ROSlaunch Command-line Arguments

A lot of the launch files that we have used throughout the lab course actually have Command-Line Arguments (CLAs) that can be (optionally) supplied when making the roslaunch call. Consider the turtlebot3_empty_world.launch file for instance...

Enter the full command and then press the SPACE and TAB keys on your keyboard like so:

roslaunch turtlebot3_gazebo turtlebot3_empty_world.launch[SPACE][TAB][TAB]

This will reveal the available command-line arguments for this launch file:

model  x_pos  y_pos  z_pos

We can therefore optionally specify custom values for these parameters in order to change what happens when the launch file is executed. Try this, for example:

roslaunch turtlebot3_gazebo turtlebot3_empty_world.launch x_pos:=1 y_pos:=1

When the Gazebo simulation is launched, the robot will be located at coordinates (1, 1) in the X-Y plane, rather than (0, 0), as would usually be the case.

Note

We assign values to roslaunch command-line arguments using the := operator.

This is all made possible through the use of <arg> tags at the start of a launch file (just after the opening <launch> tag):

<launch>
  <arg name="x_pos" default="0.0"/>
  <arg name="y_pos" default="0.0"/>
  ...

Within these tags we need to define two attributes:

1. name: the name we want to give to the CLA (name="x_pos", name="y_pos" etc...)

2. default: A default value in order to make the CLA optional

   (In the above example, the robot will be located at coordinates (0, 0) by default, unless we specify otherwise.)