<slide_break></slide_break> ## ROS Fundamentals * A distributed "operating system" * A process coordinator and an inter-process communications model * Different parts (nodes) can run on different computers * They all need to be connected by a network via TCP/IP <slide_break></slide_break> ### Connecting via Network * It could be a wired or wireless network * Each participating computer must have an IP address * An IP address can be represented as a subdomain name, e.g. robb.dyn.brandeis.edu * And all participants must be "visible" to each other on the network <slide_break></slide_break> ### What are the possible participants? * Computers (running any OS potentially) 1. The robot itself 2. Your ROS cluster account * Really any computer with ROS installed * Of course they need to be on the/a network * EDUROAM or some other network. * Somehow they need to know about each other * Virtual Private Network <slide_break></slide_break> ### **Look at our [Lab Notebook](https://campusrover.github.io/labnotebook2/)! It has lots of useful information and a good search box!** <slide_break></slide_break> ### Writing ROS software * Can be written in Python or C++. We will be using Python exclusively * Built up from multiple independent programs called Nodes * Each node is implemented as a separate program * It can use all the python features: classes, functions, imported packages, etc <slide_break></slide_break> ### ROS Nodes - the basic unit * Fundamental concept in ROS * ROS runs a number of collaborating processes called "nodes" * Think of it as a program or a process which forms part of a system 1. A real robot will have dozens to hundreds of nodes 2. You can write nodes in Python (we will do) or C++ * Nodes talk to each other by publishing and subscribing to messages <slide_break></slide_break> ### Example of a very simple node <pre><code class="language-language-python"> #!/usr/bin/env python # Don't forget to chmod +x topic_publisher.py import rospy from std_msgs.msg import Int32 # Make this into a ROS node. rospy.init_node('my_publisher_node') # Prepare to publish topic `counter` pub = rospy.Publisher('counter', Int32, queue_size=10) # sleep at 2 loops per second rate = rospy.Rate(2) count = 0 # loop until ^c while not rospy.is_shutdown(): pub.publish(count) count += 1 rate.sleep() rospy.sleep(0.5) </code></pre> <slide_break></slide_break> ### ROSCORE * Program that acts as global coordinator of all the nodes * It is run once at a known ip address or domain name * Any other node will make contact with it to announce itself * When one node talks to another, roscore helps them find each other and then steps out of the way. * Everything starts with the 'traffic-cop' `roscore` 1. Once `roscore` runs, additional nodes can run * Each node is essentially a separate program, maybe even a single file 1. A node can be written in Python or C++ (or really technically any language) 2. We will use Python mostly <slide_break></slide_break> ### "Running" a ROS application * Consists of several fairly independent nodes * They all have to be launched * Together they form the whole ROS application <slide_break></slide_break> ### ROS shell commands * Also known as the ROS `CLI` - command line interface * They take care of setting up the environment * Here are the ones you **must** learn and get used to 1. `rosrun` - runs a single ROS node 1. `roslaunch` - launches a sequence of ROS Nodes 1. `rostopic` - Inspect ROS topics 1. `roscd` - change directory to a ROS package 1. `catkin_make` (alias `cm`) <slide_break></slide_break> ### Our built-in aliases: See [labnotebook aliases](https://campusrover.github.io/labnotebook2/cheatsheets/alias/?h=aliases) <div class="callout callout-small"> <span class="callout-badge">ALERT</span> The most common explanation for weird result is that your environment variables are not set up </div> * Reset environment: `cs` * Rebuild `cm` * My own IP address: `myip` * My own vpn IP address `myvpnip` <slide_break></slide_break> ### ROS Directory Structure * This is the standard directory structure <pre><code class="language-lanugage-bash"> # top level directories. Once per system. This will already exist. ./catkin_ws /devel /build /src /cosi119_src/ /src /...etc </code></pre> Inside ./src there are directories each with a ROS package. They all have a standard shape as well and can be nested inside each other. <pre><code class="language-lanugage-bash"> # Here is an example ./turtlebot3_gazebo /launch turtlebot3_stage_1.launch . . /src run_world.py turtlebot3_drive.cpp ... and other directories CMAkeLists.txt package.xml </code></pre>