systemd services
Look (tier: observe)
systemctl status <unit>
systemctl --user status <unit> # per-user services, e.g. slated
systemctl list-units --type=service --state=failed
journalctl -u <unit> -n 100 --no-pager
journalctl --user -u <unit> -n 100 --no-pager
Change (tier: confirm)
systemctl restart <unit>
systemctl stop <unit>
systemctl enable --now <unit>
Stopping or restarting a service can interrupt what the user is doing (network, audio, display). Say which unit and why before asking. Per-user units (--user) affect only this user.
Verify
systemctl is-active <unit> prints active.