ROS Structure

catkin_ws/
├── src/                    # Source space
│   ├── package1/
│   │   ├── CMakeLists.txt
│   │   ├── package.xml
│   │   ├── src/
│   │   ├── include/
│   │   ├── launch/
│   │   └── config/
│   └── package2/
├── build/                  # Build space
├── devel/                  # Development space
└── install/                # Install space (optional)

#!/usr/bin/env python
import rospy

rospy.init_node('my_node')
rate = rospy.Rate(10)  # 10 Hz

while not rospy.is_shutdown():
    # Do work
    rate.sleep()

# Publisher
pub = rospy.Publisher('/cmd_vel', Twist, queue_size=10)
pub.publish(msg)

# Subscriber
def callback(msg):
    rospy.loginfo(msg.data)

sub = rospy.Subscriber('/scan', LaserScan, callback)

# Service Server
def handle_request(req):
    return MyServiceResponse(result)

srv = rospy.Service('my_service', MyService, handle_request)

# Service Client
rospy.wait_for_service('my_service')
client = rospy.ServiceProxy('my_service', MyService)
response = client(request)

# geometry_msgs/Twist.msg
Vector3 linear
Vector3 angular

<?xml version="1.0"?>
<package format="2">
  <name>my_package</name>
  <version>0.0.1</version>
  <description>Package description</description>

  <buildtool_depend>catkin</buildtool_depend>
  <build_depend>rospy</build_depend>
  <exec_depend>rospy</exec_depend>
</package>

cmake_minimum_required(VERSION 3.0.2)
project(my_package)

find_package(catkin REQUIRED COMPONENTS
  rospy
  std_msgs
)

catkin_package()

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

<!-- my_launch.launch -->
<launch>
  <node pkg="my_package" type="node1.py" name="node1" output="screen"/>
  <node pkg="my_package" type="node2.py" name="node2">
    <param name="rate" value="10"/>
  </node>
</launch>

# Create workspace
mkdir -p ~/catkin_ws/src
cd ~/catkin_ws

# Initialize
catkin_make

# Source environment
source devel/setup.bash

# Build specific package
catkin_make --pkg my_package

turtlebot3/
├── turtlebot3_bringup/     # Robot startup
├── turtlebot3_slam/        # SLAM mapping
├── turtlebot3_navigation/  # Autonomous nav
├── turtlebot3_description/ # URDF models
└── turtlebot3_simulations/ # Gazebo sim