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ROS2 Launch Substitute

A Launch Substitution is a promise for a value that will only be known at launch runtime, not when the Python file is evaluated.

Substitutions are only meaningful in places where launch will resolve them later, such as:

  • node name
  • parameters
  • remappings
  • log messages
  • conditions
  • command arguments
Substitution Meaning
(LaunchConfiguration)(/ROS/ros_eco/launch/python/launch_argument/) Value from launch arguments
EnvironmentVariable Value from env at runtime
PathJoinSubstitution Build paths safely
TextSubstitution Literal text
Command Run a command and capture output
FindPackageShare Locate package at runtime

Simple Demo

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Node(
    package='demo_nodes_cpp',
    executable='talker',
    name=LaunchConfiguration('node_name')
)
log substitute
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from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument, LogInfo
from launch.substitutions import LaunchConfiguration

def generate_launch_description():
    ld = LaunchDescription()

    ld.add_action(
        DeclareLaunchArgument(
            'robot_name',
            default_value='atlas'
        )
    )

    ld.add_action(
        LogInfo(
            msg=['Robot name is: ', LaunchConfiguration('robot_name')]
        )
    )

    return ld