Xacro test

launch.launch

<launch>

  <param name="robot_description" command="$(find xacro)/xacro --inorder '$(find foobar)/launch/myrobot.xacro' laser_topic:=scan_topic"/> 

  <!-- Run a python script to the send a service call to gazebo_ros to spawn a URDF robot -->
  <node name="urdf_spawner" pkg="gazebo_ros" type="spawn_model" respawn="false" output="screen"
          args="-urdf -model myrobot -param robot_description"/>
</launch>

myrobot.xacro

<robot xmlns:xacro="http://www.ros.org/wiki/xacro">
  <xacro:arg name="laser_topic" default="scan"/>
  <xacro:property name="lidar_topic" value="$(arg laser_topic)"/>
  <link name="base_link"/>

  <xacro:macro name="sick_tim551" params="name ros_topic">
    <plugin name="gazebo_ros_${name}_controller" filename="libgazebo_ros_laser.so">
      <alwaysOn>true</alwaysOn>
      <topicName>${ros_topic}</topicName>
    </plugin>
  </xacro:macro>
  <xacro:macro name="laser_scanner" params="parent topic">

    <xacro:sick_tim551 name="laser1" ros_topic="${topic}"/>

    <joint name="laser1_mount_joint" type="fixed">
      <parent link="${parent}"/>
      <child link="laser1_mount_link"/>
      <origin xyz="0 0 1" rpy="0 0 0"/>
    </joint>

  </xacro:macro>


  <laser_scanner parent="base_link" topic="${lidar_topic}"/>
</robot>

result of running xacro cli

# /opt/ros/kinetic/lib/xacro/xacro --inorder '/opt/ros/foobar/src/foobar/launch/myrobot.xacro' laser_topic:=scan_topic
<?xml version="1.0" ?>
<!-- =================================================================================== -->
<!-- |    This document was autogenerated by xacro from /opt/ros/foobar/src/foobar/launch/myrobot.xacro | -->
<!-- |    EDITING THIS FILE BY HAND IS NOT RECOMMENDED                                 | -->
<!-- =================================================================================== -->
<robot xmlns:xacro="http://www.ros.org/wiki/xacro">
  <link name="base_link"/>
  <plugin filename="libgazebo_ros_laser.so" name="gazebo_ros_laser1_controller">
    <alwaysOn>true</alwaysOn>
    <topicName>scan_topic</topicName>
  </plugin>
  <joint name="laser1_mount_joint" type="fixed">
    <parent link="base_link"/>
    <child link="laser1_mount_link"/>
    <origin rpy="0 0 0" xyz="0 0 1"/>
  </joint>
</robot>

rosparam_get_result

# rosparam get /robot_description
"<?xml version=\"1.0\" ?>\n<!-- ===================================================================================\
  \ -->\n<!-- |    This document was autogenerated by xacro from /opt/ros/foobar/src/foobar/launch/myrobot.xacro\
  \ | -->\n<!-- |    EDITING THIS FILE BY HAND IS NOT RECOMMENDED                \
  \                 | -->\n<!-- ===================================================================================\
  \ -->\n<robot xmlns:xacro=\"http://www.ros.org/wiki/xacro\">\n  <link name=\"base_link\"\
  />\n  <plugin filename=\"libgazebo_ros_laser.so\" name=\"gazebo_ros_laser1_controller\"\
  >\n    <alwaysOn>true</alwaysOn>\n    <topicName>scan_topic</topicName>\n  </plugin>\n\
  \  <joint name=\"laser1_mount_joint\" type=\"fixed\">\n    <parent link=\"base_link\"\
  />\n    <child link=\"laser1_mount_link\"/>\n    <origin rpy=\"0 0 0\" xyz=\"0 0\
  \ 1\"/>\n  </joint>\n</robot>\n\n"