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Customizing system startup (202.1)

Customizing system startup (202.1)

Candidates should be able to query and modify the behaviour of system services at various targets / run levels. A thorough understanding of the systemd, SysV Init and the Linux boot proces is required. This objective includes interacting with systemd targets and SysV init run levels.

Key Knowledge Areas

  • Systemd

  • SysV init

  • Linux Standard Base Specification (LSB)

Terms and Utilities

  • /usr/lib/systemd/

  • /etc/systemd/

  • /run/systemd/

  • systemctl

  • systemd-delta

  • /etc/inittab

  • /etc/init.d/

  • /etc/rc.d/

  • chkconfig

  • update-rc.d

  • init and telinit

Create initrd using mkinitrd


mkinitrd was discussed in a previous section, which also discusses how to create such an image manually.

To limit the size of the kernel, often initial ramdisk (initrd) images are used to preload any modules needed to access the root filesystem.

The mkinitrd is a tool which is specific to RPM based distributions (such as Red Hat, SuSE, etc.). This tool automates the process of creating an initrd file, thereby making sure that the relatively complex process is followed correctly.

In most of the larger Linux distributions the initrd image contains almost all necessary kernel modules; very few will be compiled directly into the kernel. This enables the deployment of easy fixes and patches to the kernel and its modules through RPM packages: an update of a single module will not require a recompilation or replacement of the whole kernel, but just the single module, or worst case a few dependent modules. Because these modules are contained within the initrd file, this file needs to be regenerated every time the kernel is (manually) recompiled, or a kernel (module) patch is installed. Generating a new initrd image is very simple if you use the standard tool provided on many distributions:

    # mkinitrd initrd-image kernel-version

Useful options for mkinitrd include:


  • This option displays the version number of the mkinitrd utility.


  • By specifying this switch, the utility will overwrite any existing image file with the same name.


  • This causes mkinitrd to assume the module specified was compiled into the kernel. It will not look for the module and will not show any errors if it does not exist.

--omit-lvm-modules, --omit-raid-modules, --omit-scsi-modules

  • Using these options it is possible to prevent inclusion of, respectively, LVM, RAID or SCSI modules, even if they are present, or the utility would normally include them based on the contents of /etc/fstab and/or /etc/raidtab.

Create initrd using mkinitramfs

Dissatisfied with the tool the RPM based distributions use (mkinitrd), some Debian developers wrote another utility to generate an initrd file. This tool is called mkinitramfs. The mkinitramfs tool is a shell script which generates a gzipped cpio image. It was designed to be much simpler (in code as well as in usage) than mkinitrd. The script consists of around 380 lines of code.

Configuration of mkinitramfs is done through a configuration file: initramfs.conf. This file is usually located in /etc/initramfs-tools/initramfs.conf. This configuration file is sourced by the script - it contains standard bash declarations. Comments are prefixed by a #. Variables are specified by:


Options which can be used with mkinitramfs include:

-d confdir

  • This option sets an alternate configuration directory.


  • "Keep" the temporary directory used for creating the image.

-o outfile

  • Write the resulting image to outfile.

-r root

  • Override the ROOT setting in the initramfs.conf file.

Note On Debian(-based) distributions you should always use mkinitramfs as mkinitrd is broken there for more recent kernels.

Setting the root device

Setting the root device is one of the many kernel settings. Kernel settings originate from or can be overwritten by:

  • defaults as set in the source code,

  • defaults as set by the rdev command,

  • values passed to the kernel at boot time, for example root=/dev/xyz

  • values specified in the GRUB configuration file.

