In this post, we will talk about basics of ROS. We will follow the book A Gentle Introduction to ROS. The ROS version used in the book is Indigo which is ROS1, so we will also install a ROS1 version. The latest ROS1 version is Noetic.

Install ROS1 Noetic

To install Noetic, we can follow the instruction in Ubuntu install of ROS Noetic. The steps are easy to follow and I didn't run into any issues.

If you have already installed other ROS version before and want to uninstall the packages, you could following the instruction mentioned in this post. Here are the commands:

sudo apt-get remove ros-*
sudo apt-get purge ros-*
sudo apt-get autoremove

If you run into problems with downloading packages from Ubuntu repositories, you may try to clean up the repository list first. More specifically, you could

• Save a backup file for /etc/apt/sources.list
• Remove ROS related items from /etc/apt/sources.list
• Remove ROS related files in the /etc/apt/sources.list.d/ directory.

For more information about Ubuntu repositories, please check https://help.ubuntu.com/community/Repositories/CommandLine

If the installation is successful, we should be able to run the turtle simulation by executing the following three commands in three different terminals

roscore
rosrun turtlesim turtlesim_node
rosrun turtlesim turtle_teleop_key

Basics

• Packages
• All ROS software is organized into packages.
• All ROS software is part of one package or another.
• Each package is defined by a manifest, which is a file called package.xml.
• The manifest file contains information such as the package name, version, maintainer, and dependencies.
• The directory containing package.xml is called the package directory.
• ROS node and master
• ROS nodes are running application program. They are the places where the business logic exists.
• To start a node, we can execute command rosrun package-name executable-name
• We can specify the node name when starting a node. For example:
  rosrun turtlesim turtlesim_node __name:=A
• Note that the ROS master does not allow multiple nodes with the same name.
• Node name is not necessarily the same as the executable name.
• To kill a node, we can execute the command rosnode kill node-name.
• Using kill command is better than terminate the node process manually because there are some clean-up work to do. (e.g. unregistering the node from the master).
• The communication between nodes is handled by ROS master.
• ROS master is sort of the "main" process of the application.
• We can start ROS master by execution roscore.
• You should allow the master to continue running for the entire time that you're using ROS.
• Topics and Messages
• The main communication method in ROS1 is publish/subscribe.
• Pub/sub is a many-to-many communication method.
• Messages are grouped by topics.
• ROS also provide a one-to-one communication mechanism called services.

Commands

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