Union filesystem (Unionfs)

Unionfs forms the backbone of what is known as container images. We will discuss container images in detail in the next chapter. At this time, we want to just understand a bit better what Unionfs is, and how it works. Unionfs is mainly used on Linux and allows files and directories of distinct filesystems to be overlaid to form a single coherent filesystem. In this context, the individual filesystems are called branches. Contents of directories that have the same path within the merged branches will be seen together in a single merged directory, within the new virtual filesystem. When merging branches, the priority between the branches is specified. In that way, when two branches contain the same file, the one with the higher priority is seen in the final filesystem.