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Linux Run Levels

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RHEL Runlevels Red Hat Startup Scripts Suse Runlevels Suse init scripts Creating your own init scripts  
service chkconfig /etc/inittab Changing Runlevels Linux root password recovery Booting into Rescue Mode
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Introduction

The idea of runlevels came from System V and is pretty neat idea, which unfortunately was made more complex then necessary in recent Linux distributions (especially Suse, Red Hat is more reasonable).  

In its initial form it allows, without increasing complexity of the system , substantial flexibility in defining set of running daemons for the one of several "final" stages to which system can be booted. With runelvels there can be multiple such states.  This about it as a neat generalization of MS DOS authoexec.bat file.

Each runlevel is defined by a set of daemons that it should be started on entering this runlevel (and by extension a set of daemons that it need to be shut down, when you exit this runlevel). In other words, each runlevel is configured to start its specified services when starting and to shut down its services when switching to a different runlevel.

Availability of different  system states (runlevels), each with own set of running daemons can be used for recovery and for optimizing system for running a particular application. This scheme also provides an opportunity to set a default level to which system is booted.  As a side effect of this scheme you can change from one runlevel to another dynamically without rebooting the system. This is another big plus of runlevel as implemented in System V. In this scheme shutting down the system becomes just a switch to a special (fake) runlevel. 

Switching from higher runlevel to lower, for example from 5 to 3,  usually involves killing some daemons. And vise versa, switching to high runlevel, for example from 3 to 5,  involves starting some daemons.  But please note that notions or higher runlevels and lower runlevels are human-defined. For init each runlevel is just a set of daemons to kill plus a set of daemons to start. Numeric value of the runlevel does not have any meaningful semantic in init. All runlevels are treated atomically, completely independently of each other.

So you can (for example, as a joke) switch set of daemons on runlevel 5 to runlevel 3 and vise versa (by renaming the directories) and init will eat this change as if this is what mother ordered.  Urban legend that Unix enters lower runlevels before reaching specified does not have any real ground. It is just a myth. 

For Unix OS runlevels do not have any meaningful order. They are treated by OS atomically, completely independently of each other.  Urban legend that Unix enters lower runlevels before reaching specified does not have any real ground. It is just a myth. 

Typically System V based flavors of Unix has the following four runlevels defined:

  1. Single user,
  2. Multi-user without networking,
  3. Full multiuser
  4. Full multi-user with X11. 

Meaning of runlevels differs between various Linux flavors. There are two big camp:

Runlevels in Red Hat, Suse and all other members of Red Hat family:

In addition two "pseudo-runlevels ( 0 (halt) and 6 (reboot) in linux) are assigned for shutting system down and rebooting it.  So we have 4+2=6 different runlevels.  Sometimes (in Solaris) level S is defined for runlevel that does not run any init scripts. As some levels are not used and you can define your own (see below).

Root user can switch from any runlevel to any. Switching to the single user mode and back to multi-user with networking is equivalent to "soft reboot" as it restarts all major daemons and is much quicker then full reboot of the server. 

Switching to the single user mode and back to multi-user with networking or multi user with X11 is equivalent to "soft reboot" as it restarts all major daemons and is much quicker then full reboot of the server.  

Runlevel structure in different flavors of Linux

While both Solaris and Linux are based on System V, Linux uses different set of runlevels then Solaris. Moreover meaning of runlevels differs between various Linux flavors. There are two big camps:

Structure on init scripts

There are multiple directories involved in storage of init scripts. There is one directory that stores the scripts, called /etc/init.d. And there are additional directories, one for each runlevel (/etc/rc.0.d, /etc/rc1.d, ... /etc/rc6.d) which stores links to those scripts. Links can be of two types:

Changing Runlevels

Two sets of init scripts are involved in switching runlevels: K-scripts from the runlevel on which we are currently running and S-scripts from a new (target) runlevel. It is important to understand that there are two runlevels involved in switching:

Simplifying the rc script which is responsible for changing runlevels performs the following two operations.

