Unix-like systems such as linux always have a single file system tree, regardless of
how many drives or storage devices are attached to the computer.Storage devices
are attached( mounted ) at various points on the tree according to the system administrator.
absolute pathnames
an absolute pathname begins with the root directory and follows the tree branch by branch
until the path to the desired directory or file is completed.
relative pathnames
a relative pathname starts from the working directory.
The "." symbol refers to the working directory and "the" sysmbol
refers to the working directory's parent directory.
actually, you can omit the "./".
Linux has no concept of a "file extension" like some other operating systems.