Ubuntu (like all UNIX-like systems) organizes files in a hierarchical tree, where relationships are thought of in teams of children and parent. Directories can contain other directories as well as regular files , which are the "leaves" of the tree. Any element of the tree can be references by a path name ; an absolute path name starts with the character / (identifying the root directory , which contains all other directories and files), then every child directory that must be traversed to reach the element is listed, each separated by a / sign. A relative path name is one that doesn't start with / ; in that case, the directory tree is traversed starting from a given point, which changes depending on context, called the current directory . In every directory, there are two special directories called . and .. , which refer respectively to the directory itself, and to its p...