As previously stated, every run level (except 4) has an entry in the /etc/inittab file that identifies an rc program to execute. In addition, each run level has a directory associated with it.
The directory contains rc scripts that should be executed by the rc program to start or stop processes and services when that run level becomes the current one. These rc program and rc script directories use a standard naming convention based on the run level:
For run level 0, the rc program is /etc/rc0, and /etc/rc0.d is the rc script directory.
For run level 1, the rc program is /etc/rc1, and /etc/rc1.d is the rc script directory.
For run levels 2, 3, and s, the same naming convention applies.
Run levels 5 and 6 do not have separate ...