[ROS Study Notes] (XI) How to Use Launch Files

I. Purpose of Launch Files

• Configure and start multiple nodes through an XML file.
• Automatically start ROS Master at the same time (there is no need to run roscore separately).

II. Launch File Syntax

1. <launch>

The root element in a launch file is defined using the <launch> tag.

2. <node>

Start a node:

<node pkg="package-name" type="executable-name" name="node-name" />

• pkg: Name of the package containing the node
• type: Name of the node’s executable
• name: Name used by the node at runtime
• Other optional attributes:
  • output (whether to print log information)
  • respawn (whether to restart when an error occurs)
  • require (whether a node is required to start)
  • ns (namespace defines a namespace to avoid naming conflicts)
  • args (passes arguments to each node)

3. Parameter Settings

• <param> or <param>

  Set parameters in the system and store them on the parameter server.

  <param name="output_frame" value="abcd"/>

  Load multiple parameters from a parameter file:

  <rosparam file="params.yaml" command="load" ns="params" />

• <arg>

  A local variable within a launch file

  <arg name="arg-name" default="arg-value" />

4. Remapping <remap>

Remap ROS graph resource names.

<remap from="/turtlebot/cmd_vel" to="/cmd_vel">

5. Nesting <include>

Include other launch files.

<include file="$(dirname)/other.launch">

III. Launch Example

1. Create a New Package

cd ~/catkin_ws/src
catkin_create_pkg learning_launch

2. Create a launch Folder

Create a folder named launch under the learning_launch folder.

3. Create a Launch File

Create a new file in the folder you just created.

touchs simple.launch

Its contents are:

<launch>
    <node pkg="learning_topic" type="person_subscriber" name="talker" output="screen" />
    <node pkg="learning_topic" type="person_publisher" name="listener" output="screen" /> 
</launch>

4. Run the Launch File

roslaunch learning_launch simple.launch

Both nodes are now running.