The Linux Kernel
• About 6 million lines of code • Controls memory and process management.
The Linux Kernel
Linux & the Kernel
Kernel Version Numbering: 184.108.40.206
• 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
• 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.
• Create tables for memory management.
• Dummy processes are started. These processes do not have PIDs, and cannot be killed: – Swap – Paging virtual memory – IO activity – Other (example: managing threads)
• Swapper • vhand, kflushd, kpiod, mdrecoveryd
• 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.
• 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.
• 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.
• 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
• 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.
• 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.
• 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.
• 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.
• Crontab file syntax: • Min Hr Day Month DayofWk command