Lab 3 : ROS Workspace, Package, Publisher and Subscriber

Seneca Polytechnic
SEA700 Robotics for Software Engineers

Introduction

ROS Workspace

A workspace is a directory containing ROS packages. Before using ROS, it’s necessary to source your ROS installation workspace in the terminal you plan to work in. This makes ROS’s packages available for you to use in that terminal.

ROS Package

A package is an organizational unit for your ROS code. If you want to be able to install your code or share it with others, then you’ll need it organized in a package. With packages, you can release your ROS work and allow others to build and use it easily.

For a package to be considered a catkin package it must meet a few requirements:

  • The package must contain a catkin compliant package.xml file.
    • That package.xml file provides meta information about the package.
  • The package must contain a CMakeLists.txt which uses catkin.
    • If it is a catkin metapackage it must have the relevant boilerplate CMakeLists.txt file.
  • Each package must have its own directory
    • This means no nested packages nor multiple packages sharing the same directory.

The simplest possible package may have a file structure that looks like:

my_package/
    CMakeLists.txt
    package.xml

A single workspace can contain as many packages as you want, each in their own directory. You can also have packages of different build types in one workspace (CMake, Python, etc.). You cannot have nested packages.

Best practice is to have a src directory within your workspace, and to create your packages in there. This keeps the top level of the workspace “clean”.

A trivial workspace might look like:

workspace_directory/        -- WORKSPACE
    src/                   -- SOURCE SPACE
        CMakeLists.txt       -- 'Toplevel' CMake file, provided by catkin
        package_1/
            CMakeLists.txt     -- CMakeLists.txt file for package_1
            package.xml        -- Package manifest for package_1
        ...
        package_n/
            CMakeLists.txt     -- CMakeLists.txt file for package_n
            package.xml        -- Package manifest for package_n

Procedures

Create a Workspace directory

  1. Best practice is to create a new directory for every new workspace. The name doesn’t matter, but it is helpful to have it indicate the purpose of the workspace. Let’s choose the directory name ros_ws, for “development workspace”. Open a new terminal and run:

    mkdir -p ~/ros_ws/src
cd ~/ros_ws/
catkin_make

    The catkin_make command is a convenience tool for working with catkin workspaces. Running it the first time in your workspace, it will create a CMakeLists.txt link in your src directory.

    Another best practice is to put any packages in your workspace into the src directory. The above code creates a src directory inside ros_ws.

    If you are building ROS from source to achieve Python 3 compatibility, and have setup your system appropriately (ie: have the Python 3 versions of all the required ROS Python packages installed, such as catkin) the first catkin_make command in a clean catkin workspace must be:

    catkin_make -DPYTHON_EXECUTABLE=/usr/bin/python3

    This will configure catkin_make with Python 3. You may then proceed to use just catkin_make for subsequent builds.

  2. Additionally, if you look in your current directory you should now have a 'build' and 'devel' directory. Inside the 'devel' directory you can see that there are now several setup files. Sourcing any of these files will overlay this workspace on top of your environment. Before continuing source your new setup.sh file:

    source devel/setup.bash

  3. To make sure your workspace is properly overlayed by the setup script, make sure ROS_PACKAGE_PATH environment variable includes the directory you're in.

    echo $ROS_PACKAGE_PATH

    You should see:

    /home/<youruser>/ros_ws/src:/opt/ros/melodic/share

    And other path(s) if you have them added to your source.

Create a C++ Package

  1. Navigate into ros_ws/src, and run the package creation command to create a simple C++ publisher and subscriber:

    cd ~/ros_ws/src
catkin_create_pkg cpp_pubsub std_msgs roscpp

    Your terminal will return a message verifying the creation of your package cpp_pubsub and all its necessary files and directories.

    catkin_create_pkg requires that you give it a package_name and optionally a list of dependencies on which that package depends: catkin_create_pkg <package_name> [depend1] [depend2] [depend3]

    More details on package creation can be found here.

    Write the publisher node

  2. Navigate into ros_ws/src/cpp_pubsub/src. This is the directory in any CMake package where the source files containing executables belong.

