Get the value of key.
If the key does not exist the special value nil is returned.
An error is returned if the value stored at key is not a string, because GET
only handles string values.
*Return value
Bulk string reply: the value of key, or nil when key does not exist.
*Examples
redis>
GET nonexisting
(nil)redis> SET mykey "Hello"
"OK"redis> GET mykey
"Hello"