Is there a safe shutdown command, like a "Halt" for these? Either for web Gui or ssh?
I usually unplug the rj45 cable and let the s3 came to an idle not hashing mode and my psu fan stop spinning then I turn off
Thanks for the tip.

It always seems so abrupt to just turn the things off on full stream.
Better is to ssh into the miner, then issue the following command:
poweroff -d 3
This instructs the miner to stop all processes in an orderly fashion then wait for the power switch to be turned off. The "3" instructs it to power off with a 3 second delay. You could also use "5" or "1" or whatever time interval you want. You should see the fan(s) stop as well, which is usually a good visual clue to turn off the power switch on your power supply.