  3. Download the example talker code by entering the following command:

    wget -O talker.cpp https://raw.githubusercontent.com/ros/ros_tutorials/kinetic-devel/roscpp_tutorials/talker/talker.cpp

  4. Now there will be a new file named talker.cpp. Open the file using your preferred text editor. Alternatively, create a .cpp file with the following:

    #include "ros/ros.h"
#include "std_msgs/String.h"

#include <sstream>

/**
 * This tutorial demonstrates simple sending of messages over the ROS system.
 */
int main(int argc, char **argv)
{
    /**
     * The ros::init() function needs to see argc and argv so that it can perform
     * any ROS arguments and name remapping that were provided at the command line.
     * For programmatic remappings you can use a different version of init() which takes
     * remappings directly, but for most command-line programs, passing argc and argv is
     * the easiest way to do it.  The third argument to init() is the name of the node.
     *
     * You must call one of the versions of ros::init() before using any other
     * part of the ROS system.
     */
    ros::init(argc, argv, "talker");

    /**
     * NodeHandle is the main access point to communications with the ROS system.
     * The first NodeHandle constructed will fully initialize this node, and the last
     * NodeHandle destructed will close down the node.
     */
    ros::NodeHandle n;

    /**
     * The advertise() function is how you tell ROS that you want to
     * publish on a given topic name. This invokes a call to the ROS
     * master node, which keeps a registry of who is publishing and who
     * is subscribing. After this advertise() call is made, the master
     * node will notify anyone who is trying to subscribe to this topic name,
     * and they will in turn negotiate a peer-to-peer connection with this
     * node.  advertise() returns a Publisher object which allows you to
     * publish messages on that topic through a call to publish().  Once
     * all copies of the returned Publisher object are destroyed, the topic
     * will be automatically unadvertised.
     *
     * The second parameter to advertise() is the size of the message queue
     * used for publishing messages.  If messages are published more quickly
     * than we can send them, the number here specifies how many messages to
     * buffer up before throwing some away.
     */
    ros::Publisher chatter_pub = n.advertise<std_msgs::String>("chatter", 1000);

    ros::Rate loop_rate(10);

    /**
     * A count of how many messages we have sent. This is used to create
     * a unique string for each message.
     */
    int count = 0;
    while (ros::ok())
    {
        /**
         * This is a message object. You stuff it with data, and then publish it.
         */
        std_msgs::String msg;

        std::stringstream ss;
        ss << "hello world " << count;
        msg.data = ss.str();

        ROS_INFO("%s", msg.data.c_str());

        /**
         * The publish() function is how you send messages. The parameter
         * is the message object. The type of this object must agree with the type
         * given as a template parameter to the advertise<>() call, as was done
         * in the constructor above.
         */
        chatter_pub.publish(msg);

        ros::spinOnce();

        loop_rate.sleep();
        ++count;
    }

    return 0;
}

    The Code Explained

    Now, let's break the code down.

    #include "ros/ros.h"

    ros/ros.h is a convenience include that includes all the headers necessary to use the most common public pieces of the ROS system.

    #include "std_msgs/String.h"

    This includes the std_msgs/String message, which resides in the std_msgs package. This is a header generated automatically from the String.msg file in that package. For more information on message definitions, see the msg page.

    ros::init(argc, argv, "talker");

    Initialize ROS. This allows ROS to do name remapping through the command line -- not important for now. This is also where we specify the name of our node. Node names must be unique in a running system.

    The name used here must be a base name, ie. it cannot have a / in it.

    ros::NodeHandle n;

    Create a handle to this process' node. The first NodeHandle created will actually do the initialization of the node, and the last one destructed will cleanup any resources the node was using.

    ros::Publisher chatter_pub = n.advertise<std_msgs::String>("chatter", 1000);

    Tell the master that we are going to be publishing a message of type std_msgs/String on the topic chatter. This lets the master tell any nodes listening on chatter that we are going to publish data on that topic. The second argument is the size of our publishing queue. In this case if we are publishing too quickly it will buffer up a maximum of 1000 messages before beginning to throw away old ones.

