Files and Folders

⏱ 6 min read

In one sentence: A file is a named collection of data kept in storage; a folder (or directory) is a list of files and other folders, and a path is the list of folder names that leads to one file.

Why should I care?

The photos, songs and documents on your computer are kept in files. The programs you install are stored in files too. Your files sit in folders, such as Documents, Downloads and Pictures. As a programmer, you will create, name and organize many files and folders.

Analogy

Think of a filing cabinet in an office. Each sheet of paper holds information. You put the sheets in folders with names on their labels. To find a letter, you read the labels until you find the right folder, and then the letter inside it.

(In this lesson, a file always means the computer kind, not a paper one.)

Where the analogy breaks down: a paper folder holds the paper itself. A computer folder does not hold the data of its files. It is a list of names, and for each name, the computer can find where that file's data is in storage. Also, a computer folder can hold other folders, and those can hold more folders, many levels deep.

How it actually works

A file is named data

A file is a collection of data in Storage: HDD and SSD, with a name. Inside, it is a series of bytes. A Unix paper by D. M. Ritchie and K. Thompson says: "A file contains whatever information the user places on it". A file of text, they say, is "a string of characters": characters in a row, one after another.

The name and the extension

Microsoft says Windows file names "have two parts separated by a period". (Microsoft also says the second part is optional.) In holiday.jpg, the first part is holiday. The part after the period, jpg, is the file extension: a short code for the type of file. Some examples:

The extension is a label. Microsoft says it tells the computer which app can open the file, and which icon to show for it. The real content is in the bytes.

A folder is a list of names

A folder is also called a directory. The two words mean the same thing: a Wikipedia article says that on many computers, directories are known as folders.

A 1965 paper about a computer system called Multics says: "A directory is a special file which is maintained by the file system, and which contains a list of entries." So a folder is itself stored as a special kind of file. The file system is the way files are organized in storage, and the software that manages them. It keeps track of where each file is.

A folder inside another folder is called a subfolder (or subdirectory).

Folders form a tree

Folders inside folders make a shape like an upside-down tree. At the top is a folder that is not inside any other folder: the root. On Windows, each drive, such as C:, has its own root.

A path is the way to a file

A path is the list of folder names that leads to a file, ending with the file's name. A character called a separator goes between the names.

• Windows uses a backslash \. The path to the letter above is C:\Users\Ana\Documents\letter.txt. Microsoft lists a Users folder as the default place for each user's own folder.
• Unix uses a forward slash /. Ritchie and Thompson describe a path name as "a sequence of directory names separated by slashes". A path that starts with / starts at the root. Apple says that on a Mac, the Users folder holds each user's home folder, named with the user's name. Its path is /Users. So the same letter could be at /Users/Ana/Documents/letter.txt.

So how does the computer find letter.txt? The operating system starts at the root. It looks in the root's list for Users, then in that list for Ana, and so on, until it finds the file's entry. That entry leads to where the file's bytes are in storage. (This is simplified: file systems also keep extra information, such as each file's size.)

How it came to be

In 1965, R. C. Daley and P. G. Neumann presented their paper about the file system of Multics. It describes "a basic tree hierarchy of files", with a "root directory" at the top.

Unix also used a tree. Ritchie and Thompson's paper, first published in 1974, describes a "rooted tree" of directories and paths written with slashes, such as /alpha/beta/gamma.

🧪 Try it in your browser

Web pages can ask you to choose a file, for example a photo. You will open a file chooser and look at your folders, without choosing anything.

1. In Chrome, Edge or Firefox, open this MDN Web Docs page (a website that explains web technology): https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/input/file
2. Near the top (scroll a little if needed), find the example box with the words Choose a profile picture. Click the button in that box. Its label is different in each browser.
3. A window opens: a file picker. You should see folder names, such as Documents, Downloads or Pictures. Double-click a folder to see what is inside it. This example asks for PNG and JPEG images, so other files may be hidden or grey.
4. Click Cancel (or press Esc). Do not choose a file. When you cancel, no file is chosen.

What you should see: a list of folders and files on your device. The window looks different in each browser and operating system.

On a phone: tapping the button also opens a file picker, but it may look very different. Tap Cancel or go back without choosing anything.

Common misunderstandings

• "Renaming photo.jpg to photo.png changes the file into a PNG." No. Microsoft says changing an extension "won't convert the file to another format". The bytes stay the same. Only the label changes.
• "When I open a file picker, the website can look through all my folders." No. You choose the file. MDN says the website does not even see the real path of a file you choose: it sees C:\fakepath\ before the file name, "to prevent malicious software from guessing the user's file structure". (Malicious means harmful.)

Recap

• A file is a named collection of data (bytes) in storage. The extension at the end of its name, such as .jpg, is a label that tells the computer which app to use.
• A folder, or directory, is a list of names of files and other folders. Folders inside folders form a tree, with the root at the top.
• A path, such as C:\Users\Ana\Documents\letter.txt or /Users/Ana/Documents/letter.txt, is the way from the root to one file.

Sources

• Microsoft Learn, Naming Files, Paths, and Namespaces: https://learn.microsoft.com/en-us/windows/win32/fileio/naming-a-file
• Microsoft Learn, KNOWNFOLDERID: https://learn.microsoft.com/en-us/windows/win32/shell/knownfolderid
• Microsoft Support, Common file name extensions in Windows: https://support.microsoft.com/en-us/windows/common-file-name-extensions-in-windows-da4a4430-8e76-89c5-59f7-1cdbbc75cb01
• Apple Support, Mac User Guide, Folders that come with your Mac: https://support.apple.com/guide/mac-help/mchlp1143/mac
• Apple Developer, File System Programming Guide, File System Basics: https://developer.apple.com/library/archive/documentation/FileManagement/Conceptual/FileSystemProgrammingGuide/FileSystemOverview/FileSystemOverview.html
• R. C. Daley and P. G. Neumann, A General-Purpose File System For Secondary Storage (1965): https://multicians.org/fjcc4.html
• D. M. Ritchie and K. Thompson, The UNIX Time-Sharing System (first published 1974; this link is the revised 1978 version): https://plan9.io/cm/cs/who/dmr/cacm.html
• Wikipedia, Directory (computing) (general background reference): https://en.wikipedia.org/wiki/Directory_(computing)
• MDN Web Docs, <input type="file">: https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/input/file

Written with AI · Reviewed by AI · Not yet checked by a human

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