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Applies to SUSE Linux Enterprise Server 11 SP4

10 Booting and Configuring a Linux System

Booting a Linux system involves different components. The hardware itself is initialized by the BIOS, which starts the Kernel by means of a boot loader. After this point, the boot process with init and the runlevels is completely controlled by the operating system. The runlevel concept enables you to maintain setups for everyday usage as well as to perform maintenance tasks on the system.

10.1 The Linux Boot Process

The Linux boot process consists of several stages, each represented by a different component. The following list briefly summarizes the boot process and features all the major components involved.

  1. BIOS.  After turning on the computer, the BIOS initializes the screen and keyboard and tests the main memory. Up to this stage, the machine does not access any mass storage media. Subsequently, the information about the current date, time, and the most important peripherals are loaded from the CMOS values. When the first hard disk and its geometry are recognized, the system control passes from the BIOS to the boot loader. If the BIOS supports network booting, it is also possible to configure a boot server that provides the boot loader. On x86 systems, PXE boot is needed. Other architectures commonly use the BOOTP protocol to get the boot loader.

  2. Boot Loader.  The first physical 512-byte data sector of the first hard disk is loaded into the main memory and the boot loader that resides at the beginning of this sector takes over. The commands executed by the boot loader determine the remaining part of the boot process. Therefore, the first 512 bytes on the first hard disk are referred to as the Master Boot Record (MBR). The boot loader then passes control to the actual operating system, in this case, the Linux Kernel. More information about GRUB, the Linux boot loader, can be found in Chapter 11, The Boot Loader GRUB. For a network boot, the BIOS acts as the boot loader. It gets the image to start from the boot server and starts the system. This is completely independent of local hard disks.

  3. Kernel and initramfs To pass system control, the boot loader loads both the Kernel and an initial RAM–based file system (initramfs) into memory. The contents of the initramfs can be used by the Kernel directly. initramfs contains a small executable called init that handles the mounting of the real root file system. If special hardware drivers are needed before the mass storage can be accessed, they must be in initramfs. For more information about initramfs, refer to Section 10.1.1, “initramfs. If the system does not have a local hard disk, the initramfs must provide the root file system to the Kernel. This can be done with the help of a network block device like iSCSI or SAN, but it is also possible to use NFS as the root device.

  4. init on initramfs This program performs all actions needed to mount the proper root file system, like providing Kernel functionality for the needed file system and device drivers for mass storage controllers with udev. After the root file system has been found, it is checked for errors and mounted. If this is successful, the initramfs is cleaned and the init program on the root file system is executed. For more information about init, refer to Section 10.1.2, “init on initramfs. Find more information about udev in Chapter 15, Dynamic Kernel Device Management with udev.

  5. init init handles the actual booting of the system through several different levels providing different functionality. init is described in Section 10.2, “The init Process”.

10.1.1 initramfs

initramfs is a small cpio archive that the Kernel can load to a RAM disk. It provides a minimal Linux environment that enables the execution of programs before the actual root file system is mounted. This minimal Linux environment is loaded into memory by BIOS routines and does not have specific hardware requirements other than sufficient memory. initramfs must always provide an executable named init that should execute the actual init program on the root file system for the boot process to proceed.

Before the root file system can be mounted and the operating system can be started, the Kernel needs the corresponding drivers to access the device on which the root file system is located. These drivers may include special drivers for certain kinds of hard drives or even network drivers to access a network file system. The needed modules for the root file system may be loaded by init on initramfs. After the modules are loaded, udev provides the initramfs with the needed devices. Later in the boot process, after changing the root file system, it is necessary to regenerate the devices. This is done by boot.udev with the command udevtrigger.

If you need to change hardware (for example hard disks) in an installed system and this hardware requires different drivers to be present in the Kernel at boot time, you must update initramfs. This is done in the same way as with its predecessor, init—by calling mkinitrd. Calling mkinitrd without any argument creates an initramfs. Calling mkinitrd -R creates an init. In SUSE® Linux Enterprise Server, the modules to load are specified by the variable INITRD_MODULES in /etc/sysconfig/kernel. After installation, this variable is automatically set to the correct value. The modules are loaded in exactly the order in which they appear in INITRD_MODULES. This is only important if you rely on the correct setting of the device files /dev/sd?. However, in current systems you also may use the device files below /dev/disk/ that are sorted in several subdirectories, named by-id, by-path and by-uuid, and always represent the same disk. This is also possible at install time by specifying the respective mount option.

