Updated ros2 pub tutorial with autocomplete topic prototype feature. (#5230)

* Updated ros2 pub tutorial with autocomplete topic prototype feature.

Signed-off-by: Leander Stephen D'Souza <leanderdsouza1234@gmail.com>

* Applied modifications to the ros2 topics documentation based on review.

Signed-off-by: Leander Stephen D'Souza <leanderdsouza1234@gmail.com>

---------

Signed-off-by: Leander Stephen D'Souza <leanderdsouza1234@gmail.com>

Author: leander-dsouza

source/Tutorials/Beginner-CLI-Tools/Understanding-ROS2-Topics/Understanding-ROS2-Topics.rst

@@ -244,17 +244,117 @@ Now that you have the message structure, you can publish data to a topic directl
 
 The ``'<args>'`` argument is the actual data you'll pass to the topic, in the structure you just discovered in the previous section.
 
+There are four main ways to use the ``pub`` command as shown below.
+However, the autocomplete feature described in ``c.`` and ``d.`` is not supported in Windows.
+
+a. **Publishing dictionary strings**:
+
+  In order to publish data to a topic, you need to pass the data in the form of YAML strings.
+
+  .. code-block:: bash
+
+    ros2 topic pub /turtle1/cmd_vel geometry_msgs/msg/Twist "{linear: {x: 2.0, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 1.8}}"
+
+  However, you do not need to specify the entire message, if you are just changing the linear or angular velocity, you can just specify the values you want to change.
+
+  For example, if you want to change the linear velocity to 2.0 and keep the angular velocity at 1.8, you can do the following:
+
+  .. code-block:: bash
+
+    ros2 topic pub /turtle1/cmd_vel geometry_msgs/msg/Twist "{linear: {x: 2.0}, angular: {z: 1.8}}"
+
+b. **Publishing an empty message**:
+
+  .. code-block:: bash
+
+    ros2 topic pub /turtle1/cmd_vel geometry_msgs/msg/Twist
+
+  This will publish the default values for the message type at 1 Hz.
+  In this case, this equivalent to the following command:
+
+  .. code-block:: bash
+
+    ros2 topic pub /turtle1/cmd_vel geometry_msgs/msg/Twist "{linear: {x: 0.0, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 0.0}}" --rate 1
+
+c. **Using autocomplete**:
+
+  You can trigger the autocomplete feature of your terminal by the following:
+
+  .. code-block:: bash
+
+    $ ros2 topic pub /turtle1/cmd_vel geometry_msgs/msg/Twist <TAB>
+    --keep-alive
+    --max-wait-time-secs
+    --node-name
+    --once
+    --print
+    --qos-depth
+    --qos-durability
+    --qos-history
+    --qos-liveliness
+    --qos-liveliness-lease-duration-seconds
+    --qos-profile
+    --qos-reliability
+    --rate
+    --spin-time
+    --stdin
+    --times
+    --use-sim-time
+    --wait-matching-subscriptions
+    --yaml-file
+    -1
+    -n
+    -p
+    -r
+    -s
+    -t
+    -w
+    \'linear:\^J\ \ x:\ 0.0\^J\ \ y:\ 0.0\^J\ \ z:\ 0.0\^Jangular:\^J\ \ x:\ 0.0\^J\ \ y:\ 0.0\^J\ \ z:\ 0.0\^J\'
+
+  All the options will be autocompleted by pressing the :kbd:`tab` key after the entering the first few characters of the option.
+  However, the topic message prototype will only be autocompleted after ``\'<TAB>`` is entered.
+
+  This is because the terminal does not recognize the single quote as part of the autocomplete string.
+  Hence it needs to be escaped by using ``\'`` to be recognized as part of the string.
+
+  The final autocompleted string will look like this:
+
+  .. code-block:: bash
+
+    $ ros2 topic pub /turtle1/cmd_vel geometry_msgs/msg/Twist 'linear:
+      x: 0.0
+      y: 0.0
+      z: 0.0
+    angular:
+      x: 0.0
+      y: 0.0
+      z: 0.0
+    '
+
+  This string is editable and you can change the values of the message type as required.
+
+d. **Using the raw autocompleted string**:
+
+  As mentioned above, the autocompleted string for ``geometry_msgs/msg/Twist`` looks like this:
+
+  .. code-block:: bash
+
+    \'linear:\^J\ \ x:\ 0.0\^J\ \ y:\ 0.0\^J\ \ z:\ 0.0\^Jangular:\^J\ \ x:\ 0.0\^J\ \ y:\ 0.0\^J\ \ z:\ 0.0\^J\'
+
+  This can be directly used in place of the yaml string in the command line.
+
+  .. code-block:: bash
+
+    ros2 topic pub /turtle1/cmd_vel geometry_msgs/msg/Twist \'linear:\^J\ \ x:\ 0.0\^J\ \ y:\ 0.0\^J\ \ z:\ 0.0\^Jangular:\^J\ \ x:\ 0.0\^J\ \ y:\ 0.0\^J\ \ z:\ 0.0\^J\'
+
+
 The turtle (and commonly the real robots which it is meant to emulate) require a steady stream of commands to operate continuously.
-So, to get the turtle moving, and keep it moving, you can use the following command.
-It's important to note that this argument needs to be input in YAML syntax.
-Input the full command like so:
+So, to get the turtle moving, and keep it moving, you can use the following dictionary string:
 
 .. code-block:: console
 
   ros2 topic pub /turtle1/cmd_vel geometry_msgs/msg/Twist "{linear: {x: 2.0, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 1.8}}"
 
-With no command-line options, ``ros2 topic pub`` publishes the command in a steady stream at 1 Hz.
-
 .. image:: images/pub_stream.png
 
 At times you may want to publish data to your topic only once (rather than continuously).