Variable object's position in .world file- Gazebo

Hi, I would like to specify the position of an object in a scene from a text file. Let's say I want a beer at position x,y,z, > beer: x,y,z Consider the following world file which creates a ground plane, the sun, a gaz station and a beer. **mybot.world** model://ground_planemodel://sunmodel://beerbeer-10 -8 0 0 0 0model://gas_stationgas_station-2.0 7.0 0 0 0 0 This is my launch file **benz_empty.launch** The launch command: > roslaunch mybot_gazebo benz_empty.launch How do I use variables in a .world file?