The kernel starts the first user-space process as PID 1 in a PID namespace. On a full Linux system, this init process establishes the service environment. In a container, PID 1 can instead be a small init wrapper or the application itself, but it still has special signal and child-reaping responsibilities.
Boot the System · Lesson 5
Boot Process: Init
Learn how PID 1 initializes user space, supervises services, reaps children, and coordinates shutdown.