How to execute command inside launch file?

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How do you run a command inside a launch file to set a param value?

I have launch file like:

<launch>
    <group ns="motor_arduino">
        <node pkg="rosserial_python" type="serial_node.py" name="serial_node" output="screen">
            <param name="~port" value="/dev/ttyACM0" />
            <param name="~baud" value="115200" />
        </node>
    </group>
</launch>

which allows communication with one of several Arduino Unos connected to the host computer. However, I've found that the mount path /dev/ttyACM0 is variable, and based on how the Arduino's initialize, this may be /dev/ttyACM0 or /dev/ttyACM1.

To fix this problem, I've written a script that reads the output of udevadm info --query=all --name=/dev/ttyACM* and determines the correct device path to use. However, how do I call this script from inside the launch file to set the port param dynamically?

The docs show that roslaunch does support some simply substitution commands, but I don't see a way to run an arbitrary script, e.g.

<param name="~port" value="$(identify_port.py device1)" />

gives me the error:

Unknown substitution command [identify_port.py device1]. Valid commands are ['find', 'env', 'optenv', 'anon', 'arg']