  1. All K-scripts that exist on the current runlevel l_from are processed. For each K-script for which S-script does not exists at the target runlevel l_to, the  K-script from l_from is executed.
  2. Each S script that exist at target runlevel l_to is executed

In other words when you change the runlevel first stop scripts of the current runlevel are launched, closing down some daemons running on the current runlevel that are not necessary on a new runlevel (do not have S-script defined for them). Then all start scripts of the new runlevel are run in the order defined by their numeric priorities (lexographic order).

For example, the following occurs when changing from runlevel 3 to 5:

  1. The administrator (root) enter the command init 5 which tells init to change the current runlevel to 5. 
  2. The init consults its configuration file (/etc/inittab) and determines it should start /etc/init.d/rc with the new runlevel as a parameter.
  3. 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 a K. The number following K specifies the order to start, because there are some dependencies to consider.
  4. 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 with an 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 checks /etc/inittab for changes and starts the appropriate scripts. 

The init command allows you to reboot computer using as a switch to runlevel 0. Here is an example:

# init 0

A shutdown command should generally be used instead as it prompts you  if your action is correct and can delay actual shutdown so that that user can close their programs befors reboot; you can also specify interval before reboot or exact time when it should happen. By default the command requires acknowledgement (which can be cancelled using the -y switch). The -h switch forces the system to halt, and the first argument tells it how long to wait before starting the shutdown process. minutes.  For example (you can use the word now intstead of 0):

# shutdown -hy 0

Broadcast message from root (pts/0) (Sat Nov  6 13:15:27 2004):

The system is going down for system halt NOW!
#

You can also use the init command to reboot the system immediately by entering runlevel 6.

# init 6

The "reboot" command has the same effect, but it also sends a warning message to all users.

# reboot

Broadcast message from root (pts/0) (Sat Nov  6 12:39:31 2004):

The system is going down for reboot NOW!
#

More graceful reboots can be done with the shutdown command using the -r switch and specifying a delay. For example reboot with the delay of 10 minutes:

# shutdown -ry 10

Broadcast message from root (pts/0) (Sat Nov  6 13:26:39 2004):

The system is going DOWN for reboot in 10 minutes!

Broadcast message from root (pts/0) (Sat Nov  6 13:27:39 2004):

The system is going DOWN for reboot in 9 minutes!
...
...
...
Broadcast message from root (pts/0) (Sat Nov  6 13:36:39 2004):

The system is going down for reboot NOW!

Some activities require you to force the system to log off all users, third-party applications and networking so that only the systems administrator has access to the system. This can be done by using the  command init 1. It is best to do this from the console, because if you do it from a remote terminal session you'll be logged out.

# init 1

In this case there  no prior warning to users. To broadcase a warning, you can run the shutdown command with a delay in minutes as the only argument.

# shutdown 1

Broadcast message from root (pts/0) (Sat Nov  6 13:44:59 2004):

The system is going DOWN to maintenance mode in 1 minute!

Broadcast message from root (pts/0) (Sat Nov  6 13:45:59 2004):

The system is going down to maintenance mode NOW!

...
...
#

Entering Single-user Mode At The Grub Splash Screen

You can enter single user mode directly after turning on the power to your system. The steps to do this are listed below.

1. Power on your system. Wait for the "Grub loading" message to appear and, depending on your Linux distribution, get ready to hit either any key or the ESC key to enter the grub boot menu.

Grub loading, please wait ...
Press ESC to enter the menu

or

Grub loading, please wait ...
Press any key to enter the menu

2. You will then get grub's main menu which will display a list of available kernels. Use the arrow keys to scroll to your desired version of the kernel and then press e for "edit".

Fedora Core (2.6.18-1.2239.fc5smp)
 

3. The kernel's boot menu will appear. Use the arrow keys to scroll to the "kernel" line and then press e for "edit".

root (hd0,0)
kernel /vmlinuz-2.6.18-1.2239.fc5smp ro root=LABEL=/
initrd /initrd-2.6.18-1.2239.fc5smp.img

4. A grub edit prompt will appear. Use the arrow keys to move to the end of the line and add the word "single" to the end, separated by a space. Change

grub edit> kernel /vmlinuz-2.6.18-1.2239.fc5smp ro root=LABEL=/

to

grub edit> kernel /vmlinuz-2.6.18-1.2239.fc5smp ro root=LABEL=/ single

5. Press enter to save your changes, and then b for "boot".

6. The system will continue to boot, but will go straight to the root # prompt without first asking for a username and password.

