Also known as: init · PID 1 · systemd · SysVinit
The init system is the first user-space process started by the kernel at boot, running as PID 1, responsible for starting and supervising all other system services and handling shutdown.
Because PID 1 is special — it adopts orphaned processes and its termination panics the kernel — the init system holds a uniquely central position, which is why changing it is so consistently contentious.
systemd is now the dominant choice across major distributions, providing parallel service startup, dependency ordering, socket activation, logging, and more. Its scope is precisely what critics object to, arguing it couples too many subsystems to one project.
systemd
systemd is the init system and service manager used by most major Linux distributions, handling service startup and supervision, logging, device management, and system state through a unified set of components.
Linux Distribution
A Linux distribution is a complete operating system built around the Linux kernel, bundling it with system libraries, a package manager, default utilities, and a curated set of software, along with policies for how all of it is maintained and updated.
Kernel
The kernel is the core of an operating system, responsible for managing memory, scheduling processes, and mediating all access between software and hardware. The Linux kernel is the component that makes a system 'Linux'.
cgroups
cgroups (control groups) are a Linux kernel feature that limits, accounts for, and isolates the resource usage — CPU, memory, disk I/O, network — of a group of processes.