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

Not curl, but doesn't require a HTTP interface or nc (great for something like a container where you don't have nc installed)

exec 3<>/dev/tcp/127.0.0.1/6379 && echo -e "PING\r\n" >&3 && head -c 7 <&3

Should give you

+PONG

You can read more about what's going on from this fantastic article.

I needed to add a sleep to the nc provided by @Markus to get it to work from a remote system:

(printf "PING\r\n"; sleep 1) | nc remote.redis.hostname 6379

See Request/Response protocols and RTT: Redis Pipelining for details.