A runlevel describes a certain operating state of your system. The system behaves differently on different runlevels.
| 0 | Halt |
|---|---|
| 1 | Single user mode |
| 2 | Multiuser, without NFS (The same as 3, if you do not have networking) |
| 3 | Full multiuser mode |
| 4 | unused |
| 5 | X11 |
| 6 | Reboot |
init is the parent of all processes on the system, it is executed by the kernel and is responsible for starting all other processes; it is the parent of all processes whose natural parents have died and it is responsible for reaping those when they die.
Processes managed by init are known as jobs and are defined by files in the /etc/init directory. See init for more details on configuring Upstart.
init is an event-based init daemon. This means that jobs will be automatically started and stopped by changes that occur to the system state, including as a result of jobs starting and stopping.
This is different to dependency-based init daemons which start a specified set of goal jobs, and resolve the order in which they should be started and other jobs required by iterating their dependencies.
The primary event is the startup event, emitted when the daemon has finished loading its configuration. Other useful events are the starting, started, stopping and stopped events emitted as jobs change state.
Only the administrator (root) is allowed to change the runlevel of the machine.
init [OPTION]...
Options are passed to init by placing them on the kernel command-line.
<runlevel> Change to runlevel <runlevel>.
-q, --quiet reduce output to errors only
-v, --verbose increase output to include informational messages
--help display this help and exit
--version output version information and exit
[root@server1 ~]# init 5