sysvinit

SysVinit, short for System V init, is an init system used by many Unix-like operating systems to bootstrap the user environment and manage daemons during boot, runtime, and shutdown. It implements the traditional System V Release 4 style initialization sequence and served as a common reference model for Linux and other Unixes before newer init systems emerged.

Its primary process runs as PID 1 and is responsible for starting and stopping services according to runlevels. The program reads configuration from /etc/inittab to determine the default runlevel and how to respond to state changes. Runlevels are numeric states (commonly 0 halt, 1 single-user, 2–5 multiuser with varying features, 6 reboot). For each runlevel, a directory such as /etc/rc.d/rcN.d or /etc/rcN.d contains symlinks prefixed with S (start) and K (kill) that point to scripts in /etc/init.d. When changing runlevels, init executes or halts services in the order indicated by the prefixes. The scripts in /etc/init.d implement the actual startup and shutdown behavior for individual services; they can be simple shells with responsible daemons.

SysVinit was the default init system on many Linux distributions during the 1990s and 2000s. Its simple, synchronous design made it straightforward to understand but limited in terms of parallel startup and dependency handling. In the following decades, several modern init systems introduced parallelization and explicit service dependencies, leading many distributions to replace or complement SysVinit with Upstart, systemd, or OpenRC. Some distributions continue to ship SysVinit or provide it as an option for compatibility.

Because of its long history, SysVinit remains a reference implementation for the traditional init behavior and is documented by many UNIX-like manuals. It remains relevant in contexts emphasizing simplicity and portability, and in environments where SysV-style initscripts are still in use.