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The roslaunch xml syntax doesn't have any concept of reading from or executing conditionally based on existing ROS parameters.

Is there any reason you can't use a default value in your node, when reading this parameter from the parameter server?

If you want to have a launch file that sets a user-supplied value for a parameter or uses a sane default, you could use arguments with default values:

<launch>
  <arg name="foo" default="42"/>
  <param name="foo" value="$(arg foo)"/>
</launch>