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A Description of the different Linux runlevels

Linux provides 6 different runlevels. For information on how to change the default runlevel at system startup read the Linuxtopia How do I change the default run level of my Linux system? Guide or to switch runlevel while the system is running read the Linuxtopia - How do I dynamically change my Linux run level without rebooting? Guide.

The 6 Linux runlevels are descriped in Table 1 below. Bear in mind that you are not lilely to want to have your system default to starting into some of these levels:

Table 1: Linux Runlevel Descriptions
Runlevel Description
Runlevel 0
The halt runlevel - this is the runlevel at which the system shuts down. For obvious reasons it is unlikely you would want this as your default runlevel.
Runlevel 1
Single runlevel. This causes the system to start up in a single user mode under which only the root user can log in. In this mode the system does not start any networking or X windowing, X or multi-user services. This run level is ideal for system administrators to perform system maintenance or repair activities.
Runlevel 2
Boots the system into a multi-user mode with text based console login capability. This runlevel does not, however, start the network.
Runlevel 3
Similar to runlevel 2 except that networking services are started. This is the most common runlevel for server based systems that do not require any kind of graphical desktop environment.
Runlevel 4
Undefined runlevel. This runlevel can be configured to provide a custom boot state.
Runlevel 5
Boots the system into a networked, multi-user state with X Window System capability. By default the graphical desktop environment will start at the end of the boot process. This is the most common run level for desktop or workstation use.
Runlevel 6
Reboots the system. Another runlevel that you are unlikely to want as your default.

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