Well,I typed the command just the same as what is on the ros_by_example.Here it is:rostopic pub -r 10 /cmd_vel gemoetry_msgs/Twist '{linear:{x:0.2,y:0,z:0},angular:{x:0,y:0,z:0.5}}',but it seems types don't match:rostopic: error: Argument error: while scanning a plain scalar in "<string>", line 1, column 10: {linear:{x:0.2,y:0,z:0},angular:{x:0,y:0, ... ^ found unexpected ':' in "<string>", line 1, column 11: {linear:{x:0.2,y:0,z:0},angular:{x:0,y:0,z ... ^ Please check http://pyyaml.org/wiki/YAMLColonInFlowContext for details.