On Linux systems, initialization (init) scripts manage the state of system services during system startup and shutdown. When the system goes through its runlevels, the System V init system starts and stops services as configured. While this tried-and-true technology has been around since the dawn of Unix, you can now create modern and efficient CentOS 6 init scripts by using Upstart, an event-based replacement for System V init.
Until its latest release, CentOS used the System V init system by default. SysV init scripts are simple and reliable, and guarantee a certain order of starting and stopping.
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