Q:

Any command to get active namenode for nameservice in hadoop? 

The command:
hdfs haadmin -getServiceState machine-98

Works only if you know the machine name.  Is there any command like:
hdfs haadmin -getServiceState <nameservice>

which can tell you the IP/hostname of the active namenode? 

A:

To print out the namenodes use this command: 
hdfs getconf -namenodes

To print out the secondary namenodes: 
hdfs getconf -secondaryNameNodes

To print out the backup namenodes: 
hdfs getconf -backupNodes

Note: These commands were tested using Hadoop 2.4.0. 
Update 10-31-2014:
Here is a python script that will read the NameNodes involved in Hadoop HA from the config file and determine which of them is active by using the hdfs haadmin command.  This script is not fully tested as I do not have HA configured.  Only tested the parsing using a sample file based on the Hadoop HA Documentation.  Feel free to use and modify as needed. 
#! /usr/bin/env python
# coding: UTF-8
import xml.etree. ElementTree as ET
import subprocess as SP
if __name__ == "__main__":
    hdfsSiteConfigFile = "/etc/hadoop/conf/hdfs-site.xml"

    tree = ET.parse(hdfsSiteConfigFile)
    root = tree.getroot()
    hasHadoopHAElement = False
    activeNameNode = None
    for property in root:
        if "dfs.ha.namenodes" in property.find("name").text:
            hasHadoopHAElement = True
            nameserviceId = property.find("name").text[len("dfs.ha.namenodes")+1:]
            nameNodes = property.find("value").text.split(",")
            for node in nameNodes:
                #get the namenode machine address then check if it is active node
                for n in root:
                    prefix = "dfs.namenode.rpc-address."  + nameserviceId + "." 
                    elementText = n.find("name").text
                    if prefix in elementText:
                        nodeAddress = n.find("value").text.split(":")[0]                

                        args = ["hdfs haadmin -getServiceState " + node]  
                        p = SP.Popen(args, shell=True, stdout=SP.PIPE, stderr=SP.PIPE)

                        for line in p.stdout.readlines():
                            if "active" in line.lower():
                                print "Active NameNode: " + node
                                break;
                        for err in p.stderr.readlines():
                            print "Error executing Hadoop HA command: ",err
            break            
    if not hasHadoopHAElement:
        print "Hadoop High-Availability configuration not found!" 

A:

Found this:
https://gist.github.com/cnauroth/7ff52e9f80e7d856ddb3
This works out of the box on my CDH5 namenodes, although I'm not sure other hadoop distributions will have http://namenode:50070/jmx available - if not, I think it can be added by deploying Jolokia. 
Example:
curl 'http://namenode1.example.com:50070/jmx?qry=Hadoop:service=NameNode,name=NameNodeStatus'
{
  "beans" : [ {
    "name" : "Hadoop:service=NameNode,name=NameNodeStatus",
    "modelerType" : "org.apache.hadoop.hdfs.server.namenode. NameNode",
    "State" : "active",
    "NNRole" : "NameNode",
    "HostAndPort" : "namenode1.example.com:8020",
    "SecurityEnabled" : true,
    "LastHATransitionTime" : 1436283324548
  } ]

So by firing off one http request to each namenode (this should be quick) we can figure out which one is the active one. 
It's also worth noting that if you talk WebHDFS REST API to an inactive namenode you will get a 403 Forbidden and the following JSON:
{"RemoteException":{"exception":"StandbyException","javaClassName":"org.apache.hadoop.ipc. StandbyException","message":"Operation category READ is not supported in state standby"}}

A:

You can do it in bash with hdfs cli calls, too.  With the noted caveat that this takes a bit more time since it's a few calls to the API in succession, but this may be preferable to using a python script for some.  
This was tested with Hadoop 2.6.0
get_active_nn(){
   ha_name=$1 #Needs the NameServiceID
   ha_ns_nodes=$(hdfs getconf -confKey dfs.ha.namenodes.${ha_name})
   active=""
   for node in $(echo ${ha_ns_nodes//,/ }); do
     state=$(hdfs haadmin -getServiceState $node)
     if [ "$state" == "active" ]; then
       active=$(hdfs getconf -confKey dfs.namenode.rpc-address.${ha_name}.${node})
       break
     fi
   done
   if [ -z "$active" ]; then
     >&2 echo "ERROR: no active namenode found for ${ha_name}"
     exit 1
   else
     echo $active
   fi
}

