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Linux Run Levels

The Init Program

As seen in the previous section, the kernel will start a program called init, if it finds it. The init process reads the file "/etc/inittab" and uses this file to determine how to create processes. Read the init man page for more information. Also note that init is always running and can dynamically do things and run processes based upon various signals. The administrator can also cause it to dynamically change system processes and runlevels by using the telinit program or editing the "/etc/inittab" file. Red Hat as well as most of its derivatives including Suse uses run levels in a following way:

• 0 - Halt, init 0 is similar to shutdown -h now
• 1 - Single user, runs just rc.sysinit. 
• 2 - Multiuser, without NFS (The same as 3, if you don't have networking)
• 3 - Full multi-user, console logins only. 
• 4 - Not used/undefined
• 5 - Full multi-user, with X11 and display manager as well as console logins
• 6 - Reboot

Which services are started in which runlevels can be managed with the chkconfig tool, which keeps its configuration settings in a elaborate arrangement under /etc/rc.d/.

/sbin/chkconfig --list lists all the services controlled by chkconfig and whether they are on/off for each runlevel. Setting a service A controlled by chkconfig, for levels X, Y and Z is achieved using /etc/chkconfig --level XYZ A

See Runlevel - Wikipedia, the free encyclopedia

Default level is defined in /etc/inittab, for example id:5:initdefault

The inittab file

The "/etc/inittab" file tells init which runlevel to start the system at and describes the processes to be run at each runlevel. An entry in the inittab file has the following format:

id:runlevels:action:process

• id - A unique sequence of 1-4 characters which identifies an entry in inittab.
• runlevels - Lists the runlevels for which the specified action should be taken. This field may contain multiple characters for different runlevels allowing a particular process to run at multiple runlevels. For example, 123 specifies that the process should be started in runlevels 1, 2, and 3.
• action - Describes which action should be taken. Valid actions are listed below
  • respawn - The process will be restarted whenever it terminates.
  • wait - The process will be started once when the specified runlevel is entered and init will wait for its termination.
  • once - The process will be executed once when the specified runlevel is entered
  • boot - The process will be executed during system boot. The runlevels field is ignored.
  • bootwait - Same as "boot" above, but init waits for its termination.
  • off - This does nothing.
  • ondemand - This process will be executed whenever the specified ondemand runlevel is called.
  • initdefault - Specifies the runlevel which should be entered after system boot. If none exists, init will ask for a runlevel on the console. The process field is ignored.
  • sysinit - The process will be executed during system boot. It will be executed before any boot or bootwait entries. The runlevels field is ignored.
  • powerwait - The process will be executed when init receives the SIGPWR signal. Init will wait for the process to finish before continuing.
  • powerfail - Same as powerwait but init does not wait for the process to complete.
  • powerokwait - The process will be executed when init receives the SIGPWR signal provided there is a file called "/etc/powerstatus" containing the word "OK". This means that the power has come back again.
  • ctrlaltdel - This process is executed when init receives the SIGINT signal. This means someone on the system console has pressed the "CTRL-ALT-DEL" key combination.
  • kbrequest - The process will be executed when init receives a signal from the keyboard handler that a special key combination was pressed on the console keyboard.
  • process - Specifies the process to be executed. If the process starts with the '+' character, init will not do utmp and wtmp accounting for that process. This is needed for gettys that insist on doing their own utmp/wtmp housekeeping (a historic bug).
  Below is an example file:

  	# inittab       This file describes how the INIT process should set up
  	#               the system in a certain run-level.
  	#
  	# Author:       Miquel van Smoorenburg, <miquels@drinkel.nl.mugnet.org>
  	#               Modified for RHS Linux by Marc Ewing and Donnie Barnes
  	#
  
  	# Default runlevel. The runlevels used by RHS are:
  	#   0 - halt (Do NOT set initdefault to this)
  	#   1 - Single user mode
  	#   2 - Multiuser, without NFS (The same as 3, if you do not have networking)
  	#   3 - Full multiuser mode
  	#   4 - unused
  	#   5 - X11
  	#   6 - reboot (Do NOT set initdefault to this)
  	# 
  1)	id:3:initdefault:
  
  	# System initialization.
  2)	si::sysinit:/etc/rc.d/rc.sysinit
  
  3)	l0:0:wait:/etc/rc.d/rc 0
  4)	l1:1:wait:/etc/rc.d/rc 1
  5)	l2:2:wait:/etc/rc.d/rc 2
  6)	l3:3:wait:/etc/rc.d/rc 3
  7)	l4:4:wait:/etc/rc.d/rc 4
  8)	l5:5:wait:/etc/rc.d/rc 5
  9)	l6:6:wait:/etc/rc.d/rc 6
  
