init system
also: init, PID 1, systemd, SysVinit, service manager
The first user-space process (PID 1) that the kernel starts, responsible for launching and managing all other processes on the system.
An init system is the parent of all processes on a Linux system. The kernel starts it as the first user-space program, and it then spawns other processes needed to boot the system and keep it running. Different init systems use different mechanisms to manage services and dependencies.
The most common modern init system is systemd, which uses unit files to define services, targets, and other system components. Older systems used SysVinit with shell scripts in /etc/init.d/, while some embedded systems use simpler alternatives like OpenRC or runit.
Example: When you run systemctl start nginx, the init system reads the unit file for nginx, manages its dependencies, starts the process, and tracks its state.