Reverting To Your Default runlevel From Single User Mode

The exit command forces the system to exit runlevel 1 and revert to the default runlevel for the system. You can also use the init command (for example, init 3 and init 5) to alter this default behavior:

# exit
INIT: Entering runlevel: 3
...
...
...
Fedora Core release 2 (Tettnang)
Kernel 2.6.8-1.521 on an i686
bigboy login:

Root Password Recovery

Sometimes you might forget the root password, or the previous systems administrator may move on to a new job without giving it to you. To do this by booting either directly to bash (see Boot Directly into a Shell) or, if single mode is not password protected, to a single user mode. See Linux root password recovery

Defining new runlevels

Typically you can define new runlevel by copying /etc/rc5.d or other already existing runlevel directories and then making the necessary changes.  There are two issues here:

Like in many other cases it make sense to leave existing single digit runlevels as "system runlevels" and copy the content and then modify it under a new, unused runlevel number (4, 7, 8, or 9).  Here are some possibilities that are connected with new runlevels: 

Sometimes instead of calling script that starts several products you can incorporate each step as a separate init script.  For example this can be done with such products as Oracle Fusion and LDAP server, In this case the start priorities for consequent steps should be sequential 

Comparing Runlevels

To see the difference between levels you can use diff  command, for example:

root@nti01:/etc/rc.d # diff <(ls rc2.d) <(ls rc3.d)
2,3d1
< K02cups-config-daemon
< K02haldaemon
5,6d2
< K03messagebus
< K05atd
17d12
< K44rawdevices
21,25d15
< K50xinetd
< K56acpid
< K68rpcidmapd
< K69rpcgssd
< K72autofs
29,30d18
< K74ntpd
< K75netfs
32d19
< K86nfslock
34,35d20
< K87irqbalance
< K87portmap
42d26
< K95kudzu
47a32
> S05kudzu
54a40,42
> S13irqbalance
> S13portmap
> S14nfslock
55a44,46
> S18rpcidmapd
> S19rpcgssd
> S25netfs
57a49
> S28autofs
58a51
> S44acpid
60a54,56
> S56rawdevices
> S56xinetd
> S58ntpd
65a62,63
> S95atd
> S97messagebus
66a65,66
> S98cups-config-daemon
> S98haldaemon

The default runlevel for a system is specified in the /etc/inittab file, which will contain an entry such as id:3:initdefault: if the system starts in runlevel 3, or id:5:initdefault: if it starts in runlevel 5.

There are three method of determining the current runlevel

  1.  who -r command:
    # who -r
    run-level 3  Aug 27 10:12                   last=5
  2. Using runlevel command which produces essentially the same result but in a form of just two numbers. Runlevel command retrieves information from the /var/run/utmp to locate the runlevel record.
  3. The last method is not working in mainstream flavors of Linux (Red Hat 4 & 5 and Suse 10 & 11) but is an interesting option.  In some versions of init the environment variables RUNLEVEL and PREVLEVEL are populated and changed with each change, similar to $PWD and other system variables. In this case you can simply type:
    echo $RUNLEVEL

Runlevel Utilities

One of the best ways to configure runlevels is to use an init-script utility. These tools are designed to simplify the task of maintaining files in the SysV init directory hierarchy and relieves system administrators from having to directly manipulate the numerous symbolic links in the subdirectories of /etc/rc.d/. Red Hat Enterprise Linux provides three such utilities:


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[Jan 11, 2011] Learn Linux, 101 Runlevels, shutdown, and reboot by Ian Shields

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Changing runlevels

There are several ways to change runlevels. To make a permanent change, you can edit /etc/inittab and change the default level as you just saw above.

