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I think the confusion is that in the publisher/subscriber tutorial, the message type just happens to be a string, whereas the message type you want to be publishing is not a string, but a geometry_msgs/Twist. At the top of your while loop, you should have something like

geometry_msgs::Twist msg;

then you need to fill in the message's data fields:

msg.linear.x = 10;
msg.angular.z = 5;

The syntax you're using is only for the command line tool rostopic pub.

I think the confusion is that in the publisher/subscriber tutorial, the message type just happens to be a string, whereas the message type you want to be publishing is not a string, but a geometry_msgs/Twist. At the top of your while loop, you should have something like

geometry_msgs::Twist msg;

then you need to fill in the message's data fields:

msg.linear.x = 10;
msg.angular.z = 5;

The syntax you're using is only for the command line tool rostopic pub.