Understanding the ROS filesystem level

ROS is more than a development framework. We can refer to ROS as a meta-operating system, since it offers not only tools and libraries but even OS-like functions, such as hardware abstraction, package management, and a developer toolchain. Like a real operating system, ROS files are organized on the hard disk in a particular manner, as depicted in the following figure:

Here are the explanations for each block in the filesystem:

• Packages: The ROS packages are the most basic unit of the ROS software. They contain one or more ROS programs (nodes), libraries, configuration files, and so on, which are organized together as a single unit. Packages are the atomic build item and release item in the ROS software.
• Package manifest: The package manifest file is inside a package that contains information about the package, author, license, dependencies, compilation flags, and so on. The package.xml file inside the ROS package is the manifest file of that package.
• Metapackages: The term metapackage refers to one or more related packages which can be loosely grouped together. In principle, metapackages are virtual packages that don't contain any source code or typical files usually found in packages.
• Metapackages manifest: The metapackage manifest is similar to the package manifest, the difference being that it might include packages inside it as runtime dependencies and declare an export tag.
• Messages (.msg): The ROS messages are a type of information that is sent from one ROS process to the other. We can define a custom message inside the msg folder inside a package (my_package/msg/MyMessageType.msg). The extension of the message file is .msg.
• Services (.srv): The ROS service is a kind of request/reply interaction between processes. The reply and request data types can be defined inside the srv folder inside the package (my_package/srv/MyServiceType.srv).
• Repositories: Most of the ROS packages are maintained using a Version Control System (VCS), such as Git, Subversion (svn), Mercurial (hg), and so on. The collection of packages that share a common VCS can be called repositories. The package in the repositories can be released using a catkin release automation tool called bloom.

The following screenshot gives you an idea of the files and folders of a package that we are going to create in the upcoming sections: