Booting and Shutting Down
Linux- and UNIX-based
Operating Systems
Rithy Chhay
CMSC 691X – Gary Burt
June 4, 2002
Overview





Bootstrapping
Booting PC’s
Booting in Single-User Mode
Startup Scripts
Rebooting and Shutting Down
Bootstrapping
 The process in which the kernel is loaded into
memory and begins to execute.
 Bootstrap spawns the init process which executes
“rc files” which check and mount the file system
and start system daemons.
 The system can be booted two ways:
– Automatically
 Ordinarily, systems are booted this way.
– Manually
 Used by system administrators for setup or troubleshooting.
 Computer is in “Single-User Mode”.
The Boot Process
 There are generally six distinct phases of
booting up a system
– Loading and Initializing the kernel
– Device Detection and Configuration
– Creation of Spontaneous System Processes
– Operator Intervention (Manual Boot Only)
– Execution of System Startup Scripts
– Multi-user Operation
Kernel Initialization
 The UNIX kernel is a program.
 The kernel usually resides in the root
partition of the UNIX file system.
 Most Linux distributions call this partition
either “/vmlinuz” or “/boot”.
 Other UNIX flavored systems may call this
partition “/unix”, “/vmunix”, or “/kernel”.
 Almost every vendor of Linux or UNIX calls
their kernel pathname something different.
Hardware Configuration
 The kernel inspects the installed hardware
on the system and attempts to mount the
proper drivers for each device that you tell
the kernel it should expect to find
 This is visually noticeable during the cryptic
messages that appear on the screen while
the computer boots
 Devices that have missing drivers or that did
not respond during boot-up will be disabled
and will not be accessible to UNIX until the
system is rebooted.
System Processes
 After basic initializations have been completed,
processes which are NOT created by the UNIX
fork mechanism execute.
 The number of these processes vary in the various
flavors of UNIX and Linux, but init is the common
process throughout them all.
 The purpose of these system processes is to
handle tasks that the kernel would handle but
have been created separately for scheduling or
architectural reasons.
 At this point, the bootstrapping process is
complete, but none of the processes that handle
basic operations have been created. Those
processes are taken cared of by init.
Operator Intervention (Manual Boot)
 A command-line flag is brought up to signal init to go into
single-user mode.
 From here you can choose which scripts to run, but only
the root partition is mounted so you must first mount other
partitions before you can start processed off of those
partitions.
 Daemons do not typically run in single-user mode.
 Many single-user environments are mounted as read-only
by default, therefore if “/tmp” is apart of the root partition
then programs that need to write to that location, will not
work.
 Normally, the filesystem is checked automatically by fsck,
but you must do this manually during single-user mode.
 After the single-user shell has completed, the system will
resume automated boot-up.
Multi-user Operation
 After Initialization scripts have ran, the
system is fully operational, but additional
processes have to run in order to allow
multiple users to log on to the system.
 The getty process listen for logins which is
spawned from init.
 Graphical login systems include xdm, gdm,
and dtlogin.
Booting PC’s
 PC’s have simplistic ROM codes compared
to firmware found on UNIX machines.
 UNIX firmware generally knows what
devices are installed, how to configure them
and use them on a basic level.
 Once booting begins, your machine will try
to load the first 512-byte segment from disk.
 This segment is known as the MBR (Master
Boot Record).
MBR and LILO
 The MBR is what loads the kernel from disk.
 A popular Linux boot loader is LILO.
 LILO can be installed on either the MBR or
the boot record of the Linux root partition.
 Installing LILO on the boot record allows
you to use a different boot loader for another
operating system like Windows XP.
 LILO can be configured through the “lilo”
command. Its contents are stored at
“/etc/lilo.conf”. See the man pages for
more details on how to configure LILO.
Other Boot Loaders
 GRUB, LOADLIN, boot0cfg & disklabel
are other boot loaders from either different
flavors of UNIX or Linux or third party boot
loaders which offer various options.
 UNIX and UNIX-based systems for non-PC
hardware machines generally have
proprietary system specific schemes for
booting their kernels.
Startup Scripts
 These are scripts which are run by init to
complete the bootstrap process.
 Generally speaking, there are two types of
system startup scripts.
– System V style scripts
 Used by most UNIX systems like Solaris, HP-UX and
most distributions of Linux.
– BSD style scripts
 Used by flavors of BSD such as FreeBSD, NetBSD,
OpenBSD and BSDi.
System V Style Scripts
 These startup scripts are typically centrally located
in the init.d directory located under “/etc”.
– RedHat Linux stores these files under the “/etc/rc.d”
directory.
– HP-UX stores these files under the “/sbin” directory.
 Generally, each startup script can accept two
command line arguments; start and stop.
– Start instructs the script to begin when it is suppose to
– Stop instructs the script to end when it is suppose to
Run Level States
 There are “run level” states in which represent a
particular complement of services that the system
should be running.
–
–
–
–
0: system shutdown
1 or S: single-user mode
2 – 5: multi-user mode
6: reboot
 Each run level has its own set of startup scripts.
 Typically these state directories are designated
rcn.d where n represents the specific run level
directory.
– i.e. The scripts for run level 4 would be located in the
rc4.d directory.
 Refer to the man pages regarding specific details
for your operating system’s startup scripts.
Shutting Down UNIX
 UNIX systems must be gracefully powered down.
 Failure to properly shut down can cause damage to the
system.
 There are various was to shut down a UNIX system;
generally a command to shut down the system is issued
from the command line.
 Here are a few commands:
– shutdown: Broadcasts a message to all users logged in prompting
a system shut down.
– halt: Performs essential duties required to bring the system down,
waits for the filesystem writes to complete then halts the kernel.
– reboot: Executes identically to halt with the excepting that it causes
to system to restart from scratch rather than halting the kernel.
– kill init: This is not recommended but will cause the system to
shutdown. Since init is the parent of all processes, all child
processed will be terminated when init is killed.