Starting and stopping what runs.
WHAT systemd IS
The system that starts services and manages them.
WHAT A UNIT IS
A thing systemd manages: a service, a timer, a mount.
WHAT systemctl DOES
Controls units.
WHAT THE ESSENTIAL ACTIONS ARE
status, to see whether it is running and why not start stop restart reload, where supported enable, to start at boot disable
WHAT THE DIFFERENCE BETWEEN RESTART AND RELOAD IS
Restart stops and starts; reload asks the service to re-read configuration without dropping connections.
WHAT TO PREFER
Reload, where the service supports it.
WHAT THE DIFFERENCE BETWEEN START AND ENABLE IS
Start affects now; enable affects boot.
WHY THAT CATCHES PEOPLE
A service started but not enabled disappears after a reboot.
WHAT status SHOWS
Whether it is active How long for The main process Recent log lines
WHY THOSE LOG LINES MATTER
They usually contain the reason a service failed to start.
WHAT TO DO WHEN A SERVICE WILL NOT START
Read the status output Read the fuller log Test the configuration, if the service offers a check
WHAT TO NEVER DO
Restart repeatedly hoping for a different result.
