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3. Common Commands and Tools

3.1 Starting Node Methods​

3.1.1 Launch File​

There are at least two ways to start a launch file using roslaunch:

  1. Using the ROS package path
roslaunch pkg_name launchfile_name.launch
  1. Using the absolute path to the launch file
roslaunch /absolute/path/to/launchfile.launch

Common Parameters​

  • --screen — Output node logs directly to the terminal.
  • arg:=value — Pass parameters into the launch file.

Example:

roslaunch pkg_name launchfile_name model:=urdf/myfile.urdf

Or:

roslaunch pkg_name launchfile_name model:='$(find urdf_pkg)/urdf/myfile.urdf'

roslaunch automatically checks whether the ROS master is running and starts it if necessary.

3.1.2 rosrun​

Before starting any node, you must first start the ROS master:

roscore

Start a node using:

rosrun pkg_name node_name

rosrun searches for an executable file inside the package and runs it with optional arguments.

3.1.3 Python Script Execution​

If the node is written in Python, it can be executed directly from its directory. Be sure to distinguish between Python 2 and Python 3:

python your_node.py
python3 your_node.py

3.1.4 Start a Little Turtle​

roscore
rosrun turtlesim turtlesim_node
rosrun turtlesim turtle_teleop_key

Use the arrow keys (Up, Down, Left, Right) to control the turtle.

Turtle Control

Turtlesim node output example:

[ INFO] Starting turtlesim with node name /turtlesim
[ INFO] Spawning turtle [turtle1]

3.1.5 Start Two Little Turtles​

Install Required Package​

sudo apt install ros-melodic-turtle-tf

Launch Demo​

roslaunch turtle_tf turtle_tf_demo.launch

Keyboard Control​

rosrun turtlesim turtle_teleop_key

Two Turtles Demo

Press the arrow keys to drive one turtle while the other follows.

3.2 Launch Files​

3.2.1 Overview​

In ROS, a single node usually performs a single task. A complete robot system often requires many nodes to run together. Launch files allow multiple nodes to be started simultaneously using a single command.

3.2.2 File Format​

Launch files are written in XML format:

<?xml version="1.0"?>
<launch>
</launch>

Common tags include:

<launch>
<node>
<include>
<param>
<rosparam>
<arg>
<group>
</launch>