Directory (computing)

Incomputing,adirectoryis afile systemcataloging structure which contains references to othercomputer files,and possibly other directories. On many computers, directories are known asfolders,ordrawers,analogousto aworkbenchor the traditional officefiling cabinet.The name derives from books like atelephone directorythat lists the phone numbers of all the people living in a certain area.

Screenshot of aMicrosoft Windowscommand promptwindow showing a directory listing.

Files are organized by storing related files in the same directory. In ahierarchical file system(that is, one in which files and directories are organized in a manner that resembles atree), a directory contained inside another directory is called asubdirectory.The termsparentandchildare often used to describe the relationship between a subdirectory and the directory in which it is cataloged, the latter being the parent. The top-most directory in such a filesystem, which does not have a parent of its own, is called therootdirectory.

Thefreedesktop.orgmedia type for directories within manyUnix-likesystems – including but not limited to systems usingGNOME,KDE Plasma 5,orROX Desktopas the desktop environment – is "inode/directory".This is not anIANA registered media type.

Overview

Diagram of a hierarchical directory tree. The root directory is here called "MFD", for Master File Directory. Usually a file can only be in one directory at a time, but here File 2 ishard linkedso it appears in two directories.

Historically, and even on some modernembedded systems,the file systems either had no support for directories at all or had only a "flat"directory structure,meaning subdirectories were not supported; there were only a group of top-level directories, each containing files. In modern systems, a directory can contain a mix of files and subdirectories.

A reference to a location in a directory system is called apath.

In manyoperating systems,programs have an associatedworking directoryin which they execute. Typically, file names accessed by the program are assumed to reside within this directory if the file names are not specified with an explicit directory name.

Some operating systems restrict auser'saccess only to theirhome directoryor project directory, thus isolating their activities from all other users. In early versions of Unix the root directory was the home directory of theroot user,but modern Unix usually uses another directory such as/rootfor this purpose.

In keeping withUnix philosophy,Unix systems treat directories as a type of file.Caveats include not being able to write to a directory file except indirectly by creating, renaming and removing file system objects in the directory and only being able to read from a directory file using directory-specific library routines andsystem callsthat return records, not a byte-stream.

Folder metaphor

Sample folder icon (fromKDE).

The namefolder,presenting an analogy to thefile folderused in offices, and used in a hierarchical file system design for theElectronic Recording Machine, Accounting(ERMA) Mark 1 published in 1958as well as byXerox Star,is used in almost all modernoperating systems' desktop environments. Folders are often depicted withiconswhich visually resemble physical file folders.

There is a difference between adirectory,which is afile systemconcept, and thegraphical user interfacemetaphor that is used to represent it (afolder).For example,Microsoft Windowsuses the concept ofspecial foldersto help present the contents of the computer to the user in a fairly consistent way that frees the user from having to deal with absolute directory paths, which can vary between versions of Windows, and between individual installations. Many operating systems also have the concept of "smart folders" orvirtual foldersthat reflect the results of a file system search or other operation. These folders do not represent a directory in the file hierarchy. Manyemail clientsallow the creation of folders to organize email. These folders have no corresponding representation in the filesystem structure.

If one is referring to acontainer of documents,the termfolderis more appropriate.The termdirectoryrefers to the way a structured list of document files and folders are stored on the computer. The distinction can be due to the way a directory is accessed; on Unix systems,/usr/bin/is usually referred to as a directory when viewed in acommand lineconsole,but if accessed through a graphicalfile manager,users may sometimes call it a folder.

Lookup cache

Operating systems that support hierarchical filesystems (practically all modern ones) implement a form ofcachingtoRAMof recentpathlookups. In theUnixworld, this is usually calledDirectory Name Lookup Cache(DNLC), although it is calleddcacheonLinux.

For local filesystems, DNLC entries normally expire only under pressure from other more recent entries. Fornetwork file systemsacoherence mechanismis necessary to ensure that entries have not been invalidated by other clients.

See also

• File folder

Concepts

• Directory structure
• Filename
• Filesystem Hierarchy Standard
• Home directory
• Root directory
• Virtual folder
• Working directory

Commands

• cd (command)
• chroot
• dir (command)
• mkdir
• pushd and popd
• pwd
• tree (command)

External links

• Definition of directoryby The Linux Information Project (LINFO)