<arg> tag

New in C Turtle

Introduction

The <arg> tag allows you to create more re-usable and configurable launch files by specifying values that are passed via the command-line, passing in via an <include>, or declared for higher-level files. Args are not global. An arg declaration is specific to a single launch file, much like a local parameter in a method. You must explicitly pass arg values to an included file, much like you would in a method call.

Usecase pattern

<arg> can be used in one of three ways:

• <arg name="foo" />

  • Declares the existence of foo. foo must be passed in either as a command-line argument (if top-level) or via <include> passing (if included).

  <arg name="foo" default="1" />

  • Declares foo with a default value. foo can be overriden by command-line argument (if top-level) or via <include> passing (if included).

  <arg name="foo" value="bar" />

  • Declares foo with constant value. The value for foo cannot be overridden. This usage enables internal parameterization of a launch file without exposing that parameterization at higher levels.

Attributes

• name="arg_name"

  • Name of argument.

  default="default value" (optional)

  • Default value of argument. Cannot be combined with value attribute.

  value="value" (optional)

  • Argument value. Cannot be combined with default attribute.

  doc="description for this arg" (optional) New in Indigo

  • Description of the argument. You could get this through --ros-args argument to the roslaunch command.

Examples

Passing an argument to an included file

• my_file.launch:

  <include file="included.launch">
    <!-- all vars that included.launch requires must be set -->
    <arg name="hoge" value="fuga" />
  </include>

  included.launch:

  <launch>
    <!-- declare arg to be passed in -->
    <arg name="hoge" /> 
  
    <!-- read value of arg -->
    <param name="param" value="$(arg hoge)"/>
  </launch>

Passing an argument via the command-line

roslaunch uses the same syntax as ROS remapping arguments to specify arg values.

• $ roslaunch my_file.launch hoge:=my_value      (.launch file is available at the current dir)
  $ roslaunch %YOUR_ROS_PKG% my_file.launch hoge:=my_value