Learning from ros wiki.md

Task 1 for ROS Specialization

Lets Start out with Linux commands go through the compilation we have made here LinuxCommands.md

Thats should be all for your Linux Tutorial Now back to topic : )

ROS Communication

This week you will basically be following the official tutorials 1-15 by ROS wiki here Beginner Tutorial.
We assume that you have already installed ROS and created a catkin workspace and don't forget to add these lines to your home .bashrc.

source /opt/ros/noetic/setup.bash
source ~/catkin_ws/devel/setup.bash

We will now start with creating a package. All our work will be contained inside this package.

Create package

Follow this Tutorial.

Execute this command inside the src folder of your catkin workspace

~/catkin_ws/src $ caktin_create_pkg camp roscpp rospy tf2 ## camp is the package name and roscpp rospy tf2 are basic necessary dependencies  

should create a folder

     ├── catkin_ws
         └── src/
             ├── camp/               ## package
             |    ├── include/camp/  ## Contains header files
             |    ├── src/           ## Contains source files or cpp files
             |    ├── package.xml    ## Definition of package
             |    └── CMakeLists.txt ## Definition for building and compiling your cpp files contains all dependencies
             |   ...
             └── CMakeLists.txt

Now go back to catkin_ws directory and execute

~/catkin_ws $ catkin build
~/catkin_ws $ source catkin_ws/devel/setup.bash  ## when new packages are created it is necesaary to source the workspace again

Now one can also access this pkg from anywhere using ros commands

$ roscd camp

To know what the CMakeLists.txt and package.xml does check this out CMakeLists.txt and package.xml
These will always be auto-generated but if you are using additonal dependencies/compiling cpp files you will have to make some changes in them as shown in the below tutorials.


Now there are just Seven basic important things to learn and you will never struggle thereafter

• Nodes
• Topics
• Services
• Messages and Srv files
• Publisher
• Subscriber
• Launch files

Nodes,Topics and Services

Basically there are central nodes called master that collects all the new messages and distributes them within nodes. Basically Messages are feed or reading from sensors or useful information from some device/program called Nodes.
Services compared to messages just doesnt collect or send to one node but a trasaction of information takes place. Basically it is so that a node asks the other node to do something (could be to solve a mathematical problem, or Move the motors or turn on siren) And after that is completed the other node sends a confirmation or results. This Forward and backward motion of Information is termed as Request (one who asks for something) and Response (after processing the request).

For understanding them better we recommend you to follow the Tutorial by ROS (follow the steps and play with the simulation):

• Nodes Tutorial
• Topics Tutorial
• Services Tutorial

Messages and Srv files

These Topics and Services have there own Data type called msg and srv to handle the communication effectively and robustly these are similar to your definition of struct

// structdef.h
struct {
  int id;
  string name;
  float vals[];
};

Similarly an example campmsg.msg file looks like this

## campmsg.msg
int64 id
string name
float64 val[]

Same goes for srv files it is very similar to a msg file and contains the input and output argument definitions of the service. Example campsrv.srv file

## campsrv.srv
int64 a
int64 b
---
int64 sum

These messages are sent and recieved inside individual nodes using a mechanism called as Publisher which sends message to the ros master, And Subscriber which subscribes the message from master in a Asynchronous fashion using callbacks.

The publisher publishes a value on a topic which is intercepted by an infinite loop in the subscriber and a callback function is then executed every time a message is received. For understanding them check out the tutorials.

(Try to do the tutorials both in C++ and Python. Its easier to write the code in Python but for real applications C++ runs faster and is lightweight).

• Msg and Srv Tutorial
• Publisher and Subscriber Tutorial and Run
• Service Server and Client Tutorial and Run

Launch

Basically all the nodes/scripts that you write are spawned by executing them but what if there are many such nodes to launch ?

It will be too cumbersome to open so many terminals and execute them one by one. Instead we use what is called a launch file.
This will automate spawning each node and now you will just be executing one command.

You can directly checkout its implementation here. Tutorial