3 Run levels

To understand which services are important, it is important to first understand the concept of run levels. Each run level defines a set of services to support. Some run levels are special. This is a list of run levels:

• S: this is the initial run level when a system starts up. This is not a run level that a running system should be in.
• 0: this is for “halting” the system. This is used to shutdown a system.
• 1: this is the “single user” mode. The single user mode disables all services that are useful only for a multiple user system. In other words, only core services necessary for the system are enabled.
• 2-5: these are “multiple user” modes. Run level 2 is usually the “text-only” run level, which does not start up any GUI interface.
• 6: this is for rebooting the system.

Normally, when a system starts up, it enters run level S, then run level 1, and then run level 5.