Let's look into the technical requirements for this chapter:

• ROS Melodic Morenia on Ubuntu 18.04, with Gazebo 9 (preinstalled)
• ROS packages, that is, actionlib, smach, smach_ros, and smach_viewer
  
• Timelines and test platform:
  • Estimated learning time: On average, 100 minutes
  • Project build time (inclusive of compile and run time): On average, 45 minutes
  • Project test platform: HP Pavilion laptop (Intel® Core™ i7-4510U CPU @ 2.00 GHz × 4 with 8 GB Memory and 64-bit OS, GNOME-3.28.2)

The code for this chapter is available at https://github.com/PacktPublishing/ROS-Robotics-Projects-SecondEdition/tree/master/chapter_4_ws/src.

Let's begin this chapter by understanding what ROS actions are.