linux- run level

A, init linux system is one indispensable program.
The so-called init process, which is initiated by a user-level kernel process.
After starting the kernel itself (has been loaded into memory, running, and has initialized all device drivers and data structures, etc.), it is by starting a user level program init way ,, to complete the boot process, so, init is always the first a process (its process number is always a).
The kernel has been used in the past few local init Find it, he's correct position (for linux systems) is / sbin / init. If the kernel can not find init. I will try to find a running / BIN / SH, if fails, the system will fail to start.
Two, init7 levels of meaning
0: Stop the opportunity to shut down (not to initdefault is set to 0)
1: single-user mode, only the root user to maintain
2: multi-user mode, you can not use NFS
3: Run a full multi-user mode (Standard level)
4: safe mode (rarely be used)
5: graphical (ie graphical interface)
6: reboot (Do not set initdefault 6)