How to create "parameters" inside launch files in ROS?

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Hello all,

I am a little stuck, so some help be very appreciated very much. I would like to create a list of parameters/variables in the beginning of my launch file, in such a way, that it will available for the user to change the "contents" of the variables, but the rest of the launch file will not change!

Examples: Nowadays I have the next line in my launch file:

<node pkg="rosbag" type="rosbag" name="rosbag1" args="play -l $(find guiPackageParametersToCallback)/BagFiles/2011-12-05-10-55-01.bag" required="true"/>

But the name of the recorded bag file can change so I want to do something like that:

filename = 2011-12-05-10-55-01.bag
<node pkg="rosbag" type="rosbag" name="rosbag1" args="play -l $(find guiPackageParametersToCallback)/BagFiles/filename" required="true"/>

Or in the next case:

 <node name="republish1" type="republish" pkg="image_transport" output="screen" args="compressed in:=/bb2/left/image_rect raw out:=leftCameraTopic" required="true" />

I would like to something:

inputTopic = /bb2/left/image_rect
    <node name="republish1" type="republish" pkg="image_transport" output="screen" args="compressed in:=inputTopic raw out:=leftCameraTopic" required="true" />

Since I am interested to do it in the limits of the launch file itself, I am not sure that "remap" will solve the case here.

Thanks in advance, Felix.