The most obvious to use are block devices like harddisks, SAN storage, CDs or DVDs. You can even have a NFS mounted root-disk, this requires usage of initrd and setting the nfs_root_name and nfs_root_addrs boot options. You can set or change the root device to almost anything from within the initrd environment. In order to do so, make sure that the /proc filesystem was mounted by the scripts in the initrd image. The following files are available in /proc:


The real-root-dev refers to the node number of the root file system device. It can be easily changed by writing the new number to it:

    # echo 0x301>/proc/sys/kernel/real-root-dev

This will change the real root to the filesystem on /dev/hda1. If you wish to use an NFS-mounted root, the files nfs-root-name and nfs-root-addrs have to be set using the appropriate values and the device number should be set to 0xff:

    # echo /var/nfsroot >/proc/sys/kernel/nfs-root-name
    # echo \
    # echo 255>/proc/sys/kernel/real-root-dev

Note If the root device is set to the RAM disk, the root filesystem is not moved to /initrd, but the boot procedure is simply continued by starting init on the initial RAM disk.

The Linux Boot process

There are seven phases distinguishable during boot:

  1. Kernel loader loading, setup and execution

  2. Register setup

  3. Kernel decompression

  4. Kernel and memory initialization

  5. Kernel setup

  6. Enabling of remaining CPU's

  7. Init process creation

The boot process is described in detail at Gustavo Duarte's "The Kernel Boot Process"

The kernel's final step in the boot process tries to execute these commands in order, until one succeeds:

  1. /sbin/init

  2. /etc/init

  3. /bin/init

  4. /bin/sh

If none of these succeed, the kernel will panic.

The init process

initis the parent of all processes, it reads the file/etc/inittaband creates processes based on its contents. One of the things it usually does is spawngettys` allowing users to log in. It also defines "runlevels".

A "runlevel" is a software configuration of the system which allows only a selected group of processes to exist. runlevel

runlevel 0 (reserved)

  • Runlevel 0 is used to halt the system.

runlevel 1 (reserved)

  • Runlevel 1 is used to get the system in single user mode. runlevel 1

runlevel 2-5

  • Runlevels 2,3,4 and 5 are multi-user runlevels. runlevel 2-5 multi-user runlevels

runlevel 6

  • Runlevel 6 is used to reboot the system. runlevel 6 reboot

runlevel 7-9

  • Runlevels 7, 8 and 9 can be used as you wish. Most of the Unix(/Linux) variants don't use these runlevels. On a typical Debian Linux System for instance, the /etc/rc<runlevel>.d directories, which we will discuss later, are not available for these runlevels, though that would be perfectly legal.

runlevel s or S

  • Runlevels s and S are internally the same. It brings runlevel s runlevel S the system in "single-user mode". The scripts in the /etc/rcS.d directory are executed when booting the system. Although runlevel S is not meant to be activated by the user, it can be.

runlevels A, B and C

  • Runlevels A, B and C are so called "on demand" runlevels. If the current runlevel is "2" for instance, and an init A command is executed, the scripts to start or stop processes within runlevel "A" are executed but the actual runlevel remains "2".

Configuring /etc/inittab

As mentioned before init reads the file /etc/inittab /etc/inittab to determine what it should do. An entry in this file has the following format:


Included below is an example /etc/inittab file.

    # The default runlevel.

    # Boot-time system configuration/initialization script.
    # This is run first except when booting in emergency (-b) mode.

    # What to do in single-user mode.

    # /etc/init.d executes the S and K scripts upon change
    # of runlevel.
    # Runlevel 0 is halt.
    # Runlevel 1 is single-user.
    # Runlevels 2-5 are multi-user.
    # Runlevel 6 is reboot.

    l0:0:wait:/etc/init.d/rc 0
    l1:1:wait:/etc/init.d/rc 1
    l2:2:wait:/etc/init.d/rc 2
    l3:3:wait:/etc/init.d/rc 3
    l4:4:wait:/etc/init.d/rc 4
    l5:5:wait:/etc/init.d/rc 5
    l6:6:wait:/etc/init.d/rc 6
    # Normally not reached, but fall through in case of emergency.

