SysVinitstyle

SysVinitstyle refers to the traditional System V initialization design used by many Unix-like systems. It centers on an init process (PID 1) that governs the system's boot and shutdown sequence through runlevels and a collection of startup scripts. A key feature is the use of a central configuration file, such as /etc/inittab, to declare the default runlevel and basic behavior of the init process.

In the SysVinit style, each runlevel has a set of scripts that control service startup and shutdown. These scripts are typically located in /etc/init.d or /etc/rc.d/init.d and are brought into or removed from service by symbolic links placed in per-runlevel directories like /etc/rc.d/rcN.d or /etc/rcN.d, where N denotes the runlevel. Scripts are named with S (start) or K (kill) prefixes and are executed in a defined order to start or stop services as the system transitions between runlevels. Common runlevels include 0 for halt, 6 for reboot, 1 for single-user mode, and 2–5 for multiuser states, though exact mappings vary by distribution.

Booting involves init reading /etc/inittab, determining the default runlevel, and running the appropriate startup scripts. Shutting down or rebooting alters the runlevel, triggering a sequence of K scripts to stop services before S scripts for the new state are executed. The approach is generally simple and easy to understand, but it can be serial and less aware of inter-service dependencies compared with more modern, dependency-aware init systems.

Today, SysVinitstyle has largely been supplanted by alternatives such as systemd, Upstart, and OpenRC in many distributions, though compatibility and legacy scripts often keep core concepts of SysVinit alive in various environments.