Linux Init Process / PC Boot Procedure
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- BIOS: The Basic Input/Output System is the lowest level interface between the computer and peripherals.
The BIOS performs integrity checks on memory and seeks instructions on the Master Boor Record (MBR) on the floppy drive or hard drive. - The MBR points to the boot loader (GRUB or LILO: Linux boot loader).
- Boot loader (GRUB or LILO) will then ask for the OS label which will identify which kernel to run and where it is located (hard drive and partition specified). The installation process requires to creation/identification of partitions and where to install the OS. GRUB/LILO are also configured during this process. The boot loader then loads the Linux operating system.
- See the YoLinux tutorial oncreating a boot diskfor more information on GRUB and LILO and also to learn how to put the MBR and boot loader on a floppy for system recovery.
- The first thing the kernel does is to executeinitprogram. Init is the root/parent of all processes executing on Linux.
- The first processes that init starts is a scrīpt/etc/rc.d/rc.sysinit
- Based on the appropriate run-level, scrīpts are executed to start various processes to run the system and make it functional.
| The Linux Init Processes: |
The init process is the last step in the boot procedure and identified by process id "1". Init is responsible for starting system processes as defined in the/etc/inittabfile. Init typically will start multiple instances of "getty" which waits for console logins which spawn one's user shell process. Upon shutdown, init controls the sequence and processes for shutdown. The init process is never shut down. It is a user process and not a kernel system process although it does run as root.
System Processes:
Process ID Descrīption 0 The Scheduler 1 The init process 2 kflushd 3 kupdate 4 kpiod 5 kswapd 6 mdrecoveryd
Init config file (Red Hat 7.3-9.0, Fedora Core 1-3):/etc/inittab
# Author: Miquel van Smoorenburg,
# Modified for RHS Linux by Marc Ewing and Donnie Barnes
id:3:initdefault:
# System initialization.
si::sysinit:/etc/rc.d/rc.sysinit
l0:0:wait:/etc/rc.d/rc 0
l1:1:wait:/etc/rc.d/rc 1
l2:2:wait:/etc/rc.d/rc 2
l3:3:wait:/etc/rc.d/rc 3
l4:4:wait:/etc/rc.d/rc 4
l5:5:wait:/etc/rc.d/rc 5
l6:6:wait:/etc/rc.d/rc 6
# Things to run in every runlevel.This line is only in Red Hat 7.X Used to flush disk buffers.
ud::once:/sbin/update
# Trap CTRL-ALT-DELETE
ca::ctrlaltdel:/sbin/shutdown -t3 -r now
# When our UPS tells us power has failed, schedule a shutdown for 2 minutes from now.
pf::powerfail:/sbin/shutdown -f -h +2 "Power Failure; System Shutting Down"
# If power was restored before the shutdown kicked in, cancel it.
pr:12345:powerokwait:/sbin/shutdown -c "Power Restored; Shutdown Canceled"
# Run gettys in standard runlevels
1:2345:respawn:/sbin/mingetty tty1
2:2345:respawn:/sbin/mingetty tty2
3:2345:respawn:/sbin/mingetty tty3
4:2345:respawn:/sbin/mingetty tty4
5:2345:respawn:/sbin/mingetty tty5
6:2345:respawn:/sbin/mingetty tty6
# Run xdm in runlevel 5
x:5:respawn:/etc/X11/prefdm -nodaemon
Note that this config file directs the init process to run the shell scrīpt/etc/rc.d/rc.sysinit. This scrīpt should be used as is and NOT changed. Extendrc.localand NOT this scrīpt. This will (not in exact order):
- Run/sbin/initlog
- Run devfs to generate/manage system devices
- Run network scrīpts:/etc/sysconfig/network
- Start graphical boot (If so configured):rhgb
- Start console terminals, load keymap, system fonts and print console greeting:mingetty,setsysfonts
The various virtual console sessions can be viewed with the key-stroke:ctrl-alt-F1throughF6. F7 is reserved for the GUI screen invoked in run level 5. - Mount/procand start device controllers.
- Done with boot configuration for root drive. (initrd) Unmount root drive.
- Re-mount root file system as read/write
- Direct kernel to load kernel parameters and modules:sysctl,depmod,modprobe
- Set up clock:/etc/sysconfig/clock
- Perform disk operations based on fsck configuration
- Check/mount/check/enable quotas non-root file systems:fsck,mount,quotacheck,quotaon
- Initialize logical volume management:vgscan,/etc/lvmtab
- Activate syslog, write to log files:dmesg
- Configure sound:sndconfig
- Activate PAM
- Activate swapping:swapon
Local system boot processes can be placed in file:/etc/rc.d/rc.local
The system will then boot to the runlevel set by the directiveinitdefault.
Also see:
| Linux init Run Levels: |
LINUX has six states of operation of which "0" is the shutdown state and "3" and above are fully operational with all essential processes running for user interaction. Upon system boot the LINUX system/sbin/initprogram starts other processes by performing the following:
- Init will bring up the machine by starting processes as defined in the/etc/inittabfile.