    # /sbin/getty invocations for the runlevels.
    # The "id" field MUST be the same as the last
    # characters of the device (after "tty").
    # Format:
    #  <id>:<runlevels>:<action>:<process>
    1:2345:respawn:/sbin/getty 38400 tty1
    2:23:respawn:/sbin/getty 38400 tty2
  • id

    • The id-field uniquely identifies an entry in the file /etc/inittab and can be 1-4 characters in length. For gettys and other login processes however, the id field should contain the suffix of the corresponding tty, otherwise the login accounting might not work.
  • runlevels

    • This field contains the runlevels for which the specified action should be taken.
  • action

    • respawn

    The process will be restarted whenever it terminates, respawn (e.g. getty). getty

    • wait

    The process will be started once when the specified wait runlevel is entered and init will wait for its termination.

    • once

      • The process will be executed once when the specified once unlevel is entered.
    • boot

      • The process will be executed during system boot. The boot runlevels field is ignored.
  • bootwait

    - The process will be executed during system boot, while bootwait
    `init` waits for its termination (e.g. `/etc/rc)`. The runlevels
    field is ignored.
    • off

      • This does absolutely nothing.
    • ondemand

      • A process marked with an on demand runlevel will be ondemand executed whenever the specified ondemand runlevel is called. However, no runlevel change will occur (on demand runlevels are "a", "b", and "c").
    • initdefault

      • An initdefault entry specifies the runlevel which should initdefault be entered after system boot. If none exists, init will ask for a runlevel on the console. The process field is ignored. In the example above, the system will go to runlevel 2 after boot.
    • sysinit

      • The process will be executed during system boot. It will sysinit be executed before any boot or bootwait entries. The runlevels field is ignored.
    • powerwait

      • The process will be executed when the power goes down. powerwait init is usually informed about this by a process talking to a UPS connected to the computer. UPS init will wait for the process to finish before continuing.
    • powerfail

      • As for powerwait, except that init does powerfail not wait for the process' completion.
    • powerokwait

      • This process will be executed as soon as init powerokwait is informed that the power has been restored.
    • powerfailnow

      • This process will be executed when init powerfailnow is told that the battery of the external UPS is almost empty and the power is failing (provided that the external UPS and the monitoring process are able to detect this condition).
    • ctrlaltdel

      • The process will be executed when init ctrlaltdel receives the SIGINT signal. This means that someone on the system console has pressed the CTRL-ALT-DEL key CTRL-ALT-DEL combination. Typically one wants to execute some sort of shutdown either to get into single-user level or to reboot the machine.
    • kbdrequest

      • The process will be executed when init receives a signal from the keyboard handler that a special key combination kbdrequest was pressed on the console keyboard. Basically you want to map some keyboard combination to the "KeyboardSignal" action. For example, to map Alt-Uparrow for this purpose use the following in your keymaps file: alt keycode 103 = KeyboardSignal.
  • process

    • This field specifies the process that should be executed. If the process field starts with a "+", init will not do utmp and wtmp accounting. Some gettys insist on doing their own housekeeping.

The /etc/init.d/rc script

For each of the runlevels 0-6 there is an entry in /etc/inittab that executes /etc/init.d/rc ? where "?" is 0-6, as you can see in following line from /etc/init.d/rc the earlier example above:

    l2:2:wait:/etc/init.d/rc 2

So, what actually happens is that /etc/init.d/rc is called with the runlevel as a parameter.

The directory /etc contains several, runlevel specific, directories which in their turn contain runlevel specific symbolic links to scripts in /etc/init.d/. Those directories are:

    $ ls -d /etc/rc*
    /etc/rc.boot  /etc/rc1.d  /etc/rc3.d  /etc/rc5.d  /etc/rcS.d
    /etc/rc0.d    /etc/rc2.d  /etc/rc4.d  /etc/rc6.d

As you can see, there also is a /etc/rc.boot /etc/rc.boot directory. This directory is obsolete and has been replaced by the /etc/rcN.d directory /etc/rcS.d. At boot time, the directory /etc/rcS.d is scanned first and then, for backwards compatibility, the /etc/rc.boot.

