Passing Command-line Arguments to Python Nodes via ROS Launch

COM2009 Assignment #1 Checkpoint

It helps if you've already completed Assignment #1 Part 1 before working on this.

Having built a CLI for our Python Node on the previous page, we'll now look at how this all works when we call a node using roslaunch instead.

First, create a launch file called publisher_cli.launch. It probably makes sense to create this inside your part1_pubsub package:
```
roscd part1_pubsub/launch
```
```
touch publisher_cli.launch
```
Inside this add the following:
```
<launch>
  <arg name="roslaunch_colour" default="Black" />
  <arg name="roslaunch_number" default="0.999" />

  <node pkg="tuos_examples" type="publisher_cli.py"
    name="publisher_cli_node" output="screen"
    args="-colour $(arg roslaunch_colour) -number $(arg roslaunch_number)" />
</launch>
```
Here, we are specifying command-line arguments for the launch file using <arg> tags, as discussed earlier:
```
<arg name="roslaunch_colour" default="Black" />
<arg name="roslaunch_number" default="0.999" />    
```
Next, we use a <node> tag to launch the publisher_cli.py node from the tuos_examples package. All of that should be familiar to you from Part 1. What's new here however is the additional args attribute:
```
args="-colour $(arg roslaunch_colour) -number $(arg roslaunch_number)"
```
This is how we pass the roslaunch command-line arguments into the CLI of our ROS Node:
- -colour and -number are the command-line arguments of the Python Node
- roslaunch_colour and roslaunch_number are the command-line arguments of the launch file
Run the launch file without passing any arguments first, and see what happens:
```
roslaunch part1_pubsub publisher_cli.launch
```

Then try again, this time setting alternative values for the available command-line arguments:

roslaunch part1_pubsub publisher_cli.launch roslaunch_colour:=purple roslaunch_number:=4