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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[edit] 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 represented 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 most operating systems, files can be placed 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.
[edit] Multiple root directories
Unix abstracts the nature of this tree hierarchy entirely, and in Unix and Unix-like systems, the root directory is denoted /. All filesystem entries, including mounted partitions are "branches" of this root.
Under DOS and Microsoft Windows, each partition has a drive letter assignment (labeled C:\ for a particular partition C) and there is no common root directory above that. DOS and Windows do support more abstract hierarchies, with partitions mountable within a directory of another drive, though this is rarely seen. This has been possible in DOS through the command JOIN since it first was added to DOS, and can be achieved in all Windows versions as well. 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.
[edit] /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").
[edit] VMS
In the HP (formerly Compaq, and formerly Digital Equipment Corporation) operating system VMS, the term "root directory" is used to refer to the directory in which all the user's files are stored. The user can also store directories in this directory, which in turn can contain files and other directories. The equivalent of an MS-DOS or Windows "root directory" on each disk is referred to as a Master File Directory in VMS.