The name of the symbolic link either starts with an "S" or with a "K". Let's examine the /etc/rc2.d directory:

    $ ls /etc/rc2.d
    K20gpm       S11pcmcia   S20logoutd  S20ssh      S89cron
    S10ipchains  S12kerneld  S20lpd      S20xfs      S91apache
    S10sysklogd  S14ppp      S20makedev  S22ntpdate  S99gdm
    S11klogd     S20inetd    S20mysql    S89atd      S99rmnologin

If the name of the symbolic link starts with a "K", the script is called with "stop" as a parameter to stop the process. This is the case for K20gpm, so the command becomes K20gpm stop. Let's find out what program or script is called:

    $ ls -l /etc/rc2.d/K20gpm
    lrwxrwxrwx 1 root root 13 Mar 23 2001 /etc/rc2.d/K20gpm -> ../init.d/gpm

So, K20gpm stop results in /etc/init.d/gpm stop. Let's see what happens with the "stop" parameter by examining part of the script: init scripts

    # Start Mouse event server
    case "$1" in
       sleep 3
       echo "Usage: /etc/init.d/gpm {start|stop|restart|force-reload}"
       exit 1

In the case..esac the first parameter, $1, is examined and in case its value is "stop", gpm_stop is executed.

On the other hand, if the name of the symbolic link starts with an "S", the script is called with "start" as a parameter to start the process.

The scripts are executed in a lexical sort order of the filenames. initorder of scripts

Let's say we have a daemon SomeDaemon, an accompanying script /etc/init.d/SDscript and we want SomeDaemon to be running when the system is in runlevel 2 but not when the system is in runlevel 3.

As you know by now this means we need a symbolic link, starting with an "S", for runlevel 2 and a symbolic link, starting with a "K", for runlevel 3. We've also determined that the daemon SomeDaemon is to be started after S19someotherdaemon, which implicates S20 and K80 since starting/stopping is symmetrical, i.e. that what is started first is stopped last. This is accomplished with the following set of commands:

    # cd /etc/rc2.d
    # ln -s ../init.d/SDscript S20SomeDaemon
    # cd /etc/rc3.d
    # ln -s ../init.d/SDscript K80SomeDaemon

Should you wish to manually start, restart or stop a process, it is good practice to use the appropriate script in /etc/init.d/, e.g. /etc/init.d/gpm restart to initiate the restart of the process.

update-rc.d {#scupdatercd}


Note This section only applies to Debian (based) distributions

Debian-derived Linux distributions use the update-rc.d command to install and remove the init script links mentioned in the previous section.

If you have a startup script called "foobar" in /etc/init.d/ and want to add it to the default runlevels, you can use:

    # update-rc.d foobar defaults
    Adding system startup for /etc/init.d/foobar ...
     /etc/rc0.d/K20foobar -> ../init.d/foobar
     /etc/rc1.d/K20foobar -> ../init.d/foobar
     /etc/rc6.d/K20foobar -> ../init.d/foobar
     /etc/rc2.d/S20foobar -> ../init.d/foobar
     /etc/rc3.d/S20foobar -> ../init.d/foobar
     /etc/rc4.d/S20foobar -> ../init.d/foobar
     /etc/rc5.d/S20foobar -> ../init.d/foobar

update-rc.d will create K (stop) links in rc0.d, rc1.d and rc6.d, and S (start) links in rc2.d, rc3.d, rc4.d and rc5.d.

If you do not want an installed package to start automatically, use update-rc.d to remove the startup links, for example to disable starting dovecot on boot:

    # update-rc.d -f dovecot remove
    Removing any system startup links for /etc/init.d/dovecot ...

The -f (force) option is required if the rc script still exists. If you install an updated dovecot package, the links will be restored. To prevent this create "stop" links in the startup runlevel directories:

    # update-rc.d -f dovecot stop 24 2 3 4 5 .
    Adding system startup for /etc/init.d/dovecot ...
     /etc/rc2.d/K24dovecot -> ../init.d/dovecot
     /etc/rc3.d/K24dovecot -> ../init.d/dovecot
     /etc/rc4.d/K24dovecot -> ../init.d/dovecot
     /etc/rc5.d/K24dovecot -> ../init.d/dovecot

Note Don't forget the trailing . (dot).