If you only need to bring the system up in a different runlevel for one boot, you can do this. For example, suppose you just installed a new kernel and need to build some kernel modules after the system booted with the new kernel, but before you start the X Window System. You might want to bring up the system in runlevel 3 to accomplish this. You do this at boot time by editing the kernel line (GRUB) or adding a parameter after the selected system name (LILO). Use a single digit to specify the desired runlevel (3, in this case). We'll illustrate the process with a GRUB example. Suppose your /boot/grub/menu.lst file contains the stanza shown in Listing 2.

Listing 2. Typical GRUB stanza to boot Fedora 8
                           
title Fedora (2.6.26.8-57.fc8)
        root (hd0,5)
        kernel /boot/vmlinuz-2.6.26.8-57.fc8 ro root=LABEL=FEDORA8 rhgb quiet
        initrd /boot/initrd-2.6.26.8-57.fc8.img

To bring this system up in runlevel 3, wait till the boot entries are displayed, select this entry and enter 'e' to edit the entry. Depending on your GRUB options, you may need to press a key to display the boot entries and also enter 'p' and a password to unlock editing.

...

In this example, you should now see the root, kernel, and initrd lines displayed. Move the cursor to the line starting with "kernel" and press 'e' to edit the line.

... ... ...

Finally, move the cursor to the end of the line, and add a space and the digit '3'. You may remove 'quiet' if you wish, or modify any other parameters as needed.

... ... ...

Finally, press Enter to save the changes, then type 'b' to boot the system.

Note: The steps for doing this using LILO or GRUB2 differ from those for GRUB, but the basic principle of editing the way the kernel is started remains. Even GRUB screens on other systems or other distributions may look quite different to those shown here. Prompts will usually be available to help you.

Once you have finished your setup work in runlevel 3, you will probably want to switch to runlevel 5. Fortunately, you do not need to reboot the system. You can use the telinit command to switch to another runlevel. Use the runlevel command to show both the previous runlevel and the current one. If the first output character is 'N', the runlevel has not been changed since the system was booted. Listing 3 illustrates verifying and changing the runlevel.


Listing 3. Verifying and changing the runlevel
                    
[root@pinguino ~]# runlevel
N 3
[root@pinguino ~]# telinit 5

After you enter telinit 5 you will see several messages flash by and your display will switch to the configured graphical login screen. Open a terminal window and verify that the runlevel has been changed as shown in Listing 4.


Listing 4. Confirming the new runlevel
                    
[root@pinguino ~]# runlevel
3 5

If you use the ls command to display a long listing of the telinit command, you will see that it really is a symbolic link to the init command. We illustrate this in Listing 5


Listing 5. telinit is really a symbolic link to init
                    
[root@pinguino ~]# ls -l $(which telinit)
lrwxrwxrwx 1 root root 4 2008-04-01 07:50 /sbin/telinit -> init

The init executable knows whether it was called as init or telinit and behaves accordingly. Since init runs as PID 1 at boot time, it is also smart enough to know when you subsequently invoke it using init rather than telinit. If you do, it will assume you want it to behave as if you had called telinit instead. For example, you may use init 5 instead of telinit 5 to switch to runlevel 5.

Red Hat Magazine Run-levels Create, use, modify, and master by Noah Gift

Here is a chart of Red Hat-specific run-levels:

LEVEL NAME DESCRIPTION
0 Halt Immediately shuts down system and powers it off, if it can
1 Single user Brings system to a bare essentials mode for maintenance
2 User-defined Custom
3 Multi-user with console only All services are running but X11
4 User-defined Custom
5 Multi-user with display and console All services are running including X11 (or GUI)
6 Reboot Reboots the machine

What run-level am I?

Just like life, with an operating system you need to know where you are now, in order to get to where you want to go next. If you are planning on modifying your run-level, you need to first know what your current run-level is. In order to do this, you can use one of two commands, like so:

[root@localhost ~]# who -r
         run-level 3  2008-04-29 08:17                   last=5
[root@localhost ~]# runlevel
5 3

If we look at the output of who -r, we can tell that we are currently running at run-level 3–which is multi-user, but console only.We can also tell that we were previously running at run-level 5, which is multi-user with console and X11 login.

