Linux Runlevels Explained

Posted on August 15, 2010 by Matt Aurand | Updated: March 8, 2021

Category: Tutorials | Tags: Chkconfig, Init scripts, Linux, Linux Services, Modes, Runlevels, Services, Single User mode

Reading Time: 3 minutes

A runlevel is one of the modes that a Unix-based, dedicated server or a VPS server OS will run on. Each runlevel has a certain number of services stopped or started, giving the user control over the behavior of the machine. Conventionally, seven runlevels exist, numbered from zero to six.

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.

Standard run levels for RedHat based distributions
Run Level	Mode	Action
0	Halt	Shuts down system
1	Single-User Mode	Does not configure network interfaces, start daemons, or allow non-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.

Init scripts

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.

chkconfig

The chkconfig tool is used in RedHat 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.

root@host:~ # chkconfig --list filelimits 0:off 1:off 2:on 3:on 4:on 5:on 6:off syslog 0:off 1:off 2:on 3:on 4:on 5:on 6:off gpm 0:off 1:off 2:on 3:on 4:on 5:on 6:off cpanel 0:off 1:off 2:off 3:on 4:on 5:on 6:off kudzu 0:off 1:off 2:off 3:on 4:on 5:on 6:off ntpd 0:off 1:off 2:off 3:off 4:off 5:off 6:off netfs 0:off 1:off 2:off 3:on 4:on 5:on 6:off network 0:off 1:off 2:on 3:on 4:on 5:on 6:off netplugd 0:off 1:off 2:off 3:off 4:off 5:off 6:off rawdevices 0:off 1:off 2:off 3:on 4:on 5:on 6:off ipchains 0:off 1:off 2:on 3:on 4:on 5:on 6:off iptables 0:off 1:off 2:on 3:on 4:on 5:on 6:off crond 0:off 1:off 2:on 3:on 4:on 5:on 6:off anacron 0:off 1:off 2:on 3:on 4:on 5:on 6:off cups 0:off 1:off 2:on 3:on 4:on 5:on 6:off xfs 0:off 1:off 2:on 3:on 4:on 5:on 6:off xinetd 0:off 1:off 2:off 3:on 4:on 5:on 6:off httpd 0:off 1:off 2:off 3:on 4:off 5:on 6:off

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.

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