    NodeHandle::advertise() returns a ros::Publisher object, which serves two purposes: 1) it contains a publish() method that lets you publish messages onto the topic it was created with, and 2) when it goes out of scope, it will automatically unadvertise.

    ros::Rate loop_rate(10);

    A ros::Rate object allows you to specify a frequency that you would like to loop at. It will keep track of how long it has been since the last call to Rate::sleep(), and sleep for the correct amount of time.

    In this case we tell it we want to run at 10Hz.

    int count = 0;
while (ros::ok())
{

    By default roscpp will install a SIGINT handler which provides Ctrl-C handling which will cause ros::ok() to return false if that happens.

    ros::ok() will return false if:

    • a SIGINT is received (Ctrl-C)
    • we have been kicked off the network by another node with the same name
    • ros::shutdown() has been called by another part of the application.
    • all ros::NodeHandles have been destroyed

    Once ros::ok() returns false, all ROS calls will fail.

    std_msgs::String msg;

std::stringstream ss;
ss << "hello world " << count;
msg.data = ss.str();

    We broadcast a message on ROS using a message-adapted class, generally generated from a msg file. More complicated datatypes are possible, but for now we're going to use the standard String message, which has one member: "data".

    chatter_pub.publish(msg);

    Now we actually broadcast the message to anyone who is connected.

    ROS_INFO("%s", msg.data.c_str());

    ROS_INFO and friends are our replacement for printf/cout. See the rosconsole documentation for more information.

    ros::spinOnce();

    Calling ros::spinOnce() here is not necessary for this simple program, because we are not receiving any callbacks. However, if you were to add a subscription into this application, and did not have ros::spinOnce() here, your callbacks would never get called. So, add it for good measure.

    loop_rate.sleep();

    Now we use the ros::Rate object to sleep for the time remaining to let us hit our 10Hz publish rate.

    Here's the condensed version of what's going on:

    • Initialize the ROS system
    • Advertise that we are going to be publishing std_msgs/String messages on the chatter topic to the master
    • Loop while publishing messages to chatter 10 times a second

    Write the subscriber node

    Now we need to write a node to receive the messsages.

  5. Return to ros_ws/src/cpp_pubsub/src to create the next node. Enter the following code in your terminal to download the subscriber:

    wget -O listener.cpp https://raw.github.com/ros/ros_tutorials/kinetic-devel/roscpp_tutorials/listener/listener.cpp

    Check to ensure that these files exist:

    talker.cpp  listener.cpp

  6. Open the listener.cpp with your text editor. Alternatively, create a .cpp file with the following:

    #include "ros/ros.h"
#include "std_msgs/String.h"

/**
 * This tutorial demonstrates simple receipt of messages over the ROS system.
 */
void chatterCallback(const std_msgs::String::ConstPtr& msg)
{
    ROS_INFO("I heard: [%s]", msg->data.c_str());
}

int main(int argc, char **argv)
{
    /**
     * The ros::init() function needs to see argc and argv so that it can perform
     * any ROS arguments and name remapping that were provided at the command line.
     * For programmatic remappings you can use a different version of init() which takes
     * remappings directly, but for most command-line programs, passing argc and argv is
     * the easiest way to do it.  The third argument to init() is the name of the node.
     *
     * You must call one of the versions of ros::init() before using any other
     * part of the ROS system.
     */
    ros::init(argc, argv, "listener");

    /**
     * NodeHandle is the main access point to communications with the ROS system.
     * The first NodeHandle constructed will fully initialize this node, and the last
     * NodeHandle destructed will close down the node.
     */
    ros::NodeHandle n;

    /**
     * The subscribe() call is how you tell ROS that you want to receive messages
     * on a given topic.  This invokes a call to the ROS
     * master node, which keeps a registry of who is publishing and who
     * is subscribing.  Messages are passed to a callback function, here
     * called chatterCallback.  subscribe() returns a Subscriber object that you
     * must hold on to until you want to unsubscribe.  When all copies of the Subscriber
     * object go out of scope, this callback will automatically be unsubscribed from
     * this topic.
     *
     * The second parameter to the subscribe() function is the size of the message
     * queue.  If messages are arriving faster than they are being processed, this
     * is the number of messages that will be buffered up before beginning to throw
     * away the oldest ones.
     */
    ros::Subscriber sub = n.subscribe("chatter", 1000, chatterCallback);