Using systemd targets

Instead of predefined runlevels, systemd uses targets to define the system state. These targets are represented by target units. Target units end with the .target file extensions and their only purpose is to group together other systemd units through a chain of dependencies. This also means that, in comparison to the init runlevels, multiple targets can be active at the same time.

For example, the unit, start services as the GNOME Display Manager but also depends on (which is the non-graphical system state) which in turn depends on

Before we will continue with managing and using the targets you should know which directories are used to store the default target files and how you can override them.

As with all units the default target files are stored in /usr/lib/systemd, the files in this directory are created by the vendor of the software you've installed and these should never be changed except by installation scripts. The directory where you can store you custom targets and overrides is /etc/systemd, everything written in this directory takes precendence over the files in /usr/lib/systemd.

There are multiple ways to override or append properties to unit files. You can create a completely new file with the same name, for example ssh.server, in the /etc/systemd/system. This will override the complete unit definition. If you only want to append or change some properties you can create a new directory in /etc/systemd/system with the name of the unit with .d appended, for example sshd.server.d. In this directory you can create files with a .conf extension in which you can place the properties you lik to append or override. Both of these ways can also be done by using the systemctl command, this is further described in chaper 206.3.

There is also a third location available in which you can place files to override your unit definitions, this is the /run/systemd directory. Definitions in this directory take precedence over the files in /usr/lib/systemd but not over those in /etc/systemd. Overrides in /run/systemd will only be used until the system is rebooted since all files in /run/systemd will be deleted at a reboot of the system.

systemd-delta To get an overview of all overrides active you can use the systemd-delta command. This command can return the following types:

  • masked

    • Masked units, units that can't be started.
  • equivalent

    • Overridden files that do not differ in content.
  • redirected

    • Symbolic links to other unit files.
  • overridden

    • Overridden unit files.
  • extended

    • Extended unit files using a .conf file.

You can also filter by the types above using the -t of --type= flags, these take a list of the above types. If you also want to see unchanged files you can add unchanged as a type. Other options you can use are --diff=false if you don't want systemd-delta to show the diffs of overridden files, and --no-pager if you don't want the output piped to a pager.

Now that we know where the unit files are stored and how we can override them we can take a look at changing system states.

For getting and changing the (default) system state we use the systemctl command. A full explanation of the possibilities for this command can be found in chapter 206.3.

If we want to get the default target we can run the following command:

    $ systemctl get-default

This will output the current default target which is used at boot. To get a list of all currently loaded target units you can run the following commandL

    $ systemctl list-units --type=target

This will give you a list of all currently active targets. To get all available target units you can run the following command:

    $ systemctl list-unit-files --type=target

If you want to change the default target you can use the systemctl set-default command. For example, to set the default target to you can run the following command:

    $ systemctl set-default

This command will create a symbolic link /etc/systemd/system/ which links to the target file in /usr/lib/systemd/system.

A comparison between the init runlevels and the system targets:

Runlevel Target Description
0, Shutdown and poweroff the system.
1, Set up a rescue shell.
2, Set up a non-graphical multi-user system.
3, Set up a non-graphical multi-user system.
4, Set up a non-graphical multi-user system.
5, Set up a graphical multi-user system.
6, Shutdown and reboot the system.

You can also change the system state at runtime, this can be done using the systemctl isolate command. This will stop all services not defined for the chosen target, and start all the services that are defined for the target.

For example, to go to rescue mode, you can run the following command:

    $ systemctl isolate

This will stop all services except those defined for the rescue target. Note that this command will not notify any logged in users of the action.

There are also some shortcuts available to change the system state, these have an added bonus that they will notify the logged in users of the action. For example, another way to get the system into rescue mode is the following:

    $ systemctl rescue

This will first notify the logged in users of the action and then stop all services not defined in the target. If you don't want the users to get notified you can add the --no-wall flag.

