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DeclareLaunchArgument and LaunchConfiguration

DeclareLaunchArgument

Defines an argument (a variable) that can be passed to a launch file via CLI , each argument can have default value.

LaunchConfiguration

LaunchConfiguration represents the value of a launch argument at runtime. It acts as a placeholder or reference to an argument declared with DeclareLaunchArgument

launch_argument_demo.launch.py
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from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument
from launch.substitutions import LaunchConfiguration
from launch_ros.actions import Node
from launch.actions import LogInfo

def generate_launch_description():
    ld = LaunchDescription()

    arg1_decalre = DeclareLaunchArgument('arg1', description="simple arg1 for demo", default_value='hello world')
    arg1 = LaunchConfiguration('arg1')

    log1 = LogInfo(msg=["argument demo from cli using subsitution ",arg1])

    ld.add_action(arg1_decalre)
    ld.add_action(log1)
    return ld

usage

list all launch arguments
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# Output default value
ros2 launch launch_tutorial args_log_demo.launch.py 
#
[INFO] [launch.user]: Argument 'arg1' value: hello world
run with argument
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# Show launch arguments
ros2 launch launch_tutorial args_log_demo.launch.py -s
#
Arguments (pass arguments as '<name>:=<value>'):

    'arg1':
        simple arg1 for demo
        (default: 'hello world')

# Get Argument from CLI
ros2 launch launch_tutorial args_log_demo.launch.py arg1:="data to arg1 from cli"
#
[INFO] [launch.user]: Argument 'arg1' value: data to arg1 from cli