    /**
     * ros::spin() will enter a loop, pumping callbacks.  With this version, all
     * callbacks will be called from within this thread (the main one).  ros::spin()
     * will exit when Ctrl-C is pressed, or the node is shutdown by the master.
     */
    ros::spin();

    return 0;
}

    The Code Explained

    Now, let's break it down piece by piece, ignoring some pieces that have already been explained above.

    void chatterCallback(const std_msgs::String::ConstPtr& msg)
{
    ROS_INFO("I heard: [%s]", msg->data.c_str());
}

    This is the callback function that will get called when a new message has arrived on the chatter topic. The message is passed in a boost shared_ptr, which means you can store it off if you want, without worrying about it getting deleted underneath you, and without copying the underlying data.

    ros::Subscriber sub = n.subscribe("chatter", 1000, chatterCallback);

    Subscribe to the chatter topic with the master. ROS will call the chatterCallback() function whenever a new message arrives. The 2nd argument is the queue size, in case we are not able to process messages fast enough. In this case, if the queue reaches 1000 messages, we will start throwing away old messages as new ones arrive.

    NodeHandle::subscribe() returns a ros::Subscriber object, that you must hold on to until you want to unsubscribe. When the Subscriber object is destructed, it will automatically unsubscribe from the chatter topic.

    There are versions of the NodeHandle::subscribe() function which allow you to specify a class member function, or even anything callable by a Boost.Function object. The roscpp overview contains more information.

    ros::spin();

    ros::spin() enters a loop, calling message callbacks as fast as possible. Don't worry though, if there's nothing for it to do it won't use much CPU. ros::spin() will exit once ros::ok() returns false, which means ros::shutdown() has been called, either by the default Ctrl-C handler, the master telling us to shutdown, or it being called manually.

    Again, here's a condensed version of what's going on:

    • Initialize the ROS system
    • Subscribe to the chatter topic
    • Spin, waiting for messages to arrive
    • When a message arrives, the chatterCallback() function is called

Build and Run C++ Package

  1. Now, go back to the package and open up CMakeLists.txt and ensure the following are in there. Note: Some of the functions are currently commented out and some are missing.

    ## Generate added messages and services
generate_messages(DEPENDENCIES std_msgs)

## Declare a catkin package
catkin_package()

## Build talker and listener
include_directories(include ${catkin_INCLUDE_DIRS})

add_dependencies(talker cpp_pubsub_generate_messages_cpp)
add_executable(talker src/talker.cpp)
target_link_libraries(talker ${catkin_LIBRARIES})

add_dependencies(listener cpp_pubsub_generate_messages_cpp)
add_executable(listener src/listener.cpp)
target_link_libraries(listener ${catkin_LIBRARIES})

    This will create two executables, talker and listener, which by default will go into the package directory of your devel space, located by default at ~/ros_ws/devel/lib/<package name>.

    Note that you have to add dependencies for the executable targets to message the generation targets:

    add_dependencies(talker cpp_pubsub_generate_messages_cpp)

    This makes sure message headers of this package are generated before being used. If you use messages from other packages inside your catkin workspace, you need to add dependencies to their respective generation targets as well, because catkin builds all projects in parallel. The following variable to allow you to depend on all necessary targets:

    target_link_libraries(talker ${catkin_LIBRARIES})

    You can invoke executables directly or you can use rosrun to invoke them. They are not placed in <prefix>/bin because that would pollute the PATH when installing your package to the system. If you wish for your executable to be on the PATH at installation time, you can setup an install target, see: catkin/CMakeLists.txt

  2. Before building the package, it's always a good idea to check and see if all dependencies are met.

    cd ~/ros_ws
rosdep install -i --from-path src --rosdistro melodic -y

    You should get a success return:

    #All required rosdeps installed successfully

  3. Now run catkin_make in your catkin workspace:

    cd ~/ros_ws
catkin_make

    Note: Or if you're adding as new pkg, you may need to tell catkin to force making by --force-cmake option.

