the init process (PID 1)

Every family tree has a single founding ancestor at the very top, with no parent of their own, from whom everyone else descends. On a Unix-like system that founder is the init process: the first process the kernel starts at boot, given PID 1, the ancestor of every other process, and the one with no parent in the usual sense.

When the system boots, the kernel sets itself up and then starts exactly one user process, init (on modern Linux this is usually systemd). Init then launches the core system services and login mechanisms, which in turn start more processes, building up the whole process tree from this single root. Init has two special standing duties beyond launching things. First, it is the universal adopter: when any process dies leaving orphaned children, those orphans are re-parented to init, so they always have a parent. Second, init reaps those adopted children when they finish, preventing them from lingering as zombies. Killing init is not allowed during normal operation; if PID 1 ever truly dies, the kernel panics, because the system depends on it.

Init matters because it anchors the entire process model: it is where the process tree begins, and it is the safety net that keeps orphaned processes from being lost and zombies from piling up. A useful clarification: 'init' names a role (the first process, PID 1), and different systems fill that role with different programs (classic SysV init, systemd, runit, and others). They differ in how they manage services, but all share the PID-1 duties of being the ancestor, the adopter, and the reaper of last resort.