Important: Updating initramfs or init

The boot loader loads initramfs or init in the same way as the Kernel. It is not necessary to reinstall GRUB after updating initramfs or init, because GRUB searches the directory for the right file when booting.

10.1.2 init on initramfs

The main purpose of init on initramfs is to prepare the mounting of and access to the real root file system. Depending on your system configuration, init is responsible for the following tasks.

Loading Kernel Modules

Depending on your hardware configuration, special drivers may be needed to access the hardware components of your computer (the most important component being your hard drive). To access the final root file system, the Kernel needs to load the proper file system drivers.

Providing Block Special Files

For each loaded module, the Kernel generates device events. udev handles these events and generates the required special block files on a RAM file system in /dev. Without those special files, the file system and other devices would not be accessible.

Managing RAID and LVM Setups

If you configured your system to hold the root file system under RAID or LVM, init sets up LVM or RAID to enable access to the root file system later. Find information about RAID and LVM in Chapter 15, Advanced Disk Setup.

Managing Network Configuration

If you configured your system to use a network-mounted root file system (mounted via NFS), init must make sure that the proper network drivers are loaded and that they are set up to allow access to the root file system.

If the file system resides on a networked block device like iSCSI or SAN, the connection to the storage server is also set up by the initramfs.

When init is called during the initial boot as part of the installation process, its tasks differ from those mentioned above:

Finding the Installation Medium

As you start the installation process, your machine loads an installation Kernel and a special init with the YaST installer on the installation medium. The YaST installer, which is run in a RAM file system, needs to have information about the location of the installation medium to access it and install the operating system.

Initiating Hardware Recognition and Loading Appropriate Kernel Modules

As mentioned in Section 10.1.1, “initramfs, the boot process starts with a minimum set of drivers that can be used with most hardware configurations. init starts an initial hardware scanning process that determines the set of drivers suitable for your hardware configuration. The names of the modules needed for the boot process are written to INITRD_MODULES in /etc/sysconfig/kernel. These names are used to generate a custom initramfs that is needed to boot the system. If the modules are not needed for boot but for coldplug, the modules are written to /etc/sysconfig/hardware/hwconfig-*. All devices that are described with configuration files in this directory are initialized in the boot process.

Loading the Installation System or Rescue System

As soon as the hardware is properly recognized, the appropriate drivers are loaded, and udev creates the special device files, init starts the installation system with the actual YaST installer, or the rescue system.

Starting YaST

Finally, init starts YaST, which starts package installation and system configuration.

10.2 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 10.2.1, “Runlevels”). The file also specifies which services and daemons are available in each of the runlevels. Depending on the entries in /etc/inittab, several scripts are run by init. By default, the first script that is started after booting is /etc/init.d/boot. Once the system initialization phase is finished, the system changes the runlevel to its default runlevel with the /etc/init.d/rc script. For reasons of clarity, these scripts, called init scripts, all reside in the directory /etc/init.d (see Section 10.2.2, “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 to maintain all other processes and adjust CPU time and hardware access according to requests from other programs.

10.2.1 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 Table 10.1, “Available Runlevels”. 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 10.1: Available Runlevels




System halt

S or 1

Single user mode


Local multiuser mode without remote network (NFS, etc.)


Full multiuser mode with network


User Defined, this is not used unless the administrator configures this runlevel.


Full multiuser mode with network and X display manager—KDM, GDM, or XDM


System reboot

Important: Avoid Runlevel 2 with a Partition Mounted via NFS

You should not use runlevel 2 if your system mounts a partition like /usr via NFS. The system might behave unexpectedly if program files or libraries are missing because the NFS service is not available in runlevel 2 (local multiuser mode without remote network).

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.

telinit 1 or shutdown now

The system changes to single user mode. This mode is used for system maintenance and administration tasks.

telinit 3

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.

telinit 5

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).

telinit 0 or shutdown -h now

The system halts.

telinit 6 or shutdown -r now

The system halts then reboots.

Runlevel 5 is the default runlevel in all SUSE Linux Enterprise Server standard installations. Users are prompted for login with a graphical interface or the default user is logged in automatically.

