4.3 About Run Levels

Run levels allow you to start a system with only the services that are required for a specific purpose. For example, a server can run more efficiently at run level 3, because it does not run the X Window System at that run level. It is best to perform diagnostics, backups, and upgrades at run level 1 when only root can use the system. Each run level defines the services that init stops or starts. For example, run level 3 starts network services and run level 5 starts the X Window System, whereas run level 1 stops both of these services.

The following run levels are defined by default under Oracle Linux:


Halts the system.


Single-user text mode.


Full multiuser text mode without NFS support.


Full multiuser text mode with NFS support.


Not used, but can be user defined.


Full multiuser graphical mode with an X-based login screen.


Reboots the system.