Practical Example

Now that you understand the basics, here’s how they can apply to a practical example. Follow along to see how we actually ‘do’ all of these things….

  1. First, you will need to run Ubuntu 20.04, and have ROS Noetic installed on it. For your convenience, you can download our easy-to-use image here:

    https://s3.amazonaws.com/CPR_PUBLIC/LEARN_ROS/ROS_Edu.zip

    Login (username): user

    Password: learn

  2. Get VMWare Player, and use the virtual disk above. If you don’t want to use the provided image, follow the tutorial here (after installing Ubuntu 20.04): http://wiki.ros.org/noetic/Installation/Ubuntu

    Throughout the rest of the tutorials, we will be referencing the ROS cheatsheet available here.

  3. Open a new Terminal Window (Ctrl + Alt + T). In the new terminal, type roscore (And press enter) This should produce something similar to the below.

_images/PExampleone.png

What you have just done is started the ROS Master as we described above. We can now experiment with some ROS commands.

Open a new Terminal, and type in rostopic. This will give you a list of all the options that the rostopic command can do.

For now, we are interested in rostopic list. Type in rostopic list. (And press enter). This should give you a window like the following:

_images/PExampletwo.png

The two entries listed above are ROS’s built in way of reporting and aggregating debug messages in the system. What we want to do is publish and subscribe to a message.

You can open a new Terminal again, or open a new tab on the same terminal window (Ctrl + Shift + T). In the new Terminal, type in (NOTE: This command will not work if you try to copy paste it into your terminal):

rostopic pub /hello std_msgs/String “Hello Robot"
_images/PExamplethree.png

Let’s break down the parts of that command.

rostopic pub – This commands ROS to publish a new topic.

/hello – This is the name of the new topic. (Can be whatever you want)

std_msgs/String – This is the topic type. We want to publish a string topic. In our overview examples above, it was an image data type.

“Hello Robot” – This is the actual data contained by the topic. I.E. the message itself.

Going back to the previous Terminal, we can execute rostopic list again.

We now have a new topic listed! We can also echo the topic to see the message by entering rostopic echo /hello.

_images/PExamplefour.png

We have now successfully published a topic with a message, and received that message. Type Ctrl + C to stop echoing the /hello topic. We can also look into the node that is publishing the message. Type in rosnode list. You will get a list similar to the one below. (The exact numbers beside the rostopic node may be different)

_images/PExamplefive.png

Because we asked rostopic to publish the /hello topic for us, ROS went ahead and created a node to do so. We can look into details of by typing rosnode info /rostopic/_…..(whatever numbers)

TIP: In ROS, and in Linux in general, whenever you start typing something, you can press the Tab key to auto-complete it. If there is more than one entry, double tap Tab to get the list. In the above example, all I typed was rosnode info /rost(TAB)

_images/PExamplesix.png

We can get info on our topic the same way, by typing rostopic info /hello

_images/PExampleseven.png

You will notice that the node listed under “Publishers:” is the same node we requested info about.

Up till now, we have covered the fundamentals of ROS, how to use rostopic, rosnode.

Next time, we will compile a short example program, and try it out.