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:
0Halts the system.
1Single-user text mode.
2Full multiuser text mode without NFS support.
3Full multiuser text mode with NFS support.
4Not used, but can be user defined.
5Full multiuser graphical mode with an X-based login screen.
6Reboots the system.