NodeHandle::getParam() is unable to resolve Environmental variables

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I have a launchfile in which I want to specify the path to a specific file, e.g. haarcascade_frontalface_alt.xml. Normally, I would just use the <param> tag like this:

<param name="haarcascade_filename" type="string" value="$(env ROS_ROOT)/../OpenCV/haarcascades/haarcascade_frontalface_alt.xml" />

and try to load it during initialization using NodeHandle::param():

ros::NodeHandle local_nh("~");
local_nh.param("haarcascade_filename", name_, std::string(""));

This is the way it is done for example in the face_detector stack. Assume that I now want to specify multiple files using a <rosparam> list:

<rosparam param="haarcascade_filename_list">["$(env ROS_ROOT)/../OpenCV/haarcascades/haarcascade_frontalface_alt.xml", "morestrings.."]</rosparam>

This is the way I'm trying to load all the entries of the list:

    XmlRpc::XmlRpcValue my_xmlrpc_haarlist;
    local_nh.getParam("haarcascade_filename_list", my_xmlrpc_haarlist);
    ROS_ASSERT(my_xmlrpc_haarlist.getType() == XmlRpc::XmlRpcValue::TypeArray);

    for (int32_t i = 0; i < my_xmlrpc_haarlist.size(); ++i) {
        ROS_ASSERT(my_xmlrpc_haarlist[i].getType() == XmlRpc::XmlRpcValue::TypeString);
        haar_filename_list_.push_back(static_cast<std::string> (my_xmlrpc_haarlist[i]));
    }

The problem is if I now do ROS_INFO_STREAM("Filename: " << supername_); and ROS_INFO_STREAM("Filename list: " << haar_filename_list_.at(0)); the results are not the same:

[ INFO] [1354712164.859028413]: Filename: /opt/ros/fuerte/share/ros/../OpenCV/haarcascades/haarcascade_frontalface_alt.xml
[ INFO] [1354712164.859087292]: Filename list: $(env ROS_ROOT)/../OpenCV/haarcascades/haarcascade_frontalface_alt.xml

How do I solve this issue?