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