with YaST

• To start the YaST Boot Loader module
• Start YaST, enter the root password, and then
  select System gt Boot Loader or
• Start it directly from a terminal window by
  logging in as root and entering yast2 bootloader
• Select the Section Management tab to see the
  current GRUB settings for your system

18
Figure 11-2 YaST Boot Loader module
19
Figure 11-3 Section Management
20
Figure 11-4 Clone Selected Section
21
Figure 11-4 Clone Selected Section
22
Figure 11-5 Boot Loader Installation tab
23
Boot a System Directly into a Shell

• Boot screen of the GRUB boot loader
• Lets you enter parameters that modify the
  behavior of the Linux kernel
• At the bottom of the GRUB boot screen is the Boot
  Options field
• To add a boot option, select an operating system
  and type the additional boot option in the Boot
  Options field
• With the boot parameter initnew_init_program,
  you can change the first program loaded by the
  kernel

24
Boot a System Directly into a Shell (continued)

• You are directly logged in as root without being
  asked for a password
• Change the boot configuration to require a
  password before the kernel command line can be
  edited

25
Exercise 11-1 Manage the Boot Loader

• In this exercise, you practice booting into a
  shell and modifying /boot/grub/menu.lst

26
Objective 3Manage Runlevels

• Managing runlevels is an essential part of Linux
  system administration
• In this objective, you learn what runlevels are,
  the role of the program init, and how to
  configure and change runlevels
• The init Program and Linux Runlevels
• init Scripts and Runlevel Directories
• Change the Runlevel

27
The init Program and Linux Runlevels

• The init program
• The system is initialized by /sbin/init
• Started by the kernel as the first process of the
  system
• This process, or one of its child processes,
  starts all additional processes
• SIGKILL has no effect on init
• The configuration file for init is /etc/inittab
• Part of the configuration in /etc/inittab is the
  runlevel the system uses after booting

28
The init Program and Linux Runlevels (continued)

• The runlevels
• Runlevels define the state of the system

Table 11-1 The available runlevels
29
The init Program and Linux Runlevels (continued)

• Init configuration file (/etc/inittab)
• Each line in the /etc/inittab file uses the
  following syntax idrlactionprocess
• The first entry in the /etc/inittab file contains
  the following parameters id5initdefault
• The next entry in /etc/inittab looks like this
  sibootwait/etc/init.d/boot
• The next few entries describe the actions for
  runlevels 0 to 6
• The final block of entries describes in which
  runlevels getty processes (login processes) are
  started

30
init Scripts and Runlevel Directories

• /etc/inittab defines the runlevel the system uses
  after booting is complete
• init scripts
• The /etc/init.d/ directory contains shell scripts
  that are used to perform certain tasks at bootup
  and start and stop services in the running system
• The shell scripts can be called up in the
  following ways
• Directly by init when you boot the system
• Indirectly by init when you change the runlevel
• Directly by /etc/init.d/script parameter

31
init Scripts and Runlevel Directories (continued)
Table 11-2 /etc/init.d/script parameters
32
init Scripts and Runlevel Directories (continued)

• init scripts (continued)
• Some of the more important scripts stored in
  /etc/init.d/
• boot
• boot.local
• halt
• rc
• service

33
init Scripts and Runlevel Directories (continued)

• Runlevel symbolic links
• To enter a certain runlevel, init calls the
  /etc/init.d/rc script with the runlevel as a
  parameter
• This script examines the respective runlevel
  /etc/init.d/rcx.d/ directory and starts and stops
  services depending on the links in this directory
• Each runlevel has a corresponding subdirectory in
  /etc/init.d/

34
init Scripts and Runlevel Directories (continued)

• Runlevel symbolic links (continued)
• When you view the files in a directory such as
  /etc/init.d/rc3.d/, you see two kinds of files
• Those that start with a K and those that
  start with an S
• Entering ls -l in an /etc/init.d/rcx.d/ directory
  indicates that these files are actually symbolic
  links
• By using symbolic links in subdirectories, only
  the script version in /etc/init.d/ needs to be
  modified
• Usually, two links within a runlevel directory
  point to the same script

35
init Scripts and Runlevel Directories (continued)

• When you change from your current runlevel 5 to
  the new runlevel 3, three cases are possible
• There is a Kxx link for a certain service in
  /etc/init.d/rc5.d/ and there is an Sxx link in
  /etc/init.d/rc3.d/ for the same service
• In this case, the service is neither started nor
  stopped
• There is a Kxx link for a certain service in
  /etc/init.d/rc5.d/ and there is no corresponding
  Sxx link in /etc/init.d/rc3.d/
• In this case, the script in /etc/init.d/service
  is called with the stop parameter and the service
  is stopped

36
init Scripts and Runlevel Directories (continued)

• When you change from your current runlevel 5 to
  the new runlevel 3, three cases are possible
  (continued)
• There is an Sxx link in /etc/init.d/rc3.d/ and
  there is no corresponding Kxx link for the
  service in /etc/init.d/rc5.d/
• In this case, the script in /etc/init.d/service
  is called with the start parameter and the
  service is started
• The number after the K or S determines the
  sequence in which the scripts are called

37
init Scripts and Runlevel Directories (continued)

• Activate and deactivate services for a runlevel
• Possible to manually create the symbolic links in
  the runlevel subdirectories with the ln command
• To configure runlevels with YaST
• Start the YaST Runlevel Editor module by starting
  YaST and then selecting SystemgtSystem Services
  (Runlevel)
• Or open a terminal window and, as root, enter
  yast2 runlevel
• Modes
• Simple mode
• Expert mode

38
Figure 11-6 The YaST Runlevel Editor module
39
Figure 11-7 The YaST Runlevel Editor module in
expert mode
40
init Scripts and Runlevel Directories (continued)

• Activate and deactivate services for a runlevel
  (continued)
• Normally, the default runlevel of a SUSE Linux
  system is runlevel 5
• Changes to the default runlevel take effect the
  next time you boot your computer
• To configure a service, select a service from the
  list
• Then, from the options below the list, select the
  runlevels you want associated with the service

41
Change the Runlevel

• Change the runlevel at boot
• Possible to boot to another runlevel by
  specifying the runlevel on the kernel command
  line of GRUB
• Manage runlevels from the command line
• Can change to another runlevel once the system is
  running by using the init command
• Like most modern operating systems, Linux reacts
  sensitively to being switched off without warning
• The shutdown command shuts down the system after
  the specified time

42
Exercise 11-2 Manage Runlevels

• In this exercise, you practice configuring
  runlevels

43
Summary

• After performing a POST, the BIOS typically loads
  the boot manager from the MBR
• The traditional Linux boot manager is LILO
• To manage LILO, you can edit the /etc/lilo.conf
  file and run the lilo command
• A Linux system is categorized using runlevels
• The init program is responsible for loading and
  unloading daemons in a runlevel using its
  configuration file /etc/inittab

44
Summary (continued)

• The /etc/inittab file runs scripts that start
  with S (start) or K (kill) in the
  /etc/init.d/rcrunlevel.d directory when changing
  runlevels
• You can view your current runlevel using the
  runlevel or who -r commands
• Individual daemons can be started by running the
  appropriate script in the /etc/init.d directory
  with the start argument
• The halt, poweroff, reboot, and shutdown commands
  can be used to safely change to runlevel 0 or 6