Write short notes on different types of file in Unix.

From a user perspective in a Unix system, everything is treated as a file. Even such devices such as printers and disk drives.

How can this be, you ask? Since all data is essentially a stream of bytes, each device can be viewed logically as a file.

All files in the Unix file system can be loosely categorized into 3 types, specifically:

1. ordinary files
2. directory files
3. device files ¹

While the latter two may not intuitively seem like files, they are considered "special" files.

The first type of file listed above is an ordinary file, that is, a file with no "special-ness". Ordinary files are comprised of streams of data (bytes) stored on some physical device. Examples of ordinary files include simple text files, application data files, files containing high-level source code, executable text files, and binary image files. Note that unlike some other OS implementations, files do not have to be binary Images to be executable (more on this to come).

The second type of file listed above is a special file called a directory (please don't call it a folder?). Directory files act as a container for other files, of any category. Thus we can have a directory file contained within a directory file (this is commonly referred to as a subdirectory). Directory files don't contain data in the user sense of data, they merely contain references to the files contained within them.

It is perhaps noteworthy at this point to mention that any "file" that has files directly below (contained within) it in the hierarchy must be a directory, and any "file" that does not have files below it in the hierarchy can be an ordinary file, or a directory, albeit empty.

The third category of file mentioned above is a device file. This is another special file that is used to describe a physical device, such as a printer or a portable drive. This file contains no data whatsoever, it merely maps any data coming its way to the physical device it describes.

¹ Device file types typically include: character device files, block device files, Unix domain sockets, named pipes and symbolic links. However, not all of these file types may be present across various Unix implementations.