  	# Things to run in every runlevel.
  10)	ud::once:/sbin/update
  
  	# Trap CTRL-ALT-DELETE
  11)	ca::ctrlaltdel:/sbin/shutdown -t3 -r now
  
  	# When our UPS tells us power has failed, assume we have a few minutes
  	# of power left.  Schedule a shutdown for 2 minutes from now.
  	# This does, of course, assume you have powerd installed and your
  	# UPS connected and working correctly.  
  12)	pf::powerfail:/sbin/shutdown -f -h +2 "Power Failure; System Shutting Down"
  
  	# If power was restored before the shutdown kicked in, cancel it.
  13)	pr:12345:powerokwait:/sbin/shutdown -c "Power Restored; Shutdown Cancelled"
  
  
  	# Run gettys in standard runlevels
  14)	1:2345:respawn:/sbin/mingetty tty1
  15)	2:2345:respawn:/sbin/mingetty tty2
  16)	3:2345:respawn:/sbin/mingetty tty3
  17)	4:2345:respawn:/sbin/mingetty tty4
  18)	5:2345:respawn:/sbin/mingetty tty5
  19)	6:2345:respawn:/sbin/mingetty tty6
  
  	# Run xdm in runlevel 5
  	# xdm is now a separate service
  20)	x:5:respawn:/etc/X11/prefdm -nodaemon
  

  On the left side of the file listing, above, are added numbers to help describe lines. Those lines without line numbers are either blank or begin with a "#" which means the line is a comment. Those line numbers are not part of the original file and are added here for reference purposes.
• On line 1 above you see "id:3:initdefault:". The id is "id" which stands for initdefault. Note that it is unique on all the numbered lines. The runlevel is 3 which sets the default starting runlevel to runlevel 3. The action is initdefault which tells init to make this runlevel the default runlevel. Note that the process field is blank since it is ignored by the initdefault action.
• Line 2 tells init to run the program "/etc/rc.d/rc.sysinit" during system boot, before any other processes.
• Lines 3 through 9 tell init to run the program "/etc/rc.d/rc" for runlevels 0 through 6. Note that for each line the appropriate runlevel is passed to the "/etc/rc.d/rc" script program on the command line. For example note on line 5 above the second field is the runlevel specifying 2. At the end of the line there is a space and a 2 which allows the variable 2 to be passed on the command line to the program.
• Line 10 specifies that the program "/sbin/update" will run once for every runlevel.
• Line 11 sets up the program "/sbin/shutdown" to run when someone on the system console has pressed the "CTRL-ALT-DEL" key combination.
• Line 12 specifies "/sbin/shutdown" to run if the power fails. Note that there are different options passed on the command line for lines 11 and 12 although they run the same program.
• Line 13 specified "/sbin/shutdown" will run if power is restored for any of runlevels 1 through 5.
• Lines 14 through 19 specifies the "/sbin/mingetty" program to run on 6 different terminals for runlevels 2 through 5. This means that you can run 6 virtual terminals from your keyboard simultaneously by pressing "ALT-F1" through "ALT-F6". Note pressing "ALT-F7" or above will do nothing, but the screen will not change from your current terminal.

Note the order of programs to run as specified above are:

1. /etc/rc.d/rc.sysinit
2. /etc/sbin/update
3. /etc/rc.d/rc 3 - Note: we are running runlevel 3 here.

Therefore, the next thing that the system does is to run the rc.sysinit file, save buffers to the hard drive, then run system script files for the requested runlevel which will start up many system and network services as explained in the next section.

Determining the Default Boot runlevel

The default boot runlevel is set in the file /etc/inittab with the initdefault variable. When set to 3, the system boots up with the text interface on the VGA console; when set to 5, you get the GUI. Here is a snippet of the file (delete the initdefault line you don't need):

# Default runlevel. The runlevels used by RHS are:
# 0 - halt (Do NOT set initdefault to this)
# 1 - Single user mode
# 2 - Multiuser, without NFS (The same as 3, if you do not have networking)
# 3 - Full multiuser mode
# 4 - unused
# 5 - X11
# 6 - reboot (Do NOT set initdefault to this)
# 
id:3:initdefault:                         # Console Text Mode
id:5:initdefault:                         # Console GUI Mode

Note the following:

• Most home users boot up with a Windows like GUI (runlevel 5)
• Most techies will tend to boot up with a plain text-based command-line-type interface (runlevel 3)
• Changing initdefault from 3 to 5, or vice-versa, has an effect upon your next reboot. See the following section on how to get a GUI login all the time until the next reboot.
• Of course, don't set the initdefault value to 6 or your system will constantly reboot. Setting it to 0 will never allow it to start!

Last modified: November 08, 2008