The program init is the process with process ID 1. It is responsible for initializing the system in the required way. init takes a special role. It 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
where the runlevels are defined (see
Section 8.2.1, “Runlevels”). The file also specifies which services and
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 Section 8.2.2, “Init Scripts”).
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.
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
5. See Table 8.1, “Available Runlevels”.
As an alternative, the runlevel can be
specified at boot time (at the boot prompt, for instance). Any parameters
that are not directly evaluated by the kernel itself are passed to
Table 8.1. Available Runlevels
Single user mode; from the boot prompt, only with US keyboard mapping
Single user mode
Local multiuser mode without remote network (NFS, etc.)
Full multiuser mode with network
Full multiuser mode with network and X display manager—KDM, GDM, or XDM
|Avoid Runlevel 2 with a Partition Mounted via NFS|
You should not use runlevel 2 if your system mounts a
To change runlevels while the system is running, enter telinit and the corresponding number as an argument. Only the system administrator is allowed to do this. The following list summarizes the most important commands in the runlevel area.
The system changes to single user mode. This mode is used for system maintenance and administration tasks.
All essential programs and services (including network) are started and regular users are allowed to log in and work with the system without a graphical environment.
The graphical environment is enabled. Usually a display manager like XDM, GDM, or KDM is started. If autologin is enabled, the local user is logged in to the preselected window manager (GNOME or KDE or any other window manager).
The system halts.
The system halts then reboots.
5 is the default runlevel in all SUSE Linux
standard installations. Users are prompted for login with a
graphical interface or the default user is logged in automatically.
If the default runlevel is
3, the X
Window System must be configured properly, as described in
Chapter 14, The X Window System, before the runlevel can be switched to
5. If this is done, check whether the system works in the
desired way by entering telinit
everything turns out as expected, you can use YaST to set the default
Generally, two things happen when you change runlevels. First, stop scripts of the current runlevel are launched, closing down some programs essential for the current runlevel. Then start scripts of the new runlevel are started. Here, in most cases, a number of programs are started. For example, the following occurs when changing from runlevel 3 to 5:
The administrator (
init to change to a different runlevel by entering telinit
Now rc calls all the stop scripts of the current
runlevel, but only those for which there is no start script in the new
runlevel. In this example, these are all the scripts that reside in
/etc/init.d/rc3.d (old runlevel was 3) and start with
K. The number following
specifies the order to start, because there are some dependencies to
The last things to start are the start scripts of the new runlevel. These
are, in this example, in
/etc/init.d/rc5.d and begin
S. The same procedure regarding the order in
which they are started is applied here.
When changing into the same runlevel as the current runlevel, init only
/etc/inittab for changes and starts the
appropriate steps, for example, for starting a getty on
another interface. The same functionality may be achieved with the
command telinit q.
There are two types of scripts in
This is the case
only during the boot process or if an immediate system shutdown is
initiated (power failure or a user pressing Ctrl-Alt-Del).
The execution of these scripts is defined in
All scripts are located in
Scripts that are run at boot time are called through symbolic links
/etc/init.d/boot.d. Scripts for
changing the runlevel are called through symbolic
links from one of the subdirectories (
/etc/init.d/rc6.d). This is just for clarity reasons
and avoids duplicate scripts if they are used in several runlevels. Because
every script can be executed as both a start and a stop script, these
scripts must understand the parameters
stop. The scripts also understand the
status options. These
different options are explained in
Table 8.2, “Possible init Script Options”.
Scripts that are run directly by init do not have these links. They
are run independently from the runlevel when needed.
Table 8.2. Possible init Script Options
If the service is running, stop it then restart it. If it is not running, start it.
Reload the configuration without stopping and restarting the service.
Reload the configuration if the service supports
do the same as if
Show the current status of service.
Links in each runlevel-specific subdirectory make it possible to associate
scripts with different runlevels. When installing or uninstalling packages,
these links are added and removed with the help of the program insserv (or
/usr/lib/lsb/install_initd, which is a script
calling this program). See the insserv(8) man page for details.
A short introduction to the boot and stop scripts launched first or last, respectively, follows as well as an explanation of the maintaining script.
Executed while starting the system directly using init. It
is independent of the chosen runlevel and is only executed once. Here,
pts file systems
are mounted and blogd (boot logging daemon) is activated. If the system
is booted for the first time after an update or an installation, the
initial system configuration is started.
The blogd daemon is a service started by boot and rc before any other
one. It is stopped after the actions triggered by the above scripts
(running a number of subscripts, for example) are completed. blogd writes
any screen output to the log file
but only if and when
/var is mounted read-write.
