How Computers Work · How an Operating System Works · Lesson 3 of 5

File and App Permissions

Who can open or change a file, and why apps ask before they look.

Who can read or change a file

Every file and folder on the computer has permissions. Permissions are rules about who may do what with it. The operating system checks these rules every time someone tries to open or change something.

The two most common permissions are:

• Read: you can open the file and look at it.
• Write: you can change it, save over it, or add to it.

Your own files live in your user folder, at C:\Users\YourName. Documents, Pictures, Desktop and Downloads are all inside it. Your account has full control here. Other standard users on the same computer can't open these folders.

System files are the files Windows and your programs need to run. They live in folders like C:\Windows and C:\Program Files. Everyone can read them, so programs can run. But a standard user can't change them.

If anything tries to change a system file, Windows steps in. It asks for admin approval through the User Account Control box from the last lesson. Some core files are locked even more tightly, so only Windows itself can replace them during updates.

This is why a mistake in your Documents folder can't break Windows. It is also why a bad program running in a standard account has a hard time digging in.

Curious? Right-click a file, choose Properties, and open the Security tab. You will see who has which permissions. Look, but don't change anything there.

On a Mac, choose Get Info on a file and look under Sharing & Permissions.

Why apps ask first

File permissions decide who can touch which files. There is a second layer for private things like your camera.

App permissions decide which apps may use sensitive parts of the computer. These include the camera, the microphone, your location, your contacts and your photos.

The idea is simple. An app should only get what it needs to do its job. A video call app needs the camera. A calculator does not.

On Windows 11, open Settings > Privacy & security. Under App permissions, pick Camera, Microphone or Location. You will see a list of apps with a switch next to each one. Turn off any app that has no good reason to be there.

On a Mac, apps must ask before they look. Say an app wants your camera, microphone or contacts for the first time. A box appears. The same happens for files in your Desktop or Documents folders. It says the app "would like to access" that thing. You choose Allow or Don't Allow.

You can change your mind later. Open System Settings > Privacy & Security, pick a category, and switch apps on or off. Some powerful ones, like Screen Recording and Full Disk Access, deserve extra care. Only allow apps you trust there.

Phones work the same way. You have seen these prompts on your phone for years.

A good habit: when a prompt appears, ask yourself, "Does this app need this to do what I want?" If not, say no. The app will usually still work.