I have gone through some materials available on the web regarding the unix file system but I couldn't get any proper answer.In some materials it is mentioned that the directory structure used in unix file system has a hierarchical directory structure whereas in some others it has been mentioned that actually it's a directed acyclic graph due to the presence of hard links.It is obviously hierarchical but DAG will be a better term to describe it.Can anyone explain whether it is really a DAG or hierarchical or tree structured?
It's a limited type of directed graph. If we leave aside symbolic links for a moment, then the directory structure of the filesystem is a tree, because directories cannot be hard-linked. The joint structure of directories and files is a type of restricted DAG, because the things at the leaves, i.e. the directory entries, may be hard links.
If symbolic links are taken into account, then the structure is a general graph, but because symlinks can always be recognized as such, cycles can be broken simply by treating links as a special type of file (i.e. use lstat instead of stat).