ros roslaunch launch ros nodes and load parameters from Yaml file

Example

roslaunch is an important tool that manages the start and stop of ROS nodes. It takes one or more "*.launch" files as arguments.

For this example, I will refer to the following (as asked in this question), so how can we execute those commands consecutively & automatically :

roscd stereo_camera
rosparam load marvin_cameras.yaml
rosrun stereo_camera stereo_camera __name:=bumblebeeLeft
rosrun stereo_camera stereo_camera __name:=bumblebeeCenter

roslaunch openni_launch_marvin kinect_left.launch
roslaunch openni_launch_marvin kinect_center.launch 

First of all, let's break up these commands in pieces. As it can be seen, 4 ros commands are needed : roscd, rosparam, rosrun and roslaunch. Now let's start !

This is an example of why roslaunch is powerful, In fact, all those commands could be included in one and only roslaunch file in ROS. then all what we have to do is to launch this "solution.launch" file for a consecutive and automatic call for those commands.

To start, launch files are based on XML formatting, here's a basic launch file in ROS, we will name it "basic_example.launch" and it's included in a ROS package named "roslaunch_example":

<launch>
    
</launch>

the command to execute this launch file is

$ roslaunch roslaunch_example basic_example.launch

following the specification :

$ roslaunch package_name launch_file_name.launch

Because of our launch file doesn't include any commands, nothing will be executed, more on that later...

Including nodes in Launch files :
any ROS node in any ROS package installed is call-able in launch files. to that we have to specify the package containing the node and it's name as specified in the package. for example :

rosrun stereo_camera stereo_camera __name:=bumblebeeLeft
rosrun stereo_camera stereo_camera __name:=bumblebeeCenter

stereo_camera is a node form the package stereo_camera and the arguments specified are it's name __name:=bumblebeeLeft and __name:=bumblebeeCenter.

To add those nodes, we have to add the following lines:

<launch>    
    <node name="$(arg name)" pkg="stereo_camera" type="stereo_camera" output="screen">
          <param name="name" value="bumblebeeLeft" />
    </node>

    <node name="$(arg name)" pkg="stereo_camera" type="stereo_camera" output="screen">
          <param name="name" value="bumblebeeCenter" />
    </node>
</launch>

Executing this launch file, we'll have the two nodes running.

adding a parameter to a ROS node :

As can be seen, we added a parameter "name" for the nodes as :

<param name="name" value="bumblebeeCenter" />

In fact, we can add as much parameters as we want (as created above) and then refer to them by calling "$(arg parameter_name)" instead of fixing it's value.

Specifying the output :

The output tag can be set to "screen", if you need to see the node log on the terminal or "log" to save the log to the log files in (~/.ros).

Including other ROS launch files in a ROS launch file :

As stated here, The tag enables you to import another roslaunch XML file into the current file. It will be imported within the current scope of your document, including and tags. All content in the include file will be imported except for the tag: the tag is only obeyed in the top-level file.

So, all to executes these commands

roslaunch openni_launch_marvin kinect_left.launch
roslaunch openni_launch_marvin kinect_center.launch 

from a launch file, all we have to do is adding the following lines :

<include file="$(find openni_launch_marvin)/launch/kinect_left.launch" />
<include file="$(find openni_launch_marvin)/launch/kinect_center.launch" />

roscd to a package in ROS launch file

In order to find the launch file than we want to include, we don't need to specify the full path. Instead, roslaunch provides the "$(find package_name)" directive, this way, we can refer to our launch file relative to the package racine.
In the above example, I assumed that the file "kinect_center.launch" is in the "openni_launch_marvin)/launch/" folder.

Loading parameters from YAML File :

In order to load parameters from a YAML file in ROS, ROS provides the "rosparam" tag. As stated in the wiki : "The tag enables the use of rosparam YAML files for loading and dumping parameters from the ROS Parameter Server. It can also be used to remove parameters. The tag can be put inside of a tag, in which case the parameter is treated like a private name."

Using this tag, we can load our YAML file in the launch file by adding this line :

<rosparam command="load" file="$(find marvin_cameras)/config/marvin_cameras.yaml" />

As used above, I assumed that the YAML file "marvin_cameras.yaml" is in the "marvin_cameras/config/" folder.

Assembling all pieces

Now that we have created separately our launch file contents, let's assemble them in one big launch file "solution.launch".

solution.launch

<launch> 

    <rosparam command="load" file="$(find marvin_cameras)/config/marvin_cameras.yaml" />  

    <node name="$(arg name)" pkg="stereo_camera" type="stereo_camera" output="screen">
          <param name="name" value="bumblebeeLeft" />
    </node>

    <node name="$(arg name)" pkg="stereo_camera" type="stereo_camera" output="screen">
          <param name="name" value="bumblebeeCenter" />
    </node>

    <include file="$(find openni_launch_marvin)/launch/kinect_left.launch" />
    <include file="$(find openni_launch_marvin)/launch/kinect_center.launch" />

</launch>

Now, we have our one and only roslaunch file for executing all the commands consecutively and automatically.