Runlevels on Linux

• 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.