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Transcript LinuxBootSequence

The Linux Kernel

• About 6 million lines of code • Controls memory and process management.

The Linux Kernel

Linux & the Kernel

Kernel Version Numbering:


• A is the version • B is the major revision • C is the minor revision • D are security and bug fixes • 2.6.39 was the last minor revision before 3.0 is released.

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

Kernel Startup

• Checks memory • Probes for hardware

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.

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



• Kernel passes control to init.

Kernel startup

• Create tables for memory management.

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)

Dummy processes

• Swapper • vhand, kflushd, kpiod, mdrecoveryd

Kernel Startup

• Find and mount the other partitions starting with the swap partition space.

– The /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


• 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.

init: Processes

• The basis of all Linux systems. • Run perpetually in the background waiting for input. – Services – Daemons – Shells – utilities

init:Services & Daemons

• Examples: • login prompt • X-Window server • keyboard input functionality • firewall • and all classic server programs; e-mail, DNS, FTP, telnet, ssh, etc.


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.


• Additional programs and services are started when


reads the contents of 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.


• 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.

• Read /etc/fstab • Read /etc/inittab • Run /etc/rc.sysinit

• Run /etc/rc • Run the K* and S* startup scripts.

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.

rc Scripts

• They keep track of the large number of services to be managed.

• The main rc script is /etc/rc.d/rc


• Responsible for calling the appropriate scripts in the correct order for each run level.


• 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.

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

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.

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

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.

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.

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


• 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

Core Services

• init – process id 1.

• inetd – Traditional Unix service.

• xinetd – Linux version of inetd. Used to manage some services:

• syslogd – logs system messages to /var • cron – used to start utilities at a pre described time.

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.

inetd & xinetd

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


• 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.


• Linux implementation of inetd.

• A list of the services currently offered by xinetd are in /etc/xinetd.d directory.

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.

• /sbin/syslogd • /etc/syslogd.conf

• man syslogd



• Syntax of file involves – Facility: mail, kern, daemon, lpr, other services – Priority: emerg, alert, warning, err, notice, etc.

– Log file *.emerg @loghost,childrja,root Emergency messages are sent to machine loghost and to the console session of childrja and root.


• 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.


• /etc/cron.allow

• /etc/cron.deny

• Crontab file syntax: • Min Hr Day Month DayofWk command