- The computer will be booted to the runlevel as defined by theinitdefaultdirective in the/etc/inittabfile.
id:5:initdefault:
The inittab file will allow you to capture key sequences (ctrl-alt-del), start dial in internet connections etc.
- One of these process started by init is/sbin/rc. This scrīpt runs a series of scrīpts in the directories/etc/rc.d/rc0.d/,/etc/rc.d/rc1.d/,/etc/rc.d/rc2.d/, etc
- scrīpts in these directories are executed for each boot state of operation until it becomes fully operational. scrīpts beginning with S denote startup scrīpts while scrīpts beginning with K denote shutdown (kill) scrīpts. Numbers follow these letters to denote the order of execution. (lowest to highest)
Runlevel "3" will boot to text or console mode and "5" will boot to the graphical login mode ( "4" for slackware)
Runlevel scrīpts Directory
(Red Hat/Fedora Core)State 0 /etc/rc.d/rc0.d/ shutdown/halt system 1 /etc/rc.d/rc1.d/ Single user mode 2 /etc/rc.d/rc2.d/ Multiuser with no network services exported 3 /etc/rc.d/rc3.d/ Default text/console only start. Full multiuser 4 /etc/rc.d/rc4.d/ Reserved for local use. Also X-windows (Slackware/BSD) 5 /etc/rc.d/rc5.d/ XDM X-windows GUI mode (Redhat/System V) 6 /etc/rc.d/rc6.d/ Reboot s or S Single user/Maintenance mode (Slackware) M Multiuser mode (Slackware)
One may switch init levels by issuing the init command with the appropriate runlevel. Use the command "init#" where # is one of s,S,0,1,3,5,6. The commandtelinitdoes the same.
The scrīpts for a given run level are run during boot and shutdown. The scrīpts are found in the directory/etc/rc.d/rc#.d/where the symbol#represents the run level. i.e. the run level "3" will run all the scrīpts in the directory/etc/rc.d/rc3.d/which start with the letter "S" during system boot. This starts the background processes required by the system. During shutdown all scrīpts in the directory which begin with the letter "K" will be executed. This system provides an orderly way to bring the system to different states for production and maintenance modes.
If you installed all demons (background processes), Linux will run them all. To avoid slowing down your machine, remove unneeded services from the start-up procedure. You can start/stop individual demons by changing to the directory:
- /etc/rc.d/init.d/(Red Hat/Fedora )
- /etc/init.d/(S.u.s.e.)
- /etc/init.d/(Debian)
- cd /etc/rc.d/init.d/
(or/etc/init.d/for S.u.s.e. and Debian) - httpd stop
Use the command ps -aux to view all process on your machine.
TIP:List state and run level of all services which can be started by init:chkconfig --list
or
service --status-all | grep running(Red Hat/Fedora Core based systems)
GUI tool:/usr/X11R6/bin/tksysv
| Run Level Commands: |
- Shutdown:
- Reboot:
- Enter single user mode:
- init1
| Init scrīpt Activation: |
Adding a scrīpt to the/etc/rc.d/rc#.d/directory with either an S or K prefix, adds the scrīpt to the boot or shutdown process. The scrīpts are run in numerical order. S20abc is run before S30xyz. The extensibility to the boot and shutdown procedures of the operating system is one of the strengths of UNIX. The orderly sequential initiation of processes can be coordinated for dependent processes. The orderly shutdown of processes is often required of complex programs such as databases. This is how it is done. Individual processes may be monitored, shutdown and started at any time using these scrīpts. i.e./etc/rc.d/rc2.d/httpd start. The modifiersstart, stoporstatusmay be used.
The start/stop/status scrīpts actually reside in the directory:
- /etc/rc.d/init.d/(Red Hat/Fedora)
- /etc/init.d/(S.u.s.e. and Debian)
Basic services include:
| System Service | Descrīption |
|---|---|
| anacron | Run jobs which were scheduled for execution while computer was turned off. Catch up with system duties. |
| arpwatch | Keeps track of IP address to MAC address pairings |
| atd | Run scheduled batch jobs. |
| autofs | automounts file systems on demand. |
| crond | Job sheduler for periodic tasks. |
| gpm | Allows console terminal cut and paste. (Non X-window consoles) |
| https | Apache web server. |
| iptables | Firewall rules interface to kernel. |
| keytable | Loads selected keyboard map as set in/etc/sysconfig/keyboard |
| kudzu | New hardware probe/detection during system boot. |
| lpd | Network printer services. |
| microcode_ctl | Uploads microcode to kernel and ultimately to the Intle Pentium processor. (Hardware specific.) |
| mysqld | Database services |
| named | DNS name services (Bind) |
| network | Active network services during system boot. |
| nfs | Network file system. Unix file sharing services. |
| nscd | Password and group lookup services for use with network authentication (NIS, LDAP,...). |
| ntpd | Network Time Protocol time synchronization services. |
| random | Random number generation tool used for encryption. |
| rawdevices | Enables raw IO. Useful for Oracle and software which utilizes this for high speed disk access. |
| smb | SAMBA: MS/Windows PC file sharing services |
| syslog | System log file facility. |
| ypbind | NIS file sharing/authentication infrastructure service. |
| yppasswd | NIS file sharing/authentication infrastructure service. |
| ypserv | NIS file sharing/authentication infrastructure service. |
| xfs | X-Windows font server. |
Recommended basic services:anacron, ard, autofs, crond, gpm, iptables, keytable, kudzu, microcode_ctl(Intel32 hardware only),network, random. syslog
Graphics Workstation - add:xfs
File Server for PC clients - add:smb
Print Server - add:lpdorcups
File server Linux/Unix clients - add:nfs, netfs, nfslock, portmap, ypbind, yppasswd, ypserv
Web Server - add:httpd, tux, xinetdi, sshd
GUI configuration tools:
GUI tools can help you configure the appropriate services to start and provide a descrīption of each service available:- Red Hat 8.0/9.0:/usr/bin/redhat-config-services
- Fedora/RHEL:/usr/bin/system-config-services
(Also/usr/sbin/serviceconf) - Ubuntu 6 / Debian:
- /usr/bin/services-admin
- bum(Boot Up Manager)
Red Hat/Fedora Core text console services selection tool:/usr/sbin/ntsysv
| Init scrīpt: |
A single copy of the scrīpt is located in the directory:/etc/rc.d/init.d/scrīpt-name(Red Hat/Fedora) or/etc/init.d/scrīpt-name(Debian).
