init
also: init system, systemd, SysVinit, PID 1
The first process (PID 1) that the Linux kernel starts after booting, responsible for launching all other processes and managing the system's startup sequence and shutdown.
init is the parent of all processes on a Linux system. The kernel starts it automatically during boot, and it runs until the system shuts down. Its primary job is to read configuration files that specify which services and daemons should start.
Modern Linux systems typically use systemd as their init system, though older systems used SysVinit or other alternatives. Each init system has its own configuration format: systemd uses unit files in /etc/systemd/system/, while SysVinit used shell scripts in /etc/init.d/.
When you reboot your system, init orchestrates the shutdown sequence, stopping services gracefully and unmounting filesystems. You can check which init system your system uses by running ps 1 or systemctl --version.