Rosparam dump from launch file

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roslaunch first parses your launch file and all included launch files in order and creates an ordered list of parameters to set. Parameter definitions that are parsed later overwrite earlier definitions. After all parameters are parsed, they are set on the parameter server. That means, parameters cannot reference other parameters. rosparam dump will only dump parameters that have been set before you called roslaunch.

What should work though is dumping the parameters in a node since nodes are executed after the parameters are set. Try the following:

<node name="dump_parameters" pkg="rosparam" type="rosparam" args="dump parameters_dump.yaml" />

Note that the file will be created in your ROS_HOME, i.e. ~/.ros per default. If you want to change that, use an absolute path for your output file. You can also use something like $(find <ros package>)/parameters_dump.yaml to put the file into a ROS package. Of course, that package must not be a system package but a user package since you need write permissions to its directory.

As joq and dornhege pointed out, you cannot control the order in which nodes are started. When I encountered this problem, I added the following node to my launch script:

<node name="dump_rosparams" 
      pkg="aliencontrol" 
      type="aliencontrol" 
      args="'sleep 5; rosparam dump ~/ROS/params.yaml'"/>

It simply waits five seconds for all other nodes to start, and then dumps the parameters to the given file.

If you are interested in the last set of parameters before shutdown, save them periodically until ROS shutdown:

<node name="dump_rosparams" 
      pkg="aliencontrol" 
      type="aliencontrol" 
      args="'while true; do rosparam dump ~/ROS/params.yaml; sleep 5; done'"/>

For the above options to work, you need to install the aliencontrol package. And please make sure the command is enclosed in single quotes inside XML's double quotes, because aliencontrol expects the command to execute to be a single argument.

The launch file is not a script. Nodes do not run in any guaranteed order.

All I can think of to suggest is writing a node of your own that waits until it looks like all the parameters have been set, then invokes the dump command.

Maybe someone else has a better idea.

Are you including the launch file in the generic startup launch file?

That won't work as there is no order to when things in a launch file are executed. In that case you dump launch will probably be finished before anything else is started.