Unix has, in one sense, a very static model of the world. Programs are implicitly assumed to run only briefly, so the background of files and directories can be assumed static during their execution. There is no standard, well-established way to ask the system to notify an application if and when a specified file or directory changes. This becomes a significant issue when writing long-lived user-interface software which wants to know about changes to the background.
Linux has file- and directory-change notification features,[156] and some versions of BSD have copied them, but these are not yet portable to other Unixes.
