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Chapter 9:
Understanding System
Initialization
The Complete Guide To Linux System
Administration
Objectives
• Describe how standard PC hardware starts an
operating system
• Configure how the LILO or GRUB boot loader
starts the Linux kernel
• Configure the init program and the scripts used to
start system services
• Manage system services after start-up
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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
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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
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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
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Hardware Initialization (continued)
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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
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The Boot Loader (continued)
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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
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The Boot Loader (continued)
• uname command
– Display information about kernel
– Full version number of current kernel
• uname -a
– Timestamp of kernel
• uname -v
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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
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Starting the Kernel (continued)
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Starting the Kernel (continued)
• Kernel ring buffer
– Area of memory
– Stores messages from kernel
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Using Boot Parameters
• Boot parameters
– Instruct Linux how to operate or access parts of
computer system’s hardware
– Use to activate features of Linux
• To enter
– GRUB
• Press “a”
– LILO
• Enter after operating system name
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Configuring Boot Loaders
• Configuration files
– Created by installation program
– Examine how boot loader operates
– Usually no need to change
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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
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Using LILO (continued)
• Boot image
– Each version of Linux
• Parameters
–
–
–
–
boot
delay
image
other
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Using GRUB
• Installed by default for many versions of Linux
• /boot/grub/grub.conf
– Configuration file for GRUB
– Similar in layout and syntax to lilo.conf
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Using GRUB (continued)
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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
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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
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Understanding Run Levels
(continued)
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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
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Understanding rc Scripts
(continued)
• Each file begins with K or S followed by two-digit
number
– Begin with K―stopped (killed)
– Begin with S―started
• Files symbolic links to scripts that stop and start
services
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Understanding rc Scripts
(continued)
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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
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Managing Services
• service command
– Control service
– Must know service name
– Examples:
• service lpd restart
• service named start
• service named status
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Managing Services (continued)
• chkconfig command
– 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
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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
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Managing Services Graphically
(continued)
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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
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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
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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
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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
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Shutting Down Linux (continued)
• Shut down Linux gracefully
–
–
–
–
–
Use reboot command
Use halt command
Use shutdown command
Pressing Ctrl+Alt+Del
Entering the command telinit 0
• Halt system
– Entering command telinit 6
• Restart system
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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
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Summary
• CPU BIOS and other hardware components of
computer must initialize themselves before
attempting to load operating system
• Boot loader typically first software loaded on
Linux system
• Multiple boot loaders can be installed on one
system and chained together
• Messages generated by kernel after it starts are
stored in kernel ring buffer
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Summary (continued)
• Boot loader can pass parameters to Linux kernel
• LILO and GRUB configured using text
configuration files
– Stored in /boot directory
• init program
– First process started by Linux kernel
– Runs several scripts that start numerous daemons
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Summary (continued)
• Run level mode of operation defines what
processes are started at boot time
• Must gracefully shut down Linux to avoid data
loss
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