To check the status of ZooKeeper:

$ bin/zkServer.sh status

ZooKeeper JMX enabled by default
Using config: /opt/zookeeper-i1/bin/../conf/zoo.cfg
Client port found: 2181. Client address: 0.0.0.0. Client SSL: false.
Mode: follower

To validate that ZooKeeper is listening on port 2181:

$ netstat -lan | grep 2181

tcp46      0      0  *.2181                 *.*                    LISTEN
tcp4       0      0  10.0.0.21.54989        10.0.0.21.2181         TIME_WAIT

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.