ROS Launch Files Guide

Launch files define how to start ROS nodes, set parameters, and configure the system. They use XML format with ROS-specific tags.

Basic Structure

xml

<launch>
  <!-- Comments use XML syntax -->

  <!-- Arguments (input parameters) -->
  <arg name="robot_name" default="robot1" doc="Name of the robot"/>

  <!-- Parameters on parameter server -->
  <param name="global_param" value="some_value"/>

  <!-- Node definition -->
  <node pkg="package_name" type="node_executable" name="node_name" output="screen">
    <param name="~private_param" value="$(arg robot_name)"/>
    <remap from="input" to="sensor/data"/>
  </node>

  <!-- Include other launch files -->
  <include file="$(find other_pkg)/launch/other.launch">
    <arg name="arg_name" value="arg_value"/>
  </include>
</launch>

Core Tags

`<launch>`

Root element. All other tags must be inside.

xml

<launch>
  <!-- content -->
</launch>

`<node>`

Start a ROS node.

xml

<node pkg="package_name"
      type="executable_name"
      name="node_name"
      output="screen"
      respawn="true"
      required="false"
      ns="namespace"
      args="--flag value"
      launch-prefix="gdb -ex run --args">
  <!-- node-specific params, remaps -->
</node>

Attribute	Description
`pkg`	Package containing the node (required)
`type`	Executable name (required)
`name`	Node name in ROS graph (required)
`output`	`screen` (terminal) or `log` (file)
`respawn`	Restart if node dies
`required`	Shutdown launch if node dies
`ns`	Namespace for the node
`args`	Command line arguments
`launch-prefix`	Prefix command (gdb, valgrind, etc.)

`<arg>`

Define launch file arguments.

xml

<!-- With default value -->
<arg name="robot" default="turtlebot"/>

<!-- Required (no default) -->
<arg name="config_file"/>

<!-- With documentation -->
<arg name="rate" default="10" doc="Publishing rate in Hz"/>

<!-- Usage -->
<param name="robot_name" value="$(arg robot)"/>

Passing arguments:

bash

roslaunch pkg file.launch robot:=my_robot rate:=20

`<param>`

Set parameter on parameter server.

xml

<!-- Direct value -->
<param name="speed" value="1.5" type="double"/>
<param name="enabled" value="true" type="bool"/>
<param name="name" value="robot1" type="string"/>
<param name="count" value="10" type="int"/>

<!-- From file -->
<param name="description" textfile="$(find pkg)/urdf/robot.urdf"/>

<!-- Command output -->
<param name="description" command="cat $(find pkg)/urdf/robot.urdf"/>

<!-- Private parameter (inside node tag) -->
<node ...>
  <param name="~rate" value="10"/>  <!-- becomes /node_name/rate -->
</node>

`<rosparam>`

Load/dump YAML parameter files.

xml

<!-- Load YAML file -->
<rosparam file="$(find pkg)/config/params.yaml" command="load"/>

<!-- Load into namespace -->
<rosparam file="$(find pkg)/config/params.yaml" command="load" ns="robot1"/>

<!-- Inline YAML -->
<rosparam>
  gains:
    p: 1.0
    i: 0.1
    d: 0.05
</rosparam>

<!-- Delete parameters -->
<rosparam command="delete" param="old_param"/>

`<remap>`

Remap topic/service names.

xml

<node ...>
  <remap from="input" to="/sensor/data"/>
  <remap from="cmd_vel" to="/robot1/cmd_vel"/>
</node>

`<include>`

Include another launch file.

xml

<include file="$(find other_pkg)/launch/other.launch">
  <arg name="robot" value="$(arg robot)"/>
  <arg name="sim" value="true"/>
</include>

<!-- Pass all args -->
<include file="..." pass_all_args="true"/>

<!-- Include in namespace -->
<include file="..." ns="robot1"/>

`<group>`

Group tags together for namespace or conditionals.

xml

<!-- Namespace -->
<group ns="robot1">
  <node .../>  <!-- All nodes get /robot1 prefix -->
</group>

<!-- Conditional -->
<group if="$(arg use_sim)">
  <node pkg="simulator" .../>
</group>

<group unless="$(arg use_sim)">
  <node pkg="driver" .../>
</group>

Substitution Arguments

Syntax	Description	Example
`$(arg name)`	Argument value	`$(arg robot)`
`$(find pkg)`	Package path	`$(find your_pkg)/launch`
`$(env VAR)`	Environment variable	`$(env HOME)`
`$(optenv VAR default)`	Env var with default	`$(optenv ROS_IP localhost)`
`$(dirname)`	Launch file directory	`$(dirname)/config`
`$(eval expr)`	Python expression	`$(eval arg('rate') * 2)`
`$(anon name)`	Anonymous unique name	`$(anon node)`

Conditionals

xml

<!-- if: execute when true -->
<node if="$(arg enable_feature)" .../>
<include if="$(arg use_sim)" file="..."/>
<param if="$(arg debug)" name="log_level" value="DEBUG"/>

<!-- unless: execute when false -->
<node unless="$(arg headless)" pkg="rviz" .../>

<!-- With eval for complex conditions -->
<group if="$(eval arg('mode') == 'simulation')">
  ...
</group>

Common Patterns

Multi-Robot Launch

xml

<launch>
  <arg name="robot_name" default="robot1"/>

  <group ns="$(arg robot_name)">
    <param name="tf_prefix" value="$(arg robot_name)"/>
    <include file="$(find robot_pkg)/launch/robot.launch">
      <arg name="name" value="$(arg robot_name)"/>
    </include>
  </group>
</launch>

Simulation vs Real

xml

<launch>
  <arg name="sim" default="false"/>

  <!-- Simulation -->
  <group if="$(arg sim)">
    <include file="$(find gazebo_ros)/launch/gazebo.launch"/>
    <node pkg="sim_driver" type="sim_node" name="driver"/>
  </group>

  <!-- Real hardware -->
  <group unless="$(arg sim)">
    <node pkg="real_driver" type="driver_node" name="driver"/>
  </group>
</launch>

Debug Mode

xml

<launch>
  <arg name="debug" default="false"/>

  <node pkg="my_pkg" type="my_node" name="node"
        launch-prefix="$(eval 'gdb -ex run --args' if arg('debug') else '')"/>
</launch>

Machine Deployment

xml

<launch>
  <machine name="robot" address="192.168.1.100" user="ros"
           env-loader="/home/ros/catkin_ws/devel/env.sh"/>

  <node machine="robot" pkg="driver" type="driver_node" name="driver"/>
</launch>

Best Practices

•Use arguments for configuration - Don't hardcode values
•Document arguments - Use doc attribute
•Use namespaces - Prevent name collisions
•Separate concerns - One launch file per subsystem
•Include, don't duplicate - Reuse launch files
•Use $(find) - Never hardcode paths
•Set output="screen" for debugging - See node output
•Use required="true" for critical nodes - Auto-shutdown on failure

Debugging Launch Files

bash

# Check syntax
roslaunch --ros-args pkg file.launch

# See all arguments
roslaunch --ros-args pkg file.launch

# Dry run (don't start nodes)
roslaunch pkg file.launch --dry-run

# Print generated XML
roslaunch pkg file.launch --dump-params