The SysV init runlevel system provides a standard process for controlling which programs init launches or halts when initializing a runlevel. SysV init was chosen because it is easier to use and more flexible than the traditional BSD-style init process.

The configuration files for SysV init are located in the /etc/rc.d/ directory. Within this directory, are the rc, rc.local, rc.sysinit, and, optionally, the rc.serial scripts as well as the following directories:

         init.d/ rc0.d/ rc1.d/ rc2.d/ rc3.d/ rc4.d/ rc5.d/ rc6.d/ 
      

The init.d/ directory contains the scripts used by the /sbin/init command when controlling services. Each of the numbered directories represent the six runlevels configured by default under Red Hat Enterprise Linux.