Warning: Errors in /etc/inittab May Result in a Faulty System Boot

If /etc/inittab is damaged, the system may not boot properly. Therefore, be extremely careful while editing /etc/inittab. Always let init reread /etc/inittab with the command telinit q before rebooting the machine.

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:

  1. The administrator (root) requests init to change to a different runlevel by entering telinit 5.

  2. init checks the current runlevel (runlevel) and determines it should start /etc/init.d/rc with the new runlevel as a parameter.

  3. Now rc calls the stop scripts of the current runlevel 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 (the old runlevel was 3) and start with a K. The number following K specifies the order to run the scripts with the stop parameter, because there are some dependencies to consider.

  4. The last things to start are the start scripts of the new runlevel. In this example, these are in /etc/init.d/rc5.d and begin with an S. Again, the number that follows the S determines the sequence in which the scripts are started.

When changing into the same runlevel as the current runlevel, init only checks /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.

10.2.2 Init Scripts

There are two types of scripts in /etc/init.d:

Scripts Executed Directly by init

This is the case only during the boot process or if an immediate system shutdown is initiated (power failure or a user pressing CtrlAltDel). For IBM System z systems, this is the case only during the boot process or if an immediate system shutdown is initiated (power failure or via signal quiesce). The execution of these scripts is defined in /etc/inittab.

Scripts Executed Indirectly by init

These are run when changing the runlevel and always call the master script /etc/init.d/rc, which guarantees the correct order of the relevant scripts.

All scripts are located in /etc/init.d. Scripts that are run at boot time are called through symbolic links from /etc/init.d/boot.d. Scripts for changing the runlevel are called through symbolic links from one of the subdirectories (/etc/init.d/rc0.d to /etc/init.d/rc6.d). This is just for reasons of clarity 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 start and stop. The scripts also understand the restart, reload, force-reload, and status options. These different options are explained in Table 10.2, “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 10.2: Possible init Script Options




Start service.


Stop service.


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 this. Otherwise, do the same as if restart had been given.


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 using /usr/lib/lsb/install_initd, which is a script calling this program). See man 8 insserv for more details.

All of these settings may also be changed with the help of the YaST module. If you need to check the status on the command line, use the tool chkconfig, described in the man 8 chkconfig man page.

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, the /proc and /dev/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 these scripts (running a number of subscripts, for example, making special block files available) are completed. blogd writes any screen output to the log file /var/log/boot.msg, but only if and when /var is mounted read-write. Otherwise, blogd buffers all screen data until /var becomes available. Get further information about blogd with man 8 blogd.

The boot script is also responsible for starting all the scripts in /etc/init.d/boot.d with names that start with 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. The last executed script is boot.local.


Here enter additional commands to execute at boot before changing into a runlevel. It can be compared to AUTOEXEC.BAT on DOS systems.


This script is only executed while changing into runlevel 0 or 6. Here, it is executed either as init or as init. Whether the system shuts down or reboots depends on how halt is called. If special commands are needed during the shutdown, add these to the init script.


This script calls the appropriate stop scripts of the current runlevel and the start scripts of the newly selected runlevel. Like the /etc/init.d/boot script, this script is called from /etc/inittab with the desired runlevel as parameter.

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 of init, init.d, chkconfig, and insserv. Additionally consult the man pages of startproc and killproc.

Warning: Faulty Init Scripts May Halt Your System

Faulty init scripts may hang your machine up. Edit such scripts with great care and, if possible, subject them to heavy testing in the multiuser environment. Find useful information about init scripts in Section 10.2.1, “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 procedure.

The INIT INFO block at the top is a required part of the script and must be edited. See Example 10.1, “A Minimal INIT INFO Block”.

Example 10.1: A Minimal INIT INFO Block
# 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

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-Start: and Required-Stop: lines, specify all services that need to be still running when the service itself is stopped. This information is used later to generate the numbering of script names, as found in the runlevel directories. After Default-Start: and Default-Stop:, 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 scripts in /etc/init.d/, enter the command insserv new-script-name. insserv 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 described in Section 10.2.3, “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 automatically.

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 for this script.

