System V is a System Manager for UNIX / Linux. In Linux/Unix based operating systems, init (short for initialization) is the first process that started during the system boot up by the kernel.
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.
System V is not specific to Linux. It dates back to 1983 for UNIX. System V was the successor to 1982's UNIX System III. UNIX System V and the Berkeley Software Distribution (BSD) were the two major version of UNIX.
System V versus Systemd - Systemd is a new init system and system manager which was adapted by most of the major distribution.
Discussion on the three current Linux System Managers
- Sys V - Used by all major linux distributions in the past until being replaced by Upstart and systemd.
- Upstart - developed for Ubuntu and used in Ubuntu from 9.10 to Ubuntu 14.10 & RHEL 6
- systemd - systemd is compatible with SysV and LSB init scripts. First appeared in Fedora 15.
How to tell which one your system is using: type the following at command prompt
ps -p1 | grep "init\|upstart\|systemd"