Changing run-levels

Once you know what run-level you are at, it is very simple to change to a different one. All you need to do is type: “init” followed by number of the runlevel you would like to switch to. Here is an example of switching to single user mode, or runlevel 1:

init 1

This command will change your system to single user mode, and it will ask you for the root password. When you arrive in single user mode, there are no services running, as this level it is most often used for maintenance, backup, or recovery. Once you are in single user mode it is quite common to enable, for example, network and NFS to backup your operating system, like so:

service network start; service nfs start

When you are done with your work, type in the run-level you would like to go to–perhaps run-level 5 which brings up the X11 login window:

init 5

Later in this article, we will write our own run-level and then use it to script a maintenance operation.

Permanently changing the default run-level

While changing the runlevel manually is most common, sometimes it is useful to change the default run-level from level 5 to level 3 permanently. This can help conserve resources inside of, for example, a virtual machine. You may also choose to define your own custom run-level, and wish to make that the default.

You will need to edit /etc/inittab and change this line with your favorite text editor:

id:5:initdefault:

Change ’5′ to the run-level you wish your machine to be at when it boots. To change the run-level so that it never loads the GUI on boot would look like this:

id:3:initdefault:
Note:
A word of caution on editing /etc/inittab. It is very important to keep /etc/inittab in version control, and/or keep a backup of it when you are editing the file. If you make a change incorrectly you can render your operating system unbootable.
Tip:
If you happen to get yourself in this pickle, there is a way out. You can interrupt the Grub boot loader and press “A”, and then append the word “emergency” to the end of the kernel arguments. This will boot the operating system without using init. Then, you can fix what you altered by copying back the original version of /etc/inittab.

Creating your own run-level HACK

First, a word of caution. Do not do this on a production machine, period! This section is a VERY dirty hack that you should only use on a virtual machine you can experiment with, or a machine you don’t mind rebuilding.. It is always a good idea to do testing inside of a virtual machine before doing something that could potentially render a box unbootable. This is a very quick and dirty way to alter a run-level for the purposes of learning, but perhaps you can get some ideas from it that can be used in a more production-oriented way. Ideally, some of the readers of this article will post some production quality hacks to creating custom run levels.

  1. cd to /etc/rc.d/rc4.d/
  2. do a sanity check to make sure you are running Red Hat: cat /etc/redhat-release
  3. backup existing run-level directory:
    mkdir /tmp/rc4.d.original/
    cp /etc/rc.d/rc4.d/* /tmp/rc4.d.original/
    
  4. rm -f /etc/rc.d/rc4.d/*

At this point

 cp /etc/rc.d/rc1.d/* /etc/rc.d/rc4.d/

We have now copied the run-level scripts for single user mode into our own custom run-level 4. We can hijack the the S99single script and tell it to do something different. In this example, we are going to write a custom Python script that gets forked to the background and backs up the machine over rsync. Let’s edit that file we copied:

vim /etc/rc.d/rc4.d/S99single

Change the last part of it to look like this:

# Now go to the single user level.
echo $"Telling INIT to go to single user mode."
echo "This is a custom code. Forking custom script"
/custom.py &
exec init -t1 S

We’ve inserted two lines. One echoes that we are forking off a custom script. The second line forks a python script, shown below, that backs up the machine via rsync. Note that this assumes you have set up ssh keys on the remote backup server.

custom.py script:

#!/usr/bin/env python
import time
import subprocess

rsync = "rsync -av / 10.0.1.3:/Volumes/Backup/server_backup/"
network = "service network start"
init = "init 3"

cmds = [rsync, network]

def single_user_backup():
    """Starts network service, creates backup and returns to init 3"""
    try:
        subprocess.call(network, shell=True)
        subprocess.call(rsync, shell=True)
    finally:
        subprocess.call(init, shell=True)

def main():
    """Runs program"""
    print "sleeping for 60 seconds"
    #time.sleep(60)  #Gives machine time to quiesce
    single_user_backup()

main()

The main function runs a sleep command for 60 seconds, just to give the single user mode scripts time to quiesce the box. Remember, this script is forked to the background. Next, function single_user_backup attempts to start network services and run rsync to remotely back up the whole / volume to another server. This is obviously crude and there will be lots of errors trying to back up /proc, for example, but it give you an idea of how an automated backup could work with a custom run-level. Finally, the machine gets called back to init 3, which is console only multi-user mode.