If your system is too broken to use rescue mode there is also an emergency mode available. This can be started by using the following command:

    $ systemctl emergency

If you want to start certain services at boot you have to enable them. You can enable services using the systemctl enable command. For example, to enable sshd you run the following command:

    $ systemctl enable sshd.service

To disable the service again you use systemctl disable. For example:

    $ systemctl disable sshd.service

The unit file of the service defines at which system state the service will be running. This is configured by setting the WantedBy= option under the [Install] section. If this is set to than it will run in both non-graphical as graphical mode.

The LSB standard

The Linux Standard Base (LSB) defines an interface for application programs that are compiled and packaged for LSB-conforming implementations. Hence, a program which was compiled in an LSB compatible environment will run on any distribution that supports the LSB standard. LSB compatible programs can rely on the availability of certain standard libraries. The standard also includes a list of mandatory utilities and scripts which define an environment suitable for installation of LSB-compatible binaries.

The specification includes processor architecture specific information. This implies that the LSB is a family of specifications, rather than a single one. In other words: if your LSB compatible binary was compiled for an Intel based system, it will not run on, for example, an Alpha based LSB compatible system, but will install and run on any Intel based LSB compatible system. The LSB specifications therefore consist of a common and an architecture-specific part; "LSB-generic" or "generic LSB" and "LSB-arch" or "archLSB".

The LSB standard lists which generic libraries should be available, e.g. libdl, libcrypt, libpthread and so on, and provides a list of processor specific libraries, like libc and libm. The standard also lists searchpaths for these libraries, their names and format (ELF). Another section handles the way dynamic linking should be implemented. For each standard library a list of functions is given, and data definitions and accompanying header files are listed.

The LSB defines a list of 130+ commands that should be available on an LSB compatible system, and their calling conventions and behaviour. Some examples are cp, tar, kill and gzip, and the runtime languages perl and python.

The expected behaviour of an LSB compatible system during system initialization is part of the LSB specification. So is a definition of the cron system, and are actions, functions and location of the init scripts. Any LSB compliant init script should be able to handle the following options: start, stop, restart, force-reload and status. The reload and try-restart options are optional. The standard also lists the definitions for runlevels and listings of user- and groupnames and their corresponding UID's/GID's.

Though it is possible to install an LSB compatible program without the use of a package manager (by applying a script that contains only LSB compliant commands), the LSB specification contains a description for software packages and their naming conventions.

Note LSB employs the Red Hat Package Manager standard. Debian based LSB compatible distributions may read RPM packages by using the alien command.

The LSB standards frequently refers to other well known standards, for example ISO/IEC 9945-2009 (Portable OS base, very Unix like). Any LSB conforming implementation needs to provide the mandatory portions of the file system hierarchy as specified in the Filesystem Hierarchy Standard (FHS), and a number of LSB specific requirements. See also the section on the FHS standard.

The bootscript environment and commands

Initially, Linux contained only a limited set of services and had a very simple boot environment. As Linux aged and the number of services in a distribution grew, the number of initscripts grew accordingly. After a while a set of standards emerged. Init scripts would routinely include some other script, which contained functions to start, stop and verify a process.

The LSB standard lists a number of functions that should be made available for runlevel scripts. These functions should be listed in files in the directory /lib/lsb/init-functions and need to implement (at least) the following functions:

  1. start_daemon [-f] [-n nicelevel] [-p pidfile] pathname [args...]

    runs the specified program as a daemon. The start_daemon function will check whether the program is already running. If so, it will not start another copy of the daemon unless the -f option is given. The -n option specifies a nice level.

  2. killproc [-ppidfile] pathname [signal]

    will stop the specified program, trying to terminate it using the specified signal first. If that fails, the SIGTERM signal will be sent. If a program has been terminated, the pidfile should be removed if the terminated process has not already done so.

  3. pidofproc [-p pidfile] pathname

    returns one or more process identifiers for a particular daemon, as specified by the pathname. Multiple process identifiers are separated by a single space.

In some cases, these functions are provided as stand-alone commands and the scripts simply assure that the path to these scripts is set properly. Often some logging functions and function to display status lines are also included.

