The init system that owns PID 1, starts every service in dependency order, and replaced a directory of shell scripts with declarative unit files.
6 topics
PID 1 is the first userspace process the kernel starts and the ancestor of everything else on the machine; it brings the system up and reaps the orphaned children nothing else will. On every current Debian and Ubuntu release that PID 1 is systemd, which starts units in parallel based on their dependencies, supervises long-running services, and reaches well past boot into logging, timers, mounts, and login sessions.
This chapter is the one that pays off every time a service will not start. It begins with what systemd replaced and why, then teaches the unit files that describe services and the systemctl front door that drives them. From there it covers the targets that decide what a server boots into, the timers that retire most cron jobs, and the journal that holds every line those services ever logged.