What is SystemD and why is it popular? What initialization systems were used before?

SystemD is an initialization system and service manager in Linux responsible for starting and managing processes during system boot and operation.

Why is SystemD popular:

• Parallel service startup — speeds up system boot.
• Dependency management — services are started in the correct order.
• Unified interface for managing services (systemctl).
• Logging (journald) — centralized log collection.
• cgroups support — resource control for processes.
• Unification — used in many distributions, simplifying administration.

Previously, Linux used other initialization systems:

• SysVinit — a traditional system with sequential script execution in /etc/init.d.
• Upstart — an event-driven initialization system used in Ubuntu.
• OpenRC — an initialization system used in Gentoo and some other distributions.

SystemD replaced these systems, offering a more modern, faster, and flexible approach to service management and system boot.