How to pass topic names as input argument in the command line terminal?

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Hello ! I have a problem that states "Read a rosbag, to parse a topic, that is passed as an input argument" How do I pass the input argument using "rosrun" in the command line terminal?

My code snippet:

#include <ros/ros.h>
#include <sensor_msgs/PointCloud2.h>i
#include<tf/tfMessage.h>

using namespace std;

void topicCallBack(const sensor_msgs::PointCloud2::ConstPtr&msg)
{

 cout<<"Reading message of type PointCloud2\n"<<Header is:"<< msg->header;
 if(msg->width) 
   cout<< "\nInpoint Cloud\nwidth is"<<msg->width;

  //THIS LINE WILL NOT WORK AS MSG IS OF TYPE POINT CLOUD
  if(msg->transforms.transform.translation)
    cout<< " reading message of type TF\n";

}

int main(int argc, char **argv)
{
  ros::init(argc, argv,"parser");
  ros::NodeHandle nh;

  //intializing the subscriber
  ros::Subscriber sub = nh.subscribe("/camera/depth/points",1000, topicCallBack);

ros::spin();

return 0;
}

Here I have given my topic name as "/camera/depth/points" which publishes the message of type sensor_msgs/PointCloud2. But in my command line terminal, if I pass the topic name as "/tf", how do I ask it to publish the message of type tf/tfMessage because the subscriber is of type PointCloud2? In total, I have three topics that publish messages of type PointCloud2, tf, odom messages. But I am not able to see, how to pass different topic names in the command line argument and publish the respective message type Thanks!