sbininit

sbininit is the conventional name for the initial user-space process started by the kernel during the boot sequence on many Unix-like systems. It is typically the executable located at /sbin/init and runs with PID 1, making it the root of the user-space process tree. The primary role of sbininit is to bootstrap the operating system by starting essential system services, spawning login prompts, and applying initial system configuration so that the rest of the services can come online.

Historically, sbininit and its behavior varied across Unix flavors. In BSD-style systems, init read startup scripts and rc files to bring the system to a usable state. System V-style systems used an inittab file to define runlevels and the sequence of scripts to run for each level. In both cases, init or sbininit is responsible for managing the lifecycle of processes, respawning critical daemons if needed, and coordinating orderly shutdown and reboot procedures.

In modern environments, there are several init implementations. Traditional SysVinit and BSD init have largely given way to more modern systems such as systemd, Upstart, or OpenRC in different distributions. While these alternatives differ in design and features, they typically still provide a PID 1 process and offer similar responsibilities: bringing up services, managing dependencies, and handling shutdown. Systemd, for example, runs as PID 1 and may be invoked through /sbin/init or a compatible interface, depending on the distribution.

As the first process with root privileges, sbininit is a critical component of system boot and maintenance. Its reliability and correct configuration are essential to producing a stable, secure, and recoverable operating system. See also init, /sbin/init, PID 1, runlevel, systemd.