1. Booting and boot levels

2. …The Boot Process
Linux uses the init command and a directory structure based on run-levels to start running the system and loading processes
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3. The Initialisation Process…
System States (or run levels) on early UNIX systems grew out of a need to separate how the system ran depending on the maintenance being performed
Usually, this meant that to add new hardware (or software) a system reboot was necessary
Today, with hot swappable devices, it is not strictly necessary to reboot systems in order to install new hardware or software
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4. …The initialisation Process…
The /etc/inittab file contains settings for the init process and also defines the run levels for a Linux system
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5. /etc/inittab file # Default runlevel. The runlevels used by RHS are:
# halt (Do NOT set initdefault to this)
# Single user mode
# Multiuser, without NFS (The same as 3, if you do not have networking)
# Full multiuser mode
# unused
# X11
# reboot (Do NOT set initdefault to this) #
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6. init process
init is sometimes referred to as the ‘father of all processes’
init is responsible for creating processes from a script stored in /etc/inittab
Linux init command is compatible with the System V init command
init starts as the last step of the kernel booting
init is the first command that initialises and configures the system for use
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7. init process
Init works by parsing the /etc/inittab file and by running scripts in the /etc/rc.d directories depending on the required run level
Each individual script is designed to start or stop and individual service such as networking, mail, news, web, nfs, dhcp etc.
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8. /etc/rc.d directory
The /etc/rc.d directory contains the following files:
rc, rc.local, rc.sysinit
And the following directories:
init.d
rc0.d
rc1.d
rc2.d
rc3.d
rc4.d
rc5.d
rc6.d
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9. Startup Scripts
One of the most important scripts in /etc/inttab is rc.sysinit
When init parses the inittab file, rc.sysinit is the first script found and executed
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10. rc.sysinit script Sets some initial $PATH variables
Configures Networking
Sets up Swapping for Virtual Memory
Sets the system hostname
Checks root file systems for possible repairs
Checks root filesystem quotas
Turns on user and group quotas for root file system
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11. rc.sysinit script Remounts the root filesystem read/write
Clears the mounted filesystems table /etc/mtab
Enters the root filesystem into mtab
Readies the system for loading modules
Finds module dependencies
Checks filesystems for possible repairs
Mounts all other file systems
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12. rc.sysinit script Deletes UUCP lock files
Deletes stale subsystem files
Deletes stale pid files
Sets the system clock
Turns on swapping
Initialises the serial ports
Loads Modules
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13. rc.local script
After the rc.sysinit script is run, init runs the rc.local script to carry out any site-specific operations that may be required upon system startup
rc.local may examine the local system and create a message for the login prompt based on the hardware and software versions found
If it does, it will put such details in the file /etc/issue which may be displayed on login
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14. Run Level 0: /etc/rc.d/rc0.d Starts the shutdown sequence
Kills All Processes
Turns off Virtual Memory File Swapping
Unmounts swap and mounted file systems
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15. Run Level 1: /etc/rc,d/rc1.d Single user mode, or administrative state
Used by sys admins while performing software maintenance
Nobody else can log in
Networking is turned off, but file systems are mounted
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16. Run Level 2: /etc/rc.d/rc2.d Multiuser state
Networking is enabled, NFS is disabled
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17. Run Level 3: /etc/rc.d/rc3.d
Usually the default run level specified as the first line in the /etc/inittab file
Remote file sharing is enabled along with all other desired services
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18. Run Level 4: /etc/rc4.d Usually empty, not used
If you want to define your own run level, this can be used to set up the appropriate links
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19. Run Level 5: /etc/rc.d/rc5.d
Similar to the default run level, but with the named (internet DNS server)
Usually the default run level for X11
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20. Run Level 6: /etc/rc5.d Reboot run level
Contains links similar to level 0, but logic in the halt script in init.d determines whether the system is being shut down or rebooted
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21. Run Levels…
Each run-level is defined by the services (daemons) that are normally running in that level
The services to stop and start for each run level are defined in the rcX.d directories mentioned above
These directories contain symbolic links to master scripts contained in the /etc/rc.d/init.d directory
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22. …Run Levels…
The symbolic links contained in the rcX.d directories must be named so as to start with capital K or capital S
For example: rc3.d contains the following symbolic links:
K05saslauthd K87portmap S10network S25netfs S80sendmail S95anacron
Any other files or symbolic links contained in the rcX.d directories will be ignored by the rc script
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23. …Run Levels…
In addition to the inclusion of K or S at the start of the symbolic links, the file names also carry a number between 00 and 99
The combination of K or S followed by a number means that when the files are listed alphabetically, they appear in a particular order
This order determines the order in which the scripts are run by the rc script
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24. …Run Levels
When a symbolic link beginning with K is encuntered, rc runs that script with the stop parameter
When a symbolic link beginning with S is encountered, rc runs that script with the start parameter
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25. /etc/rc/d/init.d directory
This directory contains the master copies of all the scripts that may contain symbolic links in the rcX.d directories
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26. Adding Services to Run Levels
If we want a service to start in a given run level we must:
Create or copy the start/stop/status script to the /etc/rc.d/init.d directory
Create a symbolic link in the required run level directory to start the script in the appropriate sequence
ln -s /etc/rc.d/init.d/isdn /etc/rc.d/rc3.d/S20isdn
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27. Init
Specify the run-level to put the machine into
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28. Shutdown shutdown [-krhfnc] [-t secs] time [warning message]
-k: Don’t really shut down, just warn
-r: reboot after shutdown
-h: halt after shutdown
-f: do a ‘fast’ reboot
-n: do not go through ‘init’ but go down real fast
-c: cancel a running shutdown
-t <sec>: delay between warning and kill signal
Shutdown –t60 –r now
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29. Halt
Notifies the kernel of a shutdown or reboot
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30. Reboot
Symbolic link to halt
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