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The original Unix file system supported three types of files: ordinary files, directories, and "special files", also termed device files. [1] The Berkeley Software Distribution (BSD) and System V each added a file type to be used for interprocess communication: BSD added sockets, [3] while System V added FIFO files.
The most common special file is the directory. The layout of a directory file is defined by the filesystem used. As several filesystems are available under Unix, both native and non-native, there is no one directory file layout. A directory is marked with a d as the first letter in the mode field in the output of ls -dl [5] or stat, e.g.
In early versions of Unix the root directory was the home directory of the root user, but modern Unix usually uses another directory such as /root for this purpose. In keeping with Unix philosophy , Unix systems treat directories as a type of file. [ 3 ]
Modern Linux distributions include a /sys directory as a virtual filesystem (sysfs, comparable to /proc, which is a procfs), which stores and allows modification of the devices connected to the system, [20] whereas many traditional Unix-like operating systems use /sys as a symbolic link to the kernel source tree.
Unix abstracts the nature of this tree hierarchy entirely and in Unix and Unix-like systems the root directory is denoted by the / (slash) sign. Though the root directory is conventionally referred to as /, the directory entry itself has no name – its path is the "empty" part before the initial directory separator character (/).
According to Rob Pike, the dot file was an unintended consequence of the implementation of hierarchical file systems during the UNIX 2nd Edition re-write, which introduced the . and .. file entries within all directories, and necessitated the omission of dot files from the directory listing produced by ls.
3 Unix. 4 See also. ... a directory structure is the way an operating system arranges files that are ... files and directories appear under the root directory ...
Unix directories are lists of association structures, each of which contains one filename and one inode number. The file system driver must search a directory for a particular filename and then convert the filename to the correct corresponding inode number.