Use the commandchkconfigto generate soft links to the appropriate directories for the various run levels.
#!/bin/sh
#
# Startup scrīpt forprogram
#
# chkconfig: 345 85 15- This statement tells the chkconfig command how to add or delete this process to the boot process
# descrīption:Descrīption of program
# processname:process-name
# pidfile: /var/run/process-name.pid
# Source function library.This creates the operating environment for the process to be started
. /etc/rc.d/init.d/functions
case "$1" in
start)
echo -n "Startingprocess-name: "
daemonprocess-name- Starts only one process of a given name.
echo
touch /var/lock/subsys/process-name
;;
stop)
echo -n "Shutting downprocess-name: "
killprocprocess-name
echo
rm -f /var/lock/subsys/process-name
rm -f /var/run/process-name.pid- Only if process generates this file
;;
status)
statusprocess-name
;;
restart)
$0 stop
$0 start
;;
reload)
echo -n "Reloadingprocess-name: "
killprocprocess-name-HUP
echo
;;
*)
echo "Usage: $0 {start|stop|restart|reload|status}"
exit 1
esac
exit 0
The bash scrīpt functionsdaemon, killproc and statuscan all be found in the scrīpt/etc/rc.d/init.d/functions. (Red Hat/Fedora distributions)
The scrīpt must be executable to work. (chmod +xscrīpt-name).
The scrīpt may be used to start and stop processes. i.e.:
- /etc/rc.d/init.d/httpd restart
(S.u.s.e.:/etc/init.d/apache2 restart) - /etc/rc.d/init.d/httpd stop
- /etc/rc.d/init.d/httpd start
- service httpd restart
- service httpd stop
- service httpd start
Note that two lines in the scrīpt enable thechkconfigcommand to control the scrīpt for the boot and shutdown process.
# chkconfig: 345 85 15
# descrīption:Descrīption of program
When added to the boot process using the "chkconfig --addscrīpt-name" command the start order/priority will be set to 80 while the stop/shutdown order will be set to 15. The process will be added to runlevels 3, 4 and 5. This is enabled by generating links from the location of the scrīpt (/etc/rc.d/init.d/) to the directory for the appropriate run level:/etc/rc.d/rc#.d/. The file name in the run level directory will reflect if it is used for boot (starts with an "S") or shutdown (starts with a "K")
| chkconfig: |
The (Red Hat/Fedora/IRIX)chkconfigcommand generates and breaks links between the directory/etc/rc.d/init.d/and the appropriate run level directory:/etc/rc.d/rc[0-6].d/to control boot process initiation and process shutdown.
chkconfig [--level] on | off | reset >
chkconfig --list
chkconfig --list
chkconfig --add
chkconfig --del
chkconfig --level 0123456off
Examples:
- chkconfig --level 345 httpd on- forces apache to be invoked for run levels 3, 4 and 5.
- chkconfig --add httpd- Start the web server daemon upon system boot.
- chkconfig --del sendmail- Do not start the sendmail daemon upon system boot.
- chkconfig --list- List all services and init levels.
- chkconfig --list | grep on- List all services to be started upon system boot.
Using chkconfig to administer xinetd processes.
- chkconfig wu-ftpd on- Turn on FTP service managed by xinetd.
- chkconfig ipop3 off- Turn off POP3 service managed by xinetd.
Also see:chkconfig - Linux man page.
| Related Commands: |
service Display status of system services.
Example:service --status-all
Help:service --help
Also see:service - Linux man page.
| Links and resources: |
- Linux Standard Base Specification- System Initialization (Chapter 18)
- Devfs and the init process- How devices are defined upon boot
- LinuxNetMag.com: The Boot Process
- Redhat Manual: The Boot Process, Init, and Shutdown -7.1-6.2
www.yolinux.com/TUTORIALS/LinuxTutorialInitProcess.html
Chapter 18. System Initialization
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