Substitutions in XML are not expanded intuitively when used for the purpose of executing programs.

Bug report

Required Info:

• Operating System:
  Ubuntu 22.04
• Installation type:
  Source build (humble)
• Version or commit hash:
  ros2/launch aed025e04e0b143362c69bf29a67b2ffff4c59ee
• DDS implementation:
  default, eprosima fastdds
• Client library (if applicable):
  N/A

Steps to reproduce issue

add_model_launch.xml

<launch>
  <arg name="initial_pose" default="-x 0.0 -y 0.0 -z 0.2"/>

  <arg name="model_name" default="robot"/>

  <arg name="model_type" default="robot"/>

  <node pkg="ros_gz_sim" exec="create" name="$(var model_name)_spawner"
    args="-name $(var model_name) -param robot_description $(var initial_pose)">
    <param name="robot_description"
      value="$(command 'xacro $(find-pkg-share robot_gazebo)/models/$(var model_type)/model.urdf.xacro')"/>
  </node>

</launch>

ros2 launch my_package add_model_launch.xml

Expected behavior

When the arguments to create are processed, $(var initial_pose) is expanded to [...'-x','0.0','-y','0.0','-z','0.2'] (i.e. multiple list items) for command execution. This is so that the create node executable recognises the command arguments.

Additionally, when executed with the -d flag, launch should log the command as an array of strings, as this is what is supplied to the operating system call. Example provided below.

process details: cmd=['/path/to/ros_gz_sim/lib/ros_gz_sim/create','-name','robot','-param','robot_description','-x','0.0','-y','0.0','-z','0.2','--ros-args','-r','__node:=robot_spawner','--params-file','/tmp/launch_params_958ns6s1'], cwd='None', custom_env?=True

Actual behavior

When the arguments to create are expanded $(var initial_pose) is expanded to [...'-x 0.0 -y 0.0 -z 0.2'] (.i.e a single list item) for command execution. The create command does not recognise these as a single argument string.

I had to hack the code to force it to log the command being executed. There is an implementation there, but I could not find how to turn it on. This implementation logs the command executed as a joined string, which hid this issue.

process details: cmd='/path/to/ros_gz_sim/lib/ros_gz_sim/create -name robot -param robot_description -x 0.0 -y 0.0 -z 0.2 --ros-args -r __node:=robot_spawner --params-file /tmp/launch_params_958ns6s1', cwd='None', custom_env?=True

Additional information

A work around is to ensure substitutions expand to strings without spaces in them.

i.e.

  <arg name="initial_pose_x" default="0.0"/>
  <arg name="initial_pose_y" default="0.0"/>
  <arg name="initial_pose_z" default="0.2"/>

I suspect this may also be an issue for yaml, but did not test.