The Init Process - Novell LINUX ENTERPRISE DESKTOP 10 SP2 - DEPLOYMENT GUIDE 08-05-2008 Deployment Manual

17.2 The init Process

The program init is the process with process ID 1. It is responsible for initializing the
system in the required way. init is started directly by the kernel and resists signal 9,
which normally kills processes. All other programs are either started directly by init or
by one of its child processes.
init is centrally configured in the /etc/inittab file where the runlevels are defined
(see Section 17.2.1, "Runlevels"
daemons are available in each of the levels. Depending on the entries in /etc/
inittab, several scripts are run by init. For reasons of clarity, these scripts, called
init scripts, all reside in the directory /etc/init.d (see
(page 396)).
The entire process of starting the system and shutting it down is maintained by init.
From this point of view, the kernel can be considered a background process whose task
is to maintain all other processes and adjust CPU time and hardware access according
to requests from other programs.
17.2.1 Runlevels
In Linux, runlevels define how the system is started and what services are available in
the running system. After booting, the system starts as defined in /etc/inittab in
the line initdefault. Usually this is 3 or 5. See
(page 393). As an alternative, the runlevel can be specified at boot time (by adding the
runlevel number at the boot prompt, for instance). Any parameters that are not directly
evaluated by the kernel itself are passed to init. To boot into runlevel 3, just add the
single number 3 to the boot prompt.
Table 17.1 Available Runlevels
Runlevel
0
S or 1
2
(page 393)). The file also specifies which services and
Description
System halt
Single user mode
Local multiuser mode without remote network (NFS, etc.)
Section 17.2.2, "Init Scripts"
Table 17.1, "Available Runlevels"
Booting and Configuring a Linux System 393