init

Init is the first process started by the kernel during boot on Unix-like systems. It runs with PID 1 and serves as the root of the user-space process tree, responsible for bringing the system from initial kernel initialization to a usable state. As the parent of all other processes, init also handles basic process reaping and supervision of daemons throughout runtime and shutdown.

Historically, init referred to the SysV init style, which reads a configuration that determines runlevels and starts a set of scripts to initialize services for each level. It manages the orderly start and stop of services, coordinates transitions between runlevels, and handles system shutdown. In practice, init acts as the supervisor of system services, ensuring that essential daemons are started, restarted if needed, and properly terminated.

Implementations vary. Classic SysV init is still found on some systems, while others use BSD-style rc startup scripts. In Linux, modern distributions commonly use systemd, which relies on unit files and targets (replacing runlevels) and can start services in parallel, track their state, and centralize logs via journals. Other approaches include OpenRC, runit, and s6. macOS uses launchd as its init-like system, and Windows relies on the Service Control Manager for startup tasks.

Configuration and behavior differ by implementation. SysV init uses /etc/inittab and startup scripts in /etc/rc.d or /etc/rc?.d, ordering actions according to the current runlevel. Systemd uses unit files in /etc/systemd/system or /lib/systemd/system and provides tools like systemctl to enable, start, stop, or query services. Across implementations, init’s core role remains: bootstrap user space, start and supervise services, and manage orderly system startup and shutdown.