1. SUSE Linux Enterprise Desktop Administration
Chapter 11
Manage System Initialization

2. Objectives Objective 1—Describe the Linux Load Procedure
Objective 2—GRUB (Grand Unified Bootloader)
Objective 3—Manage Runlevels
SUSE Linux Enterprise Desktop Administration

3. Objective 1—Describe 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
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4. SUSE Linux Enterprise Desktop Administration
Figure 11-1 The basic steps of booting a computer with Linux
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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
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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
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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
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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
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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
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10. Objective 2—GRUB (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
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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
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12. What a Boot Manager Is (continued)
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 1—Usually installed in the Master Boot Record (MBR) of the hard disk
Stage 2—Usually contains the actual boot loader
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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
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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
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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)
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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
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17. Configure GRUB with YaST
To start the YaST Boot Loader module
Start YaST, enter the root password, and then select System > 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
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18. SUSE Linux Enterprise Desktop Administration
Figure 11-2 YaST Boot Loader module
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19. SUSE Linux Enterprise Desktop Administration
Figure 11-3 Section Management
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20. SUSE Linux Enterprise Desktop Administration
Figure 11-4 Clone Selected Section
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21. SUSE Linux Enterprise Desktop Administration
Figure 11-4 Clone Selected Section
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22. SUSE Linux Enterprise Desktop Administration
Figure 11-5 Boot Loader Installation tab
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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 init=new_init_program, you can change the first program loaded by the kernel
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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
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25. Exercise 11-1: Manage the Boot Loader
In this exercise, you practice booting into a shell and modifying /boot/grub/menu.lst
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26. Objective 3—Manage 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
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27. The init Program and Linux Runlevels
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
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28. The init Program and Linux Runlevels (continued)
The runlevels
Runlevels define the state of the system
Table 11-1 The available runlevels
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29. The init Program and Linux Runlevels (continued)
Init configuration file (/etc/inittab)
Each line in the /etc/inittab file uses the following syntax: id:rl:action:process
The first entry in the /etc/inittab file contains the following parameters: id:5:initdefault:
The next entry in /etc/inittab looks like this: si::bootwait:/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
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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
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31. init Scripts and Runlevel Directories (continued)
Table 11-2 /etc/init.d/script parameters
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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
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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/
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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
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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
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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
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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 System>System Services (Runlevel)
Or open a terminal window and, as root, enter yast2 runlevel
Modes
Simple mode
Expert mode
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38. SUSE Linux Enterprise Desktop Administration
Figure 11-6 The YaST Runlevel Editor module
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39. SUSE Linux Enterprise Desktop Administration
Figure 11-7 The YaST Runlevel Editor module in expert mode
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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
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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
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42. Exercise 11-2: Manage Runlevels
In this exercise, you practice configuring runlevels
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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
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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
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