systemd is one of several PID 1 implementations.

System unit locations:

• /etc/systemd/system/ units installed by the system administrator
• /usr/lib/systemd/system/ units provided by installed packages

User unit locations:

• ~/.config/systemd/user/ where the user puts their own units
• /etc/systemd/user/ where system-wide user units are placed by the system administrator
• ~/.local/share/systemd/user/ where units of packages that have been installed in the home directory belong
• /usr/lib/systemd/user/ where units provided by installed packages belong

These are included under [Timer] in a foo.timer unit. OnBootSec=15min 15 minutes after boot (activated once) OnUnitActiveSec=1w every week OnCalendar=weekly every week at 12am Monday DayOfWeek Year-Month-Day Hour:Minute:Second OnCalendar extended format OnCalendar=Sat *-*-1..7 18:00:00 first Saturday of every month OnCalendar=*-*-* 4:00:00 every day at 4am Persistent=true start immediately if missed last time

systemd has journald.

Make logging nonpersistent with Storage=auto in /etc/systemd/journald.conf

Back in the days, superdaemons listened on all sockets and started up the needed services on request. Nowadays, the most common implementation is systemd's socket activation paired with the corresponding service unit file.

See https://0pointer.net/blog/projects/inetd.html for more info on this.