Linux Run Levels & Init

Once kernel and drivers are loaded, Linux starts loading the rest of the system. This starts with the First Process, known as init and it has the process id of 1 (the kernel itself has the process id of 0, which cannot be displayed by using the "ps" command).

The init process takes control of the boot operation. The init process in turn runs /etc/rc.d/rc.sysinit, which performs a number of tasks, including network configuration, SELinux status, keyboard maps, system clock, partition mounts, and host names.

The runlevels are controlled by a configuration file which init process reads from the location /etc. The name of the init configuration file is "inittab".

The init process then determines the runlevel by looking at the initdefault directive in /etc/inittab configuration file. The following are the defined runlevels. The init process remains active as long as the system is running.

Runlevel value	Description
0	Halt
1	Single-user mode
2	Multiuser, with some network services
3	Multiuser, with networking
4	Unused
5	Full Multiuser mode with X Windows (GUI login screen)
6	Reboot

Inittab configuration file

After the kernel boots, it always executes a program called "init". The init program then reads its configuration file called /etc/inittab and commences the boot process. A sample inittab file is copied below. The lines which are commencing with a "#" are comments.

### Beginning of inittab ###

# inittab This file describes how the INIT process should set up
# the system in a certain run-level.
#
# Author: Miquel van Smoorenburg,
# 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)
#
id:5: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

# Trap CTRL-ALT-DELETE
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.
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 Cancelled"

# 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

### End of inittab ###

Format of Inittab

The format of each line in inittab file is as follows:

id:runlevels:action:process

• id: A unique sequence of one to four characters that identifies this entry in the /etc/inittab file.

• runlevels: The runlevels at which the process should be invoked. For example, the runlevels entry 123 specifies something that runs at runlevels 1, 2, or 3.

• action: Describes what action should be taken. Options for this field are explained in the next table.

• Process: Names the process (or program) to execute when the runlevel is entered.

The following table specifies some valid actions for the action field.

Action Field in inittab file	Description
respawn	The process will be restarted whenever it terminates.
wait	The process will be started once when the runlevel is entered, and init will wait for its completion.
once	The process will be started once when the runlevel is entered; however, init won’t wait for termination of the process before possibly executing additional programs to be run at that particular runlevel.
boot	The process will be executed at system boot. The runlevels field is ignored in this case.
bootwait	The process will be executed at system boot, and init will wait for completion of the boot before advancing to the next process to be run.
ondemand	The process will be executed when a specific runlevel request occurs. (These runlevels are a, b, and c.) No change in runlevel occurs.
initdefault	Specifies the default runlevel for init on startup. If no default is specified, the user is prompted for a runlevel on console.
sysinit	The process will be executed during system boot, before any of the boot or bootwait entries.
powerwait	If init receives a signal from another process that there are problems with the power, this process will be run. Before continuing, init will wait for this process to finish.
powerfail	Same as powerwait, except that init will not wait for the process to finish.
Powerokwait	This process will be executed as soon as init is informed that the power has been restored.
ctrlaltdel	The process is executed when init receives a signal indicating that the user has pressed the ctrl-alt-del key combination. Keep in mind that most X Window System servers capture this key combination, and thus init may not receive this signal if the X Window System is active.