SysVinitin

SysVinitin is a traditional init system used on many Unix-like operating systems, derived from System V init. It is responsible for starting, stopping, and maintaining user-space services during boot, operation, and shutdown. The init program, PID 1, is started by the kernel and then loads its configuration from /etc/inittab, which specifies the default runlevel and the actions to take for various events.

SysVinitin operates by organizing services into runlevels, commonly 0 (halt), 1 (single-user), 2-3 (multi-user), 5 (graphical) and 6 (reboot). Each runlevel has a corresponding directory (for example, /etc/rc.d/rcN.d on some systems or /etc/rcN.d on Debian-based), containing symlinks named SNNservice to start and KNNservice to stop a service at that runlevel. The actual initialization scripts live in /etc/init.d. During a runlevel transition, init executes the appropriate S and K scripts in the correct order.

Configuration is typically defined in /etc/inittab, including the default runlevel and respawn rules for processes. The system provides a minimal interface for changing runlevels via telinit or init.

In practice, SysVinitin emphasizes simplicity and portability, with no advanced dependency resolution beyond the ordering implied by numeric prefixes. It has relatively straightforward logging and restart behavior but can be slower to boot because services are started serially.

Adoption has declined in favor of modern init systems such as systemd, Upstart, and OpenRC. Nevertheless, SysVinitin remains in use on several distributions and legacy installations, valued for its transparency and small footprint.