Changing and configuring runlevels

Changing runlevels on a running machine requires comparison of the services running in the current runlevel with those that need to be run in the new runlevel. Subsequently, it is likely that some processes need to be stopped and others need to be started.

Recall that the initscripts for a runlevel "X" are grouped in directory /etc/rc.d/rcX.d (or, on newer (LSB based) systems, in /etc/init.d/rcX.d). The filenames determine how the scripts are called: if the name starts with a "K", the script will be run with the stop option, if the name starts with a "S", the script will be run with the start option. The normal procedure during a runlevel change is to stop the superfluous processes first and then start the new ones.

The actual init scripts are located in /etc/init.d. The files you find in the rcX.d directory are symbolic links which link to these. In many cases, the start- and stop-scripts are symbolic links to the same script. This implies that such init scripts should be able to handle at least the start and stop options.

For example, the symbolic link named S06syslog in /etc/init.d/rc3.d might point to the script /etc/init.d/syslog, as may the symbolic link found in /etc/init.d/rc2.d, named K17syslog.

The order in which services are stopped or started can be of great importance. Some services may be started simultaneously, others need to start in a strict order. For example your network needs to be up before you can start the httpd. The order is determined by the names of the symbolic links. The naming conventions dictate that the names of init scripts (the ones found in the rcN.d directories) include two digits, just after the initial letter. They are executed in alphabetical order.

In the early days system administrators created these links by hand. Later most Linux distributors decided to provide Linux commands/scripts which allow the administrator to disable or enable certain scripts in certain runlevels and to check which systems (commands) would be started in which runlevel. These commands typically will manage both the aforementioned links and will name these in such a way that the scripts are run in the proper order.

The chkconfig command

Another tool to manage the proper linking of start up (init) scripts is chckconfig. On some systems (e.g. SuSE/Novell) it serves as a front-end for insserv and uses the LSB standardized comment block to maintain its administration. On older systems it maintains its own special comment section, that has a much simpler and less flexible syntax. This older syntax consists of two lines, one of them is a description of the service, it starts with the keyword description:. The other line starts with the keyword chkconfig:, and lists the run levels for which to start the service and the priority (which determines in what order the scripts will be run while changing runlevels). For example:

    # Init script for foo daemon
    # description: food, the foo daemon
    # chkconfig: 2345 55 25

This denotes that the foo daemon will start in runlevels 2, 3, 4 and 5, will have priority 55 in the queue of initscripts that are run during startup and priority 25 in the queue of initscripts that are run if the daemon needs to be stopped.

The chkconfig utility can be used to list which services will be started in which runlevels, to add a service to or to delete it from a runlevel and to add an entire service to or to delete it from the startup scripts.

Note We are providing some examples here, but be warned: there are various versions of chkconfig around. Please read the manual pages for the chkconfig command on your distribution first.

chkconfig does not automatically disable or enable a service immediately, but simply changes the symbolic links. If the cron daemon is running and you are on a Red Hat based system which is running in runlevel 2, the command

    # chkconfig --levels 2345 crond off

would change the administration but would not stop the cron daemon immediately. Also note that on a Red Hat system it is possible to specify more than one runlevel, as we did in our previous example. On Novell/SuSE systems, you may use:

    # chkconfig food 2345

and to change this so it only will run in runlevel 1 simply use

    # chkconfig food 1

    # chkconfig --list

will list the current status of services and the runlevels in which they are active. For example, the following two lines may be part of the output:

    xdm                       0:off   1:off   2:off   3:off   4:off   5:on    6:off
    xfs                       0:off   1:off   2:off   3:off   4:off   5:off   6:off

They indicate that the xfs service is not started in any runlevel and the xdm service only will be started while switching to runlevel 5.

To add a new service, let's say the foo daemon, we create a new init script and name it after the service, in this case we might use food. This script is consecutively put into the /etc/init.d directory, after which we need to insert the proper header in that script (either the old chkconfig header, or the newer LSB compliant header) and then run

    # chkconfig --add food

To remove the foo service from all runlevels, you may type:

    # chkconfig --del food

Note, that the food script will remain in the /etc/init.d/ directory.