A hierarchical file system organizes files into directories that can contain other directories. The nested structure lets you reach an item by following a path from a starting point, much like tracing branches from the root of a tree. In POSIX.1-2024, the formal description has a small but important nuance: the structure can be a directed graph rather than a strict tree.
How a hierarchical file system is organized
Directories provide the branches of the hierarchy. Each directory holds entries for files, other directories, or both; directories can therefore be nested to group related items. POSIX.1-2024 describes the arrangement this way: “Files in the system are organized in a hierarchical structure in which all of the non-terminal nodes are directories and all of the terminal nodes are any other type of file.” The Open Group, POSIX.1-2024, General Concepts, section 4.8.
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A small directory tree
root
├── documents
│ └── notes.txt
└── photos
└── trip.jpg
Here, documents and photos are directories, while notes.txt and trip.jpg are files inside them. The example shows the basic idea: directories group items and may lead to more directories.
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- Root: The starting point at the top of a rooted hierarchy. In Unix-like systems, including z/OS UNIX,
/is used as the root marker; other operating systems may use different conventions. IBM: Hierarchical file system. - Directory: A container for entries that name files or further directories.
- File: An item reached through a directory entry; in POSIX’s description, non-directory files are terminal nodes.
- Path: A sequence of names that describes how to reach an item through the hierarchy. The exact naming rules and path format depend on the operating system. Microsoft Learn: Naming a File.
Why the tree model is useful—but not exact
A tree is a clear way to picture nested directories, but it is not always a complete formal description. POSIX.1-2024 notes that multiple directory entries can refer to the same file. In that case, the connections are more accurately described as a directed graph: entries still provide ways to reach items, but one file may be reachable through more than one entry.
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Hierarchical organization is not the same as FHS
A hierarchical file system is the general organizational idea: directories nest, and paths describe how to reach entries. The Filesystem Hierarchy Standard (FHS) is narrower. It sets requirements and guidelines for where files and directories should be placed under UNIX-like systems so that users and software can predict their locations. It is a placement convention for that ecosystem, not a definition of how every operating system must organize files.
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