How to get information about the message which is used by the topic

General Support
1

Hi there, I am bit confused about how to find out the message which is used by the particular topic. For instance, we have an exercise where we are told to get the info of the message that is used by the /odom topic. Here we can create the .py script to initialize /odom topic but how can we know beforehand that which type of message it will use, thanks

2

Hi there,

So there are a couple of commands that will be useful when investigating topics/messages

First is
~$ rostopic list

This will show all of the active ros topics. The output will look something like this

/camera/depth/camera_info
/camera/depth/image_raw
/camera/depth/points
/camera/parameter_descriptions
/camera/parameter_updates
/camera/rgb/camera_info
/camera/rgb/image_raw
/clock
/cmd_vel
/gazebo/link_states
/gazebo/model_states
/gazebo/parameter_descriptions
/gazebo/parameter_updates
/gazebo/performance_metrics
/gazebo/set_link_state
/gazebo/set_model_state
/imu
/joint_states
/odom
/rosout
/rosout_agg
/scan
/tf

Now Instead of listing the topics we can get additional information about a single topic by running the following command

~$ rostopic info /odom

The output will look something like

Type: nav_msgs/Odometry

Publishers: 
 * /gazebo (http://ubuntu:33969/)

Subscribers: None

Here you can see that the message type is nav_msgs/Odometry

You can even go one step further to see how the message is structure with

~$ rosmsg show nav_msgs/Odometry

which produces the following output

std_msgs/Header header
  uint32 seq
  time stamp
  string frame_id
string child_frame_id
geometry_msgs/PoseWithCovariance pose
  geometry_msgs/Pose pose
    geometry_msgs/Point position
      float64 x
      float64 y
      float64 z
    geometry_msgs/Quaternion orientation
      float64 x
      float64 y
      float64 z
      float64 w
  float64[36] covariance
geometry_msgs/TwistWithCovariance twist
  geometry_msgs/Twist twist
    geometry_msgs/Vector3 linear
      float64 x
      float64 y
      float64 z
    geometry_msgs/Vector3 angular
      float64 x
      float64 y
      float64 z
  float64[36] covariance
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3

I already found the solution myself before the answer(I used rostopic info) and still many thanks.

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