To validate that ZooKeeper is listening on port 2181: $ netstat -l | grep 2181 tcp6 0 0 [::]:2181 [::]:* LISTEN
To list all active connections to ZooKeeper: $ netstat -ape | grep 2181
$ netstat -ape | awk '{if ($5 == "localhost:2181") print $4, $9;}'

You can visit this page for the list of the four letter words commands https://zookeeper.apache.org/doc/current/zookeeperAdmin.html#sc_zkCommands

To be able to execute the four letter words commands of ZooKeeper, you need to add the following property to "zoo.cfg" file:
4lw.commands.whitelist=*
You can also set Java system property: zookeeper.4lw.commands.whitelist

You can run one of four letter words commands of ZooKeeper using nc command: $ echo "srvr" | nc localhost 2181 Zookeeper version: 3.6.1-390fe37ea45dee01bf87dc1c042b5e3dcce88653, built on 05/03/2019 12:07 GMT Latency min/avg/max: 0/0/0 Received: 3 Sent: 2 Connections: 1 Outstanding: 0 Zxid: 0x0 Mode: standalone Node count: 5
$ echo "stat" | nc localhost 2181 Zookeeper version: 3.6.1-390fe37ea45dee01bf87dc1c042b5e3dcce88653, built on 05/03/2019 12:07 GMT Clients: /127.0.0.1:37924[0](queued=0,recved=1,sent=0) Latency min/avg/max: 0/0/0 Received: 4 Sent: 3 Connections: 1 Outstanding: 0 Zxid: 0x0 Mode: standalone Node count: 5
$ echo "cons" | nc localhost 2181 /127.0.0.1:37926[0](queued=0,recved=1,sent=0)
You can also use telnet command to connect to the client port and execute the command srvr: $ telnet localhost 2181 Trying 127.0.0.1... Connected to localhost. Escape character is '^]'. srvr Zookeeper version: 3.6.1-390fe37ea45dee01bf87dc1c042b5e3dcce88653, built on 05/03/2019 12:07 GMT Latency min/avg/max: 0/0/0 Received: 7 Sent: 6 Connections: 1 Outstanding: 0 Zxid: 0x0 Mode: standalone Node count: 5 Connection closed by foreign host.