Convert XACRO in URDF

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Now I am facing a new problem, if you could help me, so I will be grateful! The problem is the following. I have constructed the robot not in URFD but in XACRO....so the tutorial says me to convert the Xacro in Urdf extension....well I tried but so the following error message appears to me on the terminal: xacro:

Traditional processing is deprecated. Switch to --inorder processing!
To check for compatibility of your document, use option --check-order.
For more infos, see http://wiki.ros.org/xacro#Processing_Order

So, I have clicked in the link, I have read the topics and later I tried the command

rosrun xacro xacro.py --inorder pan_tilt.xacro > pan_tilt_generated.urdf

instead the original command suggested by the book

rosrun xacro xacro.py pan_tilt.xacro > pan_tilt_generated.urdf

This last command was the command that generated the error depicted above, and the first command also didn't give me a correct feedback, in other words, the xacro was not converted again in urdf.

Could someone explain me what is happening? I am not understanding...

Thanks

Update:

I forgot to entry the error message when I type the command

rosrun xacro xacro.py pan_tilt.xacro >pan_tilt_generated.urdf

The message error is:

No such file or directory: pan_tilt.xacro None None

But The file exist, and the directory too, the pan_tilt.xacro is inside the urdf directory.