Otherwise, blogd buffers all screen data until
becomes available. Get further information about blogd on the blogd(8)
boot is also responsible for starting all
the scripts in
/etc/init.d/boot.d with a name that
S. There, the file systems are checked and
loop devices are configured if needed. The system time is also set. If an
error occurs while automatically checking and repairing the file system,
the system administrator can intervene after first entering the root
password. Last executed is the script
This script is executed when changing from single user mode to any other runlevel and is responsible for a number of basic settings, such as the keyboard layout and initialization of the virtual consoles.
You can create your own scripts and easily integrate them into the scheme
described above. For instructions about formatting, naming, and organizing
custom scripts, refer to the specifications of the LSB and to the man pages
insserv. Additionally consult the man pages of
|Faulty init Scripts May Halt Your System|
Faulty init scripts may hang your machine. Edit such scripts with great care and, if possible, subject them to heavy testing in the multiuser environment. Some useful information about init scripts can be found in Section 8.2.1, “Runlevels”.
To create a custom init script for a given program or service, use the file
/etc/init.d/skeleton as a template. Save a copy of
this file under the new name and edit the relevant program and filenames,
paths, and other details as needed. You may also need to enhance the script
with your own parts, so the correct actions are triggered by the init
INIT INFO block at the top is a required part of the
script and must be edited. See
Example 8.1, “A Minimal INIT INFO Block”.
Example 8.1. A Minimal INIT INFO Block
### BEGIN INIT INFO # Provides: FOO # Required-Start: $syslog $remote_fs # Required-Stop: $syslog $remote_fs # Default-Start: 3 5 # Default-Stop: 0 1 2 6 # Description: Start FOO to allow XY and provide YZ ### END INIT INFO
In the first line of the
INFO block, after
Provides:, specify the name of the program or service
controlled by this init script. In the
Required-Stop: lines, specify all services that need
to be started or stopped before the service itself is started
or stopped. This information is used later to generate the numbering of
script names, as found in the runlevel directories. After
specify the runlevels in which the service should automatically be started
or stopped. Finally, for
Description:, provide a short
description of the service in question.
To create the links from the runlevel directories
/etc/init.d/rc?.d/) to the corresponding
/etc/init.d/, enter the command
insserv program evaluates the
INIT INFO header to create
the necessary links for start and stop scripts in the runlevel directories
/etc/init.d/rc?.d/). The program also takes care of
the correct start and stop order for each runlevel by including the
necessary numbers in the names of these links. If you prefer a graphical
tool to create such links, use the runlevel editor provided by YaST, as
Section 8.2.3, “Configuring System Services (Runlevel) with YaST”.
If a script already present in
/etc/init.d/ should be
integrated into the existing runlevel scheme, create the links in the
runlevel directories right away with insserv or by enabling the
corresponding service in the runlevel editor of YaST. Your changes are
applied during the next reboot—the new service is started
Do not set these links manually. If something is wrong in the
INFO block, problems will arise when
insserv is run later for some other service.
The manually-added service will be
removed with the next run of insserv.
After starting this YaST module with+ + , it displays an overview listing all the available services and the current status of each service (disabled or enabled). Decide whether to use the module in or in . The default should be sufficient for most purposes. The left column shows the name of the service, the center column indicates its current status, and the right column gives a short description. For the selected service, a more detailed description is provided in the lower part of the window. To enable a service, select it in the table then select . The same steps apply to disable a service.
For detailed control over the runlevels in which a service is started or stopped or to change the default runlevel, first select initdefault” (the runlevel into which the system boots by default) is displayed at the top. Normally, the default runlevel of a SUSE Linux system is runlevel 5 (full multiuser mode with network and X). A suitable alternative might be runlevel 3 (full multiuser mode with network).. The current default runlevel or “
This YaST dialog allows the selection of one of the runlevels (as listed in Table 8.1, “Available Runlevels”) as the new default. Additionally use the table in this window to enable or disable individual services and daemons. The table lists the services and daemons available, shows whether they are currently enabled on your system, and, if so, for which runlevels. After selecting one of the rows with the mouse, click the check boxes representing the runlevels ( , , , , , , , and ) to define the runlevels in which the selected service or daemon should be running. Runlevel 4 is initially undefined to allow creation of a custom runlevel. A brief description of the currently selected service or daemon is provided below the table overview.
With, decide whether a service should be activated. checks the current status. lets you select whether to apply your changes to the system or to restore the settings that existed before starting the runlevel editor. Selecting saves the changed settings to disk.
|Faulty Runlevel Settings May Damage Your System|
Faulty runlevel settings may render a system unusable. Before applying your changes, make absolutely sure that you know their consequences.