Tip:
Again, this is just an idea for a backup script, but not one I would actually run in production in my wildest dreams. One problem with this technique is that because of symbolic links in run level 1, we actually, changed run level 1 and our run level 4. This is not acceptable, obviously, for any sane user, but it is acceptable as a way to have fun with a disposable virtual machine!

If you can think of a more realistic backup script that would work from a custom run-level, I would love to see it. Create a how to on your blog, and then post a response to this article. Also, it would interesting to see other things such as database backups and migration done with custom run-levels as well. Leave a comment and let me know what you’d do.

Running your own run-level

To run the newly created run-level, you only need to type:

init 4

You will then see the custom print statements we inserted. The machine
will sleep for 60 seconds, and then run the rsync backup.

Summary

This article covered quite a bit of ground in a short while. We went over what a run-level was, how to tell what run-level you are at, how to change run-levels, and, finally, how to make your own run-level with custom, frankenstein quality, code. Hopefully, this showed you some new tricks and spurs some ideas for further innovation with run-levels.

References

[Jun 09, 2010] Booting and Shutting Down Your SuSE Linux Enterprise Server init and Understanding Runlevels

InformIT

init and Understanding Runlevels

It is difficult to discuss the init program without first having an understanding of what it controls. The init process is invoked by the kernel itself and becomes the parent of all other processes on a machine. At startup, the /etc/inittab file is interpreted by init and all identified routines are processed. The default runlevel for the system is maintained within inittab.

You can think of a Linux operating system runlevel as the "application state" of the machine. For each distinct state, select processes are functional. Table 3.1 displays the known list of runlevels and their impact on user access.

TABLE 3.1: RUNLEVELS

Runlevel Script Directory Definition
0 rc0.d System is stopped
S rc1.d Single user mode accessed from the boot prompt
1 rc1.d Single user mode
2 rc2.d Multiuser mode but no network available
3 rc3.d Multiuser mode with full network access
4 rc4.d N/A, an unused runlevel
5 rc5.d Multiuser network enabled including the X Windows System
6 rc6.d Forces a machine to reboot

You initiate transitions between states by running the init routine followed by the runlevel. Each transition, in turn, changes which processes are running. The application startups or shutdowns triggered by a change in the runlevel are managed through a series of script files. The directory /etc/init.d is the base directory for a collection of directories, each of which, represents a distinct runlevel. Within each level’s directory resides a collection of appropriate scripts for the applications required for the specific runlevel.

When the system enters a particular state, it initiates all the appropriate routines for that state by executing all the script files that begin with the letter S (Start) in the corresponding directory.

Similarly, when the system is leaving a source level for another, the scripts residing in the source directory beginning with the letter K (Kill) are processed. This ensures that all applications that are supposed to run only in a particular level are shut down properly before the system moves onto the next runlevel.

The one exception is one of efficiency. If a start script for a specific application exists in the target level, the stop (Kill) script in the source level is ignored.

As an example, you can examine the change of state from runlevel 3 and runlevel 5 on the xdm application. Xdm is the X Windows System display manager. Referring to Table 3.1, X is available only at runlevel 5. Listing 3.3 show the files associated with xdm in the /etc/init.d/rc5.d directory.

LISTING 3.3: xdm FILES ASSOCIATED WITH RUNLEVEL 5

Castor:/etc/init.d # ls -l rc5.d | grep -e xdm
lrwxrwxrwx  1 root root  6 Jan 5 04:53 K06xdm -> ../xdm
lrwxrwxrwx  1 root root  6 Jan 5 04:53 S16xdm -> ../xdm
Castor:/etc/init.d #

From Listing 3.3, it is possible to see that xdm is both started and stopped as the system enters or leaves runlevel 5. What is also apparent is that both the startup and shutdown scripts are simply links to the same script residing in /etc/init.d. This linking minimizes duplication of the scripts, ensuring a consistent application startup and shutdown independent of target runlevel.