10.2.3 Configuring System Services (Runlevel) with YaST

After starting this YaST module with YaST › System › System Services (Runlevel), 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 Simple Mode or in Expert Mode. The default Simple Mode 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 Enable. 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 Expert Mode. The current default runlevel or initdefault (the runlevel into which the system boots by default) is displayed at the top. Normally, the default runlevel of a SUSE Linux Enterprise Server system is runlevel 5 (full multiuser mode with network and X). A suitable alternative might be runlevel 3 (full multiuser mode with network).

This YaST dialog allows the selection of one of the runlevels (as listed in Table 10.1, “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 (B, 0, 1, 2, 3, 5, 6, and S) to define the runlevels in which the selected service or daemon should be running. Runlevel 4 is undefined to allow creation of a custom runlevel. A brief description of the currently selected service or daemon is provided below the table overview.

Warning: Faulty Runlevel Settings May Damage Your System

Faulty runlevel settings may make your system unusable. Before applying your changes, make absolutely sure that you know their consequences.

System Services (Runlevel)
Figure 10.1: System Services (Runlevel)

With Start, Stop, or Refresh, decide whether a service should be activated. Refresh status checks the current status. Set or Reset lets you select whether to apply your changes to the system or to restore the settings that existed before starting the runlevel editor. Selecting OK saves the changed settings to disk.

10.3 System Configuration via /etc/sysconfig

The main configuration of SUSE Linux Enterprise Server is controlled by the configuration files in /etc/sysconfig. The individual files in /etc/sysconfig are only read by the scripts to which they are relevant. This ensures that network settings, for example, only need to be parsed by network-related scripts.

There are two ways to edit the system configuration. Either use the YaST sysconfig Editor or edit the configuration files manually.

10.3.1 Changing the System Configuration Using the YaST sysconfig Editor

The YaST sysconfig editor provides an easy-to-use front-end for system configuration. Without any knowledge of the actual location of the configuration variable you need to change, you can just use the built-in search function of this module, change the value of the configuration variable as needed and let YaST take care of applying these changes, updating configurations that depend on the values set in sysconfig and restarting services.

Warning: Modifying /etc/sysconfig/* Files Can Damage Your Installation

Do not modify the /etc/sysconfig files if you lack previous experience and knowledge. It can do considerable damage to your system. The files in /etc/sysconfig include a short comment for each variable to explain what effect they actually have.

System Configuration Using the sysconfig Editor
Figure 10.2: System Configuration Using the sysconfig Editor

The YaST sysconfig dialog is split into three parts. The left part of the dialog shows a tree view of all configurable variables. When you select a variable, the right part displays both the current selection and the current setting of this variable. Below, a third window displays a short description of the variable's purpose, possible values, the default value and the actual configuration file from which this variable originates. The dialog also provides information about which configuration script is executed after changing the variable and which new service is started as a result of the change. YaST prompts you to confirm your changes and informs you which scripts will be executed after you leave the dialog by selecting Finish. Also select the services and scripts to skip for now, so they are started later. YaST applies all changes automatically and restarts any services involved for your changes to take an effect.

10.3.2 Changing the System Configuration Manually

To manually change the system configuration, proceed as follows

  1. Become root.

  2. Bring the system into single user mode (runlevel 1) with telinit 1.

  3. Change the configuration files as needed with an editor of your choice.

    If you do not use YaST to change the configuration files in /etc/sysconfig, make sure that empty variable values are represented by two quotation marks (KEYTABLE="") and that values with blanks in them are enclosed in quotation marks. Values consisting of one word only do not need to be quoted.

  4. Execute SuSEconfig to make sure that the changes take effect.

  5. Bring your system back to the previous runlevel with a command like telinit default_runlevel. Replace default_runlevel with the default runlevel of the system. Choose 5 if you want to return to full multiuser with network and X or choose 3 if you prefer to work in full multiuser with network.

This procedure is mainly relevant when changing systemwide settings, such as the network configuration. Small changes should not require going into single user mode, but you may still do so to make absolutely sure that all the programs concerned are correctly restarted.

Tip: Configuring Automated System Configuration

To disable the automated system configuration by SuSEconfig, set the variable ENABLE_SUSECONFIG in /etc/sysconfig/suseconfig to no. Do not disable SuSEconfig if you want to use the SUSE installation support. It is also possible to disable the autoconfiguration partially.

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