ROS Parameter Server 
====================== 

Configuration information in ROS is usually saved to the Parameter server. The Parameter sever is a collection of values that can be accessed upon request through the command prompt, nodes or launch files. Parameters are intended to be fairly static, globally available values such as integers, floats, strings or bool values.

Parameters From the Terminal
-----------------------------

Parameters can be can be viewed and changed directly from a terminal using various rosparam commands. The simplest command lists all of the currently active parameters. 

.. code-block:: bash

  rosparam list

With a Husky gazebo simulation running, the following is an example of the output of rosparam list and gives us a good idea the type of values that are best suited for the parameter server.

The rosparam get command can be used to determine the value of a parameter.

.. code-block:: bash

  rosparam get 

For example, we can see that gravity in the z direction in Gazebo is set to -9.8, and we are currently running ROS hydro.

It is also possible to change a parameter using rosparam set

.. code-block:: bash

  rosparam set  

Say we wanted to simulate Husky driving on the moon, then we could change the gravity parameter is gazebo very easily

.. code-block:: bash

 rosparam set /gazebo/gravit_z -1.6

Accessing Parameters from Nodes
-------------------------------

It is often the case that your nodes will have to access the parameter server during start up to retrieve configuration information, or set a parameter value. This can be done quite easily in both C++ or Python, to set a parameter use:


**C++**     

.. code-block:: C

  void:ros::param::set(parameter_name, input_value)

**Python**

.. code-block:: Python

  rospy.param_set(parameter_name, input_value)

Similarly, to retrieve a parameter value from the parameter server

**C++**

.. code-block:: C

  void:ros::param::get(parameter_name)

**Python**

.. code-block:: Python

  rospy.param_get(parameter_name)

Accessing Parameter from Launch Files
--------------------------------------

The final source where you may need to access the parameter server is from a launch file. Setting a parameter value during a launch file is common practice to conveniently initialize parameters on start up. This can be done in your launch file using

.. code-block:: bash

  <param name="param-name" value="param-value" />

You may also use YMAL format along with the parameter tags, which is an easy to read 1 to 1 format for setting parameter

.. code-block:: bash

  string: 'foo'
  integer: 1234
  float: 1234.5
  boolean: true
  list: [1.0, mixed list]
  dictionary: {a: b, c: d}