Installation guide

The inittab file is composed of an unlimited number of lines, each with
four fields; each field is separated by a colon. The fields and syntax for
entries in the inittab file are as follows:
Identifier: Runlevel: Action: Command
Identifier
This 14-character field uniquely identifies an object entry.
Runlevel
This 20-character field defines the run levels in which the object entry
is to be processed. The
Runlevel variable corresponds to a
configuration of processes in a system. Each process spawned by the
init command is assigned one or more run levels in which it is
allowed to exist. The run levels are as follows:
0
Specifies the halt state
s or S
Specifies single-user mode
2
Specifies multiuser mode without network services
3
Specifies multiuser mode with network services
The Runlevel field can define multiple run levels for a process by
specifying more than one run level character in any combination.
Action
This 20-character field tells init how to treat the specified process.
The most common actions that init recognizes are as follows:
Customizing the System Environment 4–5