The main purpose of init is to start and stop essential processes on the system. There are three major implementations of init in Linux, System V, Upstart and systemd.
Linux Sys V starts and stops processes sequentially. Sys V scripts start and stop services referencing runlevels which are set from 0 to 6.
- 0: Shutdown
- 1: Single User Mode
- 2: Multiuser mode without networking
- 3: Multiuser mode with networking
- 4: Unused
- 5: Multiuser mode with networking and GUI
- 6: Reboot
The scripts are located in /etc/rc.d/rc[runlevel number].d/ or /etc/init.d. Scripts that start with S(start) or K(kill) will run on startup and shutdown, respectively.
List services
service --status-all
Start a service
sudo service networking start
Stop a service
sudo service networking stop
Restart a service
sudo service networking restart
System V is on its way out with respect to Linux.