1. The Linux Kernel About 6 million lines of code
Controls memory and process management.

2. The Linux Kernel

3. Linux & the Kernel

4. Kernel Version Numbering: 2.6.38.8
A.B.C.D
A is the version
B is the major revision
C is the minor revision
D are security and bug fixes
was the last minor revision before 3.0 is released.

5. Linux Boot Sequence
After the BIOS/UEFI instructions are given to start the OS,
A compressed version of the kernel is loaded into the first megabyte of ram.
The complete kernel is expanded and loaded
The kernel starts

6. Kernel Startup
Checks memory
Probes for hardware

7. Kernel Hardware Probe
Some drivers are compiled into the kernel other drivers are loaded when the kernel starts.
Everything recognized by the os has a file node associated with it. The kernel will establish nodes for each required device.

8. Starting the kernel Find and set up swap (virtual memory)
A hardware probe is done to determine what drivers should be initialized.
Read /etc/fstab to mount the root file system.
Mount other devices
Start a program called init.
Kernel passes control to init.

9. Kernel startup
Create tables for memory management.

10. Kernel Startup
Dummy processes are started. These processes do not have PIDs, and cannot be killed:
Swap
Paging virtual memory
IO activity
Other (example: managing threads)

11. Dummy processes
Swapper
vhand, kflushd, kpiod, mdrecoveryd

12. Kernel Startup
Find and mount the other partitions starting with the swap partition space.
The /etc/fstab

13. /etc/fstab
/dev/root
/proc –A virtual file-system (in memory) used for managing processes. This allows loadable runtime modules to be inserted and removed as needed.
/dev/pts – A VFS used for managing devices

14. init The first non-dummy process, PID = 1.
First process to run and is always running.
The kernel passes process management to init.
Init reads its configuration file: /etc/inittab
Directly or indirectly spawns all other processes until shutdown.

15. init: Processes The basis of all Linux systems.
Run perpetually in the background waiting for input.
Services
Daemons
Shells
utilities

16. init:Services & Daemons
Examples:
login prompt
X-Window server
keyboard input functionality
firewall
and all classic server programs; , DNS, FTP, telnet, ssh, etc.

17. init the runlevel is determined by inittab: 0. Halt the system
1. Enter single-user mode (no networking)
2. Multiuser mode, without NFS.
3. Full multiuser mode
4. Unused
5. Same as runlevel 3 but with X-windows.
6. Reboot.

18. /etc/inittab
Additional programs and services are started when init reads the contents of inittab.

19. inittab Each line has the same syntax: Id:runlevels:action:process
Id A unique sequence of 1-4 characters to identify the line in the file (a macro).
Runlevels are any combination of numbers from 0-6. If blank it implies all run levels.
Action- what should happen.
Process – the actual process to run.

20. Action Respawn – restart whenever it terminates.
Wait – don’t continue until it completes.
Once – will not wait for completion.
boot – run at boot time.
Bootwait – start at boot time and wait for completion before continuing.
ondemand, initdefault, sysinit, powerwait, powerfail, powerkwait, ctrlaltdel.

21. Read /etc/fstab
Read /etc/inittab
Run /etc/rc.sysinit
Run /etc/rc
Run the K* and S* startup scripts.

22. inittab and sysinit
The file rc.sysinit is a script run once at boot time to set up services specific to the computer system:
Hostname
Networking
Etc.

23. rc Scripts
They keep track of the large number of services to be managed.
The main rc script is /etc/rc.d/rc

24. /etc/rc.d/rc
Responsible for calling the appropriate scripts in the correct order for each run level.

25. /etc/rc.d/rc[0-6].d
There is an additional directory for each different runlevel: 0-6.
Each directory has the scripts needed to run the specified boot level.
The files in these subdirectories have two classifications: kill and start.

26. Kill and Start Services.
The letter K is the prefix to all services to be killed.
The letter S is the prefix to all the services to be Started.
The K & S prefixed names are links to the actual service scripts located in /etc/rc.d/init.d

27. Order of services A number follows the S or K prefix.
The number determines the order of execution.
The rc file accesses the correct run-level directory and executes the files in numerical order.
K files are first, then S files.

28. K and S scripts
Every script in the /etc/rc.d/init.d directory is run with one of two options:
stop if the prefix is K
start if the prefix is S
/etc/rc.d/init.d/xfs start
/etc/rc/d/init.d/sound stop

29. Adding services
If a service need be installed at boot time, you can edit the /etc/rc.d/rc.local file.
Or add functionality by adding a script to the /etc/rc.d/init.d directory.

30. Contents of Custom RC script
A description of the scripts purpose.
Verify the program really exists before attempting to start it.
Parse for start and stop command line options.
Put your script in the /etc/rc.d/init.d directory.

31. Link the New RC script
Add new service script to the /etc/rc.d/init.d directory.
cd to the correct run level directories,
cd /etc/rc.d/rc5.d
Add the link
ln –s ../init.d/mynewservice S98mynewservice

32. /etc/rc.d/rc.local
You can also add a service by modifying the /etc/rc.d/rc.local file.
/etc/rc.d/rc[0-6].d/[K,S]99local

33. Core Services init – process id 1. inetd – Traditional Unix service.
xinetd – Linux version of inetd. Used to manage some services: xinetd.org/faq.html
syslogd – logs system messages to /var
cron – used to start utilities at a pre-described time.

34. Review: init The parent process of all processes.
Started by the kernel at boot time.
If a process dies before all of its children complete, then the children inherit init as the parent (PPID = 1).
Controls the run level with /etc/inittab.

35. inetd & xinetd
Daemons – independent background processes that poll for events.
Events sent to the daemons determine how the daemon behaves at any given time.

36. inetd
Pre-Linux tool.
The supervisor of network server-related processes.
Rather than running many daemons taking up memory, initd polls for daemon requests. When a particular daemon gets an event, initd will activate the appropriate daemon.

37. xinetd Linux implementation of inetd.
A list of the services currently offered by xinetd are in /etc/xinetd.d directory.

38. syslogd daemon
With programs/services disconnected from a terminal, where does their standard output go?
Syslogd routes the output of services to text files.
Most log files are in /var/log
Works in a heterogeneous environment.

39. syslogd
/sbin/syslogd
/etc/syslogd.conf
man syslogd

40. /etc/syslog.conf Syntax of file involves
Facility: mail, kern, daemon, lpr, other services
Priority: emerg, alert, warning, err, notice, etc.
Log file
Emergency messages are sent to machine loghost and to the console session of childrja and root.

41. cron Used to schedule programs to run.
Cron wakes up once every minute and checks all the crontab files on the system.
If an entry in one of the crontab files matches the date and time, then the designated process is run.

42. crontab /etc/cron.allow /etc/cron.deny Crontab file syntax:
Min Hr Day Month DayofWk command