Linux Run Levels

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The default runlevel for a system is specified in the /etc/inittab file, which will contain an entry such as id:3:initdefault: if the system starts in runlevel 3, or id:5:initdefault: if it starts in runlevel 5.

There are three method of determining the current runlevel

who -r command:

# who -r
run-level 3  Aug 27 10:12                   last=5

Using runlevel command which produces essentially the same result but in a form of just two numbers. Runlevel command retrieves information from the /var/run/utmp to locate the runlevel record.
The last method is not working in mainstream flavors of Linux (Red Hat 4 & 5 and Suse 10 & 11) but is an interesting option. In some versions of init from the environment variables RUNLEVEL and PREVLEVEL are populated and changed with each change, similar to $PWD and other system variables, for example:
```
echo $RUNLEVEL
```

The Init Program

After the kernel starts a program called init, the init process reads the file "/etc/inittab" and uses this file to determine how to create processes. It does it in stages called runlevels.

The administrator can also dynamically change the current runlevel by using the init program. the default runlevel into which the system is booted can be changed by editing the "/etc/inittab" file.

Red Hat as well as 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 set of link to scripts contained in /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:

/etc/rc.d/rc.sysinit
/etc/sbin/update
/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!

Old News

Cool Solutions Runlevels in SUSE Linux

Posted: 3 May 2005
Linux operating systems utilize runlevels to determine the services that should be running and to allow specific work to be done on the system. For example, runlevel one is designated single-user mode without networking and is used for critical system maintenance and troubleshooting. Runlevel three is multi-user text mode. Runlevel five is multi-user graphical mode.

Switching between runlevels is accomplished using the init command. Entering:

# init 5

will change to runlevel five. Changing to runlevel six will reboot the machine and runlevel zero will shut down the machine.

Entering runlevel at the command prompt will output the previous and current runlevels. For example:

#runlevel
N 5

indicates that the machine is in runlevel five. The 'N' indicates that the machine was booted into level five, or no previous runlevel was entered.

As mentioned previously, the different runlevels are configured to run a specific set of services. Each runlevel is configured to start its specified services when starting and to shut down its services when switching to a different runlevel.

As a general rule, turn off all services that are not needed at a given runlevel. Managing the services started in each runlevel is simple and can be done graphically in YaST or at the command line with the chkconfig command.

Managing Runlevel Services With YaST

YaST offers a robust and easy to use runlevel service configuration tool. Access the tool at YaST > System > Runlevel Editor. Within the tool, turn services on or off by selecting the service and then selecting the Enable/Disable buttons.

Selecting expert mode allows the administrator to configure the state of a service in each runlevel.

For example, use the YaST runlevel editor to configure the OpenSLP daemon, slpd, to start in runlevels three and five only. Do the following:

Select YaST > System > Runlevel Editor.

Select the Expert Mode radio button.

In the services list, select slpd.

Use the check boxes to select runlevels three and five. Deselect all other runlevels.

Save changes and exit.

Managing Runlevel Services With chkconfig

Most Linux distributions include the chkconfig command for managing runlevel services. The syntax for chkconfig is specified in the chkconfig man page.

For example, use chkconfig to configure the OpenSLP daemon, slpd, to start in runlevels three and five only. Do the following:

1.Check if slpd is on in the current runlevel

# chkconfig slpd
slpd on

2.Check slpd's configuration for every runlevel.

# chkconfig -l slpd
slpd 0:off 1:off 2:off 3:off 4:off 5:off 6:off

3.Turn slpd on in runlevels three and five.

#chkconfig slpd 35

2.2. Understanding Runlevels

Unlike most non-UNIX operating systems which only have 2 modes of functionality (on and off), UNIX operating systems, including Linux, have different runlevels such as "maintenance" runlevel or "multi-user" runlevel, etc.

Runlevels are numbered from 0 to 6 and will vary from one Linux distribution to another. The description for each runlevel functionality is sometimes documented in /etc/inittab.

Table 2.1. Linux runlevels

Runlevel Purpose

0 Shuts down the machine safely. The operating system will also attempt to poweroff the system if possible

1 Single user mode. Only one terminal is available for the (single) user root. All other users are logged out

2^[a] Multi-user mode, but does not start NFS. Network services like email or web services are also stopped

3^[a] Full multi-user mode. Selected network services are all on.

4^[a] Not defined and generally unused

5^[a] Like runlevel 3 but runs a Display Manager as well

6 restarts the machine safely

^[a]These runlevels may vary in functionality between Linux flavours. But these descriptions correspond to the current LSB specification.
2.2.1. INIT Controls Runlevels

Both init and telinit are used to switch from one runlevel to another. Remember that init is the first program launched after the kernel has accessed the root device.

At boot time init is instructed which runlevel to reach in /etc/inittab with the line:
id:5:initdefault:
When the system is started it is possible to change runlevels by invoking init (or telinit which is a symbolic link pointing at init).

For example we switch to runlevel 4 with either of the next commands:
init 4

telinit 4
The PID for init is always 1. It is possible to find out which runlevel the system is currently in with the command runlevel
runlevel
N 5
The first number is the previous runlevel (or N if not applicable) and the second number is the current runlevel.

Runlevel	Purpose
0	Shuts down the machine safely. The operating system will also attempt to poweroff the system if possible
1	Single user mode. Only one terminal is available for the (single) user `root`. All other users are logged out
2^[a]	Multi-user mode, but does not start NFS. Network services like email or web services are also stopped
3^[a]	Full multi-user mode. Selected network services are all on.
4^[a]	Not defined and generally unused
5^[a]	Like runlevel 3 but runs a Display Manager as well
6	restarts the machine safely
^[a]These runlevels may vary in functionality between Linux flavours. But these descriptions correspond to the current LSB specification.