    Now you have written a simple publisher and subscriber.

  4. Run ROS master.

    roscore

  5. In a new terminal, source the setup files:

    source ./devel/setup.bash

  6. Now run the talker node from ros_ws:

    rosrun cpp_pubsub talker

    The terminal should start publishing info messages every 0.5 seconds, like so:

    [ INFO] [1727906572.247429209]: hello world 0
[ INFO] [1727906572.347872260]: hello world 1
[ INFO] [1727906572.448580826]: hello world 2
[ INFO] [1727906572.548227290]: hello world 3
[ INFO] [1727906572.650658485]: hello world 4

  7. Open another terminal, source the setup files from inside ros_ws again, and then start the listener node:

    . devel/setup.bash
rosrun cpp_pubsub listener

    The listener will start printing messages to the console, starting at whatever message count the publisher is on at that time, like so:

    [ INFO] [1727906765.424901599]: I heard: [hello world 10]
[ INFO] [1727906765.525323426]: I heard: [hello world 11]
[ INFO] [1727906765.625240241]: I heard: [hello world 12]
[ INFO] [1727906765.728040103]: I heard: [hello world 13]
[ INFO] [1727906765.824066051]: I heard: [hello world 14]

  8. Enter Ctrl+C in each terminal to stop the nodes from spinning.

Create a Python Package

  1. Navigate into ros_ws/src, and run the package creation command to create a simple Python publisher and subscriber:

    cd ~/ros_ws/src
catkin_create_pkg py_pubsub std_msgs rospy

    Your terminal will return a message verifying the creation of your package py_pubsub and all its necessary files and directories.

    Write the publisher node

  2. Navigate to your package ros_ws/src/py_pubsub and let's first create a scripts directories to store our Python scripts in and navigate into it:

    cd py_pubsub

    Create a directory.

    mkdir scripts
cd scripts

  3. Download the example talker code and make it executable by entering the following command:

    wget https://raw.github.com/ros/ros_tutorials/kinetic-devel/rospy_tutorials/001_talker_listener/talker.py
chmod +x talker.py

  4. Now there will be a new file named talker.py. Open the file using your preferred text editor. Alternatively, create a .py file with the following:

    #!/usr/bin/env python
# license removed for brevity
import rospy
from std_msgs.msg import String

def talker():
    pub = rospy.Publisher('chatter', String, queue_size=10)
    rospy.init_node('talker', anonymous=True)
    rate = rospy.Rate(10) # 10hz
    while not rospy.is_shutdown():
        hello_str = "hello world %s" % rospy.get_time()
        rospy.loginfo(hello_str)
        pub.publish(hello_str)
        rate.sleep()

if __name__ == '__main__':
    try:
        talker()
    except rospy.ROSInterruptException:
        pass

  5. Add the following to your CMakeLists.txt. This makes sure the python script gets installed properly, and uses the right python interpreter.

    catkin_install_python(PROGRAMS scripts/talker.py
    DESTINATION ${CATKIN_PACKAGE_BIN_DESTINATION}
)

    The Code Explained

    Now, let's break the code down.

    #!/usr/bin/env python

    Every Python ROS Node will have this declaration at the top. The first line makes sure your script is executed as a Python script.

    import rospy
from std_msgs.msg import String

    You need to import rospy if you are writing a ROS Node. The std_msgs.msg import is so that we can reuse the std_msgs/String message type (a simple string container) for publishing.

    pub = rospy.Publisher('chatter', String, queue_size=10)
rospy.init_node('talker', anonymous=True)

    This section of code defines the talker's interface to the rest of ROS.

    pub = rospy.Publisher("chatter", String, queue_size=10) declares that your node is publishing to the chatter topic using the message type String. String here is actually the class std_msgs.msg.String. The queue_size argument limits the amount of queued messages if any subscriber is not receiving them fast enough.

