Directory (computing)

In computing, a directory is a file system cataloging structure that contains references to other computer files, and possibly other directories. On many computers, directories are known as folders or drawers, analogous to a workbench or the traditional office filing cabinet. The name derives from books like a telephone directory that lists the phone numbers of all the people living in a certain area.

Files are organized by storing related files in the same directory. In a hierarchical file system (that is, one in which files and directories are organized in a manner that resembles a tree), a directory contained inside another directory is called a subdirectory. The terms parent and child are 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 the root directory.

The freedesktop.org media type for directories within many Unix-like systems – including but not limited to systems using GNOME, KDE Plasma 5, or ROX Desktop as the desktop environment – is "inode/directory". This is not an IANA registered media type.

Historically, and even on some modern embedded 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 was 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 a path.

In many operating systems, programs have an associated working directory in 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 a user's access only to their home directory or project directory, thus isolating their activities from all other users. 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. 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 and system calls that return records, not a byte-stream.