how does systemd actually start things?

I’ve typed systemctl start thousands of times without really knowing what happens after I hit enter. Here’s what I pieced together.

units all the way down

Everything systemd manages is a unit. A service is a unit, a mount is a unit, a timer is a unit. They’re defined in plain text files, which is nice — you can read them.

systemctl cat sshd.service

dependencies are the interesting part

The thing I didn’t appreciate: systemd builds a dependency graph and starts things in parallel wherever it can. Wants= and Requires= describe ordering intent, and After= / Before= describe actual ordering.

That’s the bit I always forget, so now it’s written down.