    The next line, rospy.init_node(NAME, ...), is very important as it tells rospy the name of your node -- until rospy has this information, it cannot start communicating with the ROS Master. In this case, your node will take on the name talker. NOTE: the name must be a base name, i.e. it cannot contain any slashes "/".

    anonymous = True ensures that your node has a unique name by adding random numbers to the end of NAME. Refer to Initialization and Shutdown - Initializing your ROS Node in the rospy documentation for more information about node initialization options.

    rate = rospy.Rate(10) # 10hz

    This line creates a Rate object rate. With the help of its method sleep(), it offers a convenient way for looping at the desired rate. With its argument of 10, we should expect to go through the loop 10 times per second (as long as our processing time does not exceed 1/10th of a second!)

    while not rospy.is_shutdown():
    hello_str = "hello world %s" % rospy.get_time()
    rospy.loginfo(hello_str)
    pub.publish(hello_str)
    rate.sleep()

    This loop is a fairly standard rospy construct: checking the rospy.is_shutdown() flag and then doing work. You have to check is_shutdown() to check if your program should exit (e.g. if there is a Ctrl-C or otherwise). In this case, the "work" is a call to pub.publish(hello_str) that publishes a string to our chatter topic. The loop calls rate.sleep(), which sleeps just long enough to maintain the desired rate through the loop.

    (You may also run across rospy.sleep() which is similar to time.sleep() except that it works with simulated time as well (see Clock).)

    This loop also calls rospy.loginfo(str), which performs triple-duty: the messages get printed to screen, it gets written to the Node's log file, and it gets written to rosout. rosout is a handy tool for debugging: you can pull up messages using rqt_console instead of having to find the console window with your Node's output.

    std_msgs.msg.String is a very simple message type, so you may be wondering what it looks like to publish more complicated types. The general rule of thumb is that constructor args are in the same order as in the .msg file. You can also pass in no arguments and initialize the fields directly, e.g.

    msg = String()
msg.data = str

    or you can initialize some of the fields and leave the rest with default values:

    String(data=str)

    You may be wondering about the last little bit:

    try:
    talker()
except rospy.ROSInterruptException:
    pass

    In addition to the standard Python __main__ check, this catches a rospy.ROSInterruptException exception, which can be thrown by rospy.sleep() and rospy.Rate.sleep() methods when Ctrl-C is pressed or your Node is otherwise shutdown. The reason this exception is raised is so that you don't accidentally continue executing code after the sleep().

    Write the subscriber node

    Now we need to write a node to receive the messages.

  6. Return to ros_ws/src/py_pubsub/scripts to create the next node. Enter the following code in your terminal:

    wget https://raw.github.com/ros/ros_tutorials/kinetic-devel/rospy_tutorials/001_talker_listener/listener.py
chmod +x listener.py

    Now the directory should have these files:

    listener.py  talker.py

  7. Open the listener.py with your text editor. Alternatively, create a .py file with the following:

    #!/usr/bin/env python
import rospy
from std_msgs.msg import String

def callback(data):
    rospy.loginfo(rospy.get_caller_id() + "I heard %s", data.data)

def listener():

    # In ROS, nodes are uniquely named. If two nodes with the same
    # name are launched, the previous one is kicked off. The
    # anonymous=True flag means that rospy will choose a unique
    # name for our 'listener' node so that multiple listeners can
    # run simultaneously.
    rospy.init_node('listener', anonymous=True)

    rospy.Subscriber("chatter", String, callback)

    # spin() simply keeps python from exiting until this node is stopped
    rospy.spin()

if __name__ == '__main__':
    listener()

  8. Then, edit the catkin_install_python() call in your CMakeLists.txt so it looks like the following:

    catkin_install_python(PROGRAMS scripts/talker.py scripts/listener.py
    DESTINATION ${CATKIN_PACKAGE_BIN_DESTINATION}
)

    The Code Explained

    The code for listener.py is similar to talker.py, except we've introduced a new callback-based mechanism for subscribing to messages.

    rospy.init_node('listener', anonymous=True)

rospy.Subscriber("chatter", String, callback)

# spin() simply keeps python from exiting until this node is stopped
rospy.spin()

    This declares that your node subscribes to the chatter topic which is of type std_msgs.msgs.String. When new messages are received, callback is invoked with the message as the first argument.

