When you want to use curl, you need REST over RESP, like webdis, tinywebdis or turbowebdis. See https://github.com/markuman/tinywebdis#turbowebdis-tinywebdis–cherrywebdis

$ curl -w '\n' http://127.0.0.1:8888/ping
{"ping":"PONG"}

Without a REST interface for redis, you can use netcat for example.

$ (printf "PING\r\n";) | nc <redis-host> 6379 
+PONG

For password protected redis you can use netcat like this:

$ (printf "AUTH <password>\r\n";) | nc <redis-host> 6379
+PONG

With netcat you have to build the RESP protocol by your self. See http://redis.io/topics/protocol

update 2018-01-09

I’ve build a powerfull bash function which pings the redis instance at any cost over tcp

    function redis-ping() {
            # ping a redis server at any cost
            redis-cli -h $1 ping 2>/dev/null || \
                    echo $((printf "PING\r\n";) | nc $1 6379 2>/dev/null || \
                    exec 3<>/dev/tcp/$1/6379 && echo -e "PING\r\n" >&3 && head -c 7 <&3)
    }

usage redis-ping localhost