Root directory
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In computer file systems, the root directory is the first or top-most directory in a hierarchy. It can be likened to the root of a tree—the starting point where all branches originate.
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Metaphor
To use the example of a physical file cabinet, if the separate drawers in the file cabinet are represented as the highest level of sub-directories in the file system, then the room the file cabinet is in may be representated as the root directory. That is, the other directories may be inside it, but the root directory cannot go in any other directories, at least in that file system. In many operating systems, including Microsoft Windows and Unix, you may place files inside the root directory, as well in its sub-directories. One may envision this as placing paper files in the room but not in the drawer of any file cabinet.
Multiple root directories
Unix abstracts the nature of this tree hierarchy entirely, and in Unix the root directory is denoted /. All filesystem entries, including mounted partitions are "leaves" of this root. However under DOS and Windows, this behavior is different: each partition has a separate root directory (labeled C:\ for a particular partition C) and there is no common root directory above that.
Strictly speaking, recent versions of Windows do support more abstract hierarchies, with partitions mountable within a directory of another drive, though this is rarely seen. In some contexts, it is also possible to refer to a root directory containing all mounted drives, although it cannot contain files directly as it does not exist on any file system. For instance, when linking to a local file using the "file:" URI scheme, the syntax is of the form "file:///C:/...", where "file://" is the standard prefix, and the third '/' represents the root of the local system.
In Unix and similar operating systems, each process has its own idea of what the root directory is. For most processes this is the same as the system's actual root directory, but it can be changed by calling the chroot system call. This is typically done for security purposes to restrict which files a process may access to just a subset of the file hierarchy.
/root
On many Unices, there is also a directory which is named /root. Confusingly, it is not a root directory in the sense of this article, but rather the home directory of the Superuser (conventionally known as "root").
See also
fr:Répertoire racine et:Juurkaust nl:Root directory pl:Katalog główny