    We also changed up the call to rospy.init_node() somewhat. We've added the anonymous=True keyword argument. ROS requires that each node have a unique name. If a node with the same name comes up, it bumps the previous one. This is so that malfunctioning nodes can easily be kicked off the network. The anonymous=True flag tells rospy to generate a unique name for the node so that you can have multiple listener.py nodes run easily.

    The final addition, rospy.spin() simply keeps your node from exiting until the node has been shutdown. Unlike roscpp, rospy.spin() does not affect the subscriber callback functions, as those have their own threads.

Build and run Python Package

We use CMake as our build system and, yes, you have to use it even for Python nodes. This is to make sure that the autogenerated Python code for messages and services is created.

  1. Before building the package, it's always a good idea to check and see if all dependencies are met.

    cd ~/ros_ws
rosdep install -i --from-path src --rosdistro melodic -y

    You should get a success return:

    #All required rosdeps installed successfully

  2. Go to your catkin workspace and run catkin_make:

    cd ~/ros_ws
catkin_make

  3. Run ROS master.

    roscore

  4. Open a new terminal, navigate to ros_ws, and source the setup files:

    source ./devel/setup.bash

  5. Now run the talker node:

    rosrun py_pubsub talker.py

    The terminal should start publishing info messages every 0.5 seconds, like so:

    [INFO] [1727910097.196673]: hello world 1727910097.2
[INFO] [1727910097.293260]: hello world 1727910097.29
[INFO] [1727910097.394363]: hello world 1727910097.39
[INFO] [1727910097.493223]: hello world 1727910097.49
[INFO] [1727910097.592866]: hello world 1727910097.59

  6. Open another terminal, source the setup files from inside ros_ws again, and then start the listener node:

    rosrun py_pubsub listener.py

    The listener will start printing messages to the console, starting at whatever message count the publisher is on at that time, like so:

    [INFO] [1727910166.665430]: /listener_5173_1727910161732I heard hello world 1727910166.66
[INFO] [1727910166.761804]: /listener_5173_1727910161732I heard hello world 1727910166.76
[INFO] [1727910166.870095]: /listener_5173_1727910161732I heard hello world 1727910166.87
[INFO] [1727910166.963474]: /listener_5173_1727910161732I heard hello world 1727910166.96
[INFO] [1727910167.071514]: /listener_5173_1727910161732I heard hello world 1727910167.07

  7. Stop the listener and try to run the C++ listener from earlier:

    rosrun cpp_pubsub listener

    You should see a similar same output.

  8. Enter Ctrl+C in each terminal to stop the nodes from spinning.

Lab Question

  1. Write a new controller (C++ or Python) for turtlesim that replace turtle_teleop_key. Since the turtlesim node is the subscriber in this example, you’ll only need to write a single publisher node.

    Create a new package called lab3_turtlesim. You can create a new workspace called lab3_ws or use your existing workspace.

    catkin_create_pkg lab3_turtlesim roscpp geometry_msgs

    or

    catkin_create_pkg lab3_turtlesim rospy geometry_msgs

    Your node should do the following:

    • Accept a command line argument specifying the name of the turtle it should control.
      • Running rosrun lab3_turtlesim turtle_controller turtle1 will start a controller node that controls turtle1.
    • Use w, a, s, d to control the turtle by publish velocity control messages on the appropriate topic whenever the user presses those keys on the keyboard, as in the original turtle_teleop_key. Capturing individual keystrokes from the terminal is slightly complicated, so feel free to use keyboard input such as scanf() or input() instead.

    Hint: You'll need to use the Twist message type in the geometry_msgs package.

    To test, spawn multiple turtles and open multiple instances of your new turtle controller node, each linked to a different turtle.

Once you've completed all the above steps, ask the lab professor or instructor over and demostrate that you've completed the lab and written down all your observations. You might be asked to explain some of the concepts you've learned in this lab.

Reference