Hands ROS Topics Intro

Shell Commands

# bringup command (on the robot itself)
$ roslaunch turtlebot3_bringup turtlebot3_robot.launch

# run teleop application
$ roslaunch turtlebot3_teleop turtlebot3_teleop_key.launch

# rostopic command to send cmd_vel to robot
$ rostopic pub /cmd_vel geometry_msgs/Twist "linear:
  x: 0.0
  y: 0.0
  z: 0.0
angular:
  x: 0.0
  y: 0.0
  z: 0.0"

ROS Shell utilities

• roscore: Start the ROS Core program
• rostopic: Work with topics
  • rostopic list: list all ros topics that roscore knows about
  • rostopic echo: print out any topics that are published
  • rostopic info: ask ros what it knows about a certain topic
• rqt_graph - display topics and subscriptions in a nice graphical format
• roslaunch - tool to launch collections of ROS nodes in one command. Requires a ‘package launch file’
• rosversion -d - what ros version is installed?

FAQ

• Linux: Use Ubuntu 16.04 exactly
• ROS Install for Turtlebot3: Follow the installation instructions for the TurtleBot3 exactly
• roscore: You must have roscore running before you try to run any ros-related program
• Execution: In order to execute a program/script on Linux, you must chmod +x filename.py
• shebang: Also include the ‘shebang’: #!/usr/bin/env python

Linux Commands

• apt family of commands, see: https://itsfoss.com/apt-command-guide/

Troubleshooting

If you get an error message that Ubuntu is running Lo-res mode or ubuntu just stalls during bootup, then:

• Ctrl-Alt F1 # get to terminal
• sudo apt-get update # update the apt-get app itself
• sudo apt-get upgrade # upgrade packages
• sudo apt-get install -f # apt-get fix its database
• restart

Commands

$ roslaunch turtlebot3_gazebo turtlebot3_stage_4.launch
$ roslaunch turtlebot3_navigation turtlebot3_navigation.launch map_file:=stage4
$ roslaunch turtlebot3_gazebo turtlebot3_simulation.launch
$ roslaunch turtlebot3_slam turtlebot3_slam.launch slam_methods:=gmapping
$ rosbag record rosbag record -O stage4.bag /scan /tf /odom
$ rosbag info stage4.bag
$ rosparam set use_sim_time true
$ rosbag play --clock stage4.bag
$ rosrun map_server map_saver -f stage4
$ rosrun map_server map_server stage4.yaml
$ roslaunch turtlebot3_gazebo turtlebot3_simulation.launch

Demo

• Lets write a sample topic publisher

 #!/usr/bin/env python
 import rospy
 from std_msgs.msg import Int32
 rospy.init_node('topic_publisher')
 pub = rospy.Publisher('counter', Int32. queue_size=10)
 rate = rospy.Rate(2)
 count = 0
 while not rospy.is_shutdown():
     pub.publish(count)
    count += 1
    rate.sleep()

• rostopic list
• rostopic echo /counter
• rostopic info /counter
• Now lets write a sample subscriber

 #!/usr/bin/env python
 import rospy
 from std_msgs.msg import Int32
 def callback(msg):
   print msg.data
 
 rospy.init_node('topic_subscriber')
 sub = rospy.Subscriber('counter', Int32, callback)
 rospy.spin()

• rostopic info /counter
• rqt_graph