PONG if no argument is provided, otherwise return a copy of the
argument as a bulk.
This command is often used to test if a connection is still alive, or to measure
If the client is subscribed to a channel or a pattern, it will instead return a multi-bulk with a "pong" in the first position and an empty bulk in the second position, unless an argument is provided in which case it returns a copy of the argument.
Simple string reply, and specifically
PONG, when no argument is provided.
Bulk string reply the argument provided, when applicable.
"PONG"redis> PING "hello world"