The init process can therefore be defined as a tool used to control which applications are active on a system. You can identify which applications are active within a runlevel by looking at content of the subdirectory for the level in question.

Cool Solutions Runlevels in SUSE Linux

Posted: 3 May 2005

Linux operating systems utilize runlevels to determine the services that should be running and to allow specific work to be done on the system. For example, runlevel one is designated single-user mode without networking and is used for critical system maintenance and troubleshooting. Runlevel three is multi-user text mode. Runlevel five is multi-user graphical mode.

Switching between runlevels is accomplished using the init command. Entering:

# init 5

will change to runlevel five. Changing to runlevel six will reboot the machine and runlevel zero will shut down the machine.

Entering runlevel at the command prompt will output the previous and current runlevels. For example:

#runlevel
N 5

indicates that the machine is in runlevel five. The 'N' indicates that the machine was booted into level five, or no previous runlevel was entered.

As mentioned previously, the different runlevels are configured to run a specific set of services. Each runlevel is configured to start its specified services when starting and to shut down its services when switching to a different runlevel.

As a general rule, turn off all services that are not needed at a given runlevel. Managing the services started in each runlevel is simple and can be done graphically in YaST or at the command line with the chkconfig command.

Managing Runlevel Services With YaST

YaST offers a robust and easy to use runlevel service configuration tool. Access the tool at YaST > System > Runlevel Editor. Within the tool, turn services on or off by selecting the service and then selecting the Enable/Disable buttons.

Selecting expert mode allows the administrator to configure the state of a service in each runlevel.

For example, use the YaST runlevel editor to configure the OpenSLP daemon, slpd, to start in runlevels three and five only. Do the following:

  1. Select YaST > System > Runlevel Editor.
  2. Select the Expert Mode radio button.
  3. In the services list, select slpd.
  4. Use the check boxes to select runlevels three and five. Deselect all other runlevels.
  5. Save changes and exit.

Managing Runlevel Services With chkconfig

Most Linux distributions include the chkconfig command for managing runlevel services. The syntax for chkconfig is specified in the chkconfig man page.

For example, use chkconfig to configure the OpenSLP daemon, slpd, to start in runlevels three and five only. Do the following:

1.Check if slpd is on in the current runlevel

# chkconfig slpd
slpd on

2.Check slpd's configuration for every runlevel.

# chkconfig -l slpd
slpd 0:off 1:off 2:off 3:off 4:off 5:off 6:off

3.Turn slpd on in runlevels three and five.

#chkconfig slpd 35

Understanding Runlevels

Unlike most non-UNIX operating systems which only have 2 modes of functionality (on and off), UNIX operating systems, including Linux, have different runlevels such as "maintenance" runlevel or "multi-user" runlevel, etc.

Runlevels are numbered from 0 to 6 and will vary from one Linux distribution to another. The description for each runlevel functionality is sometimes documented in /etc/inittab.

Runlevel Purpose
0 Shuts down the machine safely. The operating system will also attempt to poweroff the system if possible
1 Single user mode. Only one terminal is available for the (single) user root. All other users are logged out
2[a] Multi-user mode, but does not start NFS. Network services like email or web services are also stopped
3[a] Full multi-user mode. Selected network services are all on.
4[a] Not defined and generally unused
5[a] Like runlevel 3 but runs a Display Manager as well
6 restarts the machine safely
[a] These runlevels may vary in functionality between Linux flavours. But these descriptions correspond to the current LSB specification.

2.2.1. INIT Controls Runlevels

Both init and telinit are used to switch from one runlevel to another. Remember that init is the first program launched after the kernel has accessed the root device.

At boot time init is instructed which runlevel to reach in /etc/inittab with the line:

id:5:initdefault:

When the system is started it is possible to change runlevels by invoking init (or telinit which is a symbolic link pointing at init).

For example we switch to runlevel 4 with either of the next commands:

init 4
telinit 4

The PID for init is always 1. It is possible to find out which runlevel the system is currently in with the command runlevel

runlevel
N 5

The first number is the previous runlevel (or N if not applicable) and the second number is the current runlevel.

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