Linux Run Levels

Linux run levels details

After the Linux kernel has booted, the init program reads the /etc/inittab file to determine the behavior for each runlevel. Unless the user specifies another value as a kernel boot parameter, the system will attempt to enter (start) the default runlevel.

Here is an example of stanard run Levels under Centos Linux.

RL           Mode                               Action

0              Halt                                  Shuts down system

1              Single-User Mode            Does not configure network interfaces, start daemons,no root logins

2              Multi-User Mode               Does not configure network interfaces or start daemons.

3              Multi-User Mode 

                with  Networking               Starts the system normally.

4              Undefined                          Not used/User-definable

5              X11                                    As runlevel 3 + display manager(X)

6              Reboot                              Reboots the system

Most Linux servers lack a graphical user interface and therefore start in runlevel 3. Servers with a GUI and desktop Unix systems start runlevel 5. When a server is issued a reboot command, it enters runlevel 6.

To see what level is currently using your Distro, you can use the who -r command.

Linux also uses the init scripts to initialize proper service to the user. Init (short for initialization) is the program on Unix and Unix-like systems that spawns all other processes. It runs as a daemon and typically has PID 1.

The /etc/inittab file is used to set the default run level for the system. This is the runlevel that a system will start up on upon reboot. The applications that are started by init are located in the /etc/rc.d folder. Within this directory there is a separate folder for each run level, eg rc0.d, rc1.d, and so on.

The chkconfig tool is used in Red Hat based systems (like CentOS) to control what services are started at which runlevels. Running the command chkconfig –list will display a list of services whether they are enabled or disabled for each runlevel.

On the next graphic we can list the current scripts that have been configured to run at certain levels, or not to run on certain run levels in a Centos enviroment.

Single User mode is a mode that a multi-user system (like a Linux server) can be booted into the operating system as a superuser. Booting a system into this mode does not start networking, but can be used to make changes to any configuration files on the server. One of the most common usages for single-user mode is to change the root password for a server on which the current password is unknown.

Runlevels are an important part of the core of the Linux operating system. While not something the average administrator will work with on a daily basis, understanding runlevels gives the administrator another layer of control and flexibility over the servers they manage.

As we have seen here, the traditional method of booting a Linux system is based on the UNIX System V init process. It involves loading an initial RAM disk (initrd) and then passing control to a program called init, a program that is usually installed as part of the sysvinit package. The init program is the first process in the system and has PID (Process ID) 1. It runs a series of scripts in a predefined order to bring up the system. If something that is expected is not available, the init process typically waits until it is. While this worked adequately for systems where everything is known and connected when the system starts, modern systems with hot-pluggable devices, network file systems, and even network interfaces that may not be available at start time present new challenges