Computer Basics - Computer File Permissions and Access Control

Introduction

Computer file permissions and access control are mechanisms used by operating systems to control who can access files and folders and what actions they are allowed to perform. These mechanisms are important because computers often contain different types of information, including personal documents, application files, system files, and shared data. Without proper access control, unauthorized users or applications could modify, delete, or view important information.

File permissions determine the operations that a particular user or group can perform on a file or folder. Common operations include reading a file, modifying its contents, executing a program, creating new files, and deleting existing files. Access control provides the broader framework through which an operating system decides whether a particular user or process is permitted to access a resource.

What Are File Permissions?

File permissions are rules associated with files and directories that specify what users are allowed to do with them. The exact permission system depends on the operating system.

For example, a document may allow its owner to read and modify it while allowing other users only to read it. A system file may be restricted so that ordinary users cannot modify it.

The primary purpose of file permissions is to provide controlled access to computer resources. They help prevent accidental changes as well as unauthorized access.

Common Types of File Permissions

Three fundamental permissions commonly found in operating systems are read, write, and execute.

Read permission allows a user or program to view the contents of a file. For a directory, read permission generally allows the user to see the names of files and folders contained within it.

Write permission allows a user or program to modify a file. Depending on the operating system and resource involved, write access to a directory can also allow files to be created, removed, or renamed.

Execute permission allows a file to be executed as a program or script. For directories in systems such as Linux, execute permission generally relates to being able to access or traverse the directory.

Users and Groups

Access control often works by associating permissions with users and groups.

A user represents an individual account that accesses the computer. Different users can have different levels of access to the same resources.

A group is a collection of users. Permissions can be assigned to a group instead of configuring every user separately. This is particularly useful in schools, offices, companies, and other environments where several users need similar access.

For example, an organization could create a group called "Accounts" and give members of that group permission to access financial documents. Other employees could be prevented from accessing those files.

File Ownership

Many operating systems associate files with an owner. The owner is generally the user account that created the file or the account to which ownership was assigned.

Ownership helps the operating system determine which permissions should apply to a particular user.

For example, suppose a user creates a file called project.txt. The operating system can identify that user's account as the owner. The owner may be given permission to read and modify the file, while other users may receive more limited permissions.

File ownership is therefore an important part of access control.

Access Control

Access control refers to the process of determining whether a user, application, or process is allowed to access a particular resource.

The resource could be a file, folder, printer, database, application, or other computer resource.

A typical access-control decision involves several elements:

  1. Identifying the user or process requesting access.

  2. Identifying the resource being requested.

  3. Determining the requested operation, such as read, write, or execute.

  4. Checking the applicable permissions or access rules.

  5. Allowing or denying the requested operation.

For example, when a user attempts to open a protected file, the operating system checks the user's identity and the permissions associated with that file before providing access.

File Permissions in Windows

Microsoft Windows uses an access-control system based on security identifiers and access control lists. Permissions can be assigned to individual users and groups.

Common Windows permissions include:

  • Full control

  • Modify

  • Read and execute

  • List folder contents

  • Read

  • Write

For example, a user may have permission to read a folder but not modify its contents. Another user may have full control over the same folder.

Windows also supports NTFS permissions, which provide detailed control over files and folders stored on NTFS file systems.

File Permissions in Linux

Linux and other Unix-like operating systems commonly use a permission model involving the owner, group, and others.

For a file, permissions are commonly represented in three sets:

Owner: Permissions granted to the user who owns the file.

Group: Permissions granted to users belonging to the file's associated group.

Others: Permissions granted to users who are neither the owner nor members of the associated group.

For example, a permission representation might look like:

rwxr-xr--

Here:

  • r represents read permission.

  • w represents write permission.

  • x represents execute permission.

  • - indicates that a particular permission is not granted.

The first three characters represent the owner's permissions, the next three represent the group's permissions, and the final three represent permissions for other users.

Numeric Permission Representation

Linux permissions can also be represented using numbers.

The commonly used values are:

  • Read = 4

  • Write = 2

  • Execute = 1

The values are added together to represent a combination of permissions.

For example:

7 = 4 + 2 + 1

Therefore, 7 represents read, write, and execute permissions.

Similarly:

5 = 4 + 1

This represents read and execute permissions.

A permission setting such as 755 therefore represents:

  • Owner: 7 — read, write, execute

  • Group: 5 — read, execute

  • Others: 5 — read, execute

This numerical representation is frequently used when managing permissions in Linux systems.

Why File Permissions Are Important

File permissions provide several important benefits.

Protection of confidential information: Permissions can prevent unauthorized users from viewing sensitive documents.

Prevention of accidental modification: Restricting write access can prevent users from unintentionally changing important files.

System protection: Operating systems can prevent ordinary users from modifying critical system files.

Controlled collaboration: Organizations can give specific groups access to shared folders while restricting other users.

Data integrity: Limiting modification rights reduces the possibility of unauthorized changes to important information.

Application security: Applications and services can be given only the permissions they need to perform their functions.

Example of File Access Control

Consider a school computer containing a folder called StudentRecords.

The administrator might configure the folder so that:

  • Administrators can read, modify, create, and delete files.

  • Teachers can read and update student records.

  • Students can view only information specifically made available to them.

  • Other users have no access.

This arrangement ensures that users receive only the access necessary for their responsibilities.

Principle of Least Privilege

One important concept in access control is the principle of least privilege. It means that a user or program should receive only the permissions required to perform its intended task.

For example, if an employee only needs to view a document, giving that employee write permission may be unnecessary. Read-only access would provide the required functionality while reducing the possibility of accidental or unauthorized modification.

The principle of least privilege is widely used in operating-system security and other areas of computer security.

File Permissions and Security

File permissions are an important part of computer security, but they are not the only security mechanism. Strong passwords, user authentication, encryption, antivirus protection, network security, backups, and other security controls may also be required.

Permissions can help prevent unauthorized access, but they must be configured correctly. Poorly configured permissions can expose sensitive information or allow unauthorized users to modify important files.

Difference Between Authentication and Authorization

Authentication and authorization are closely related but have different purposes.

Authentication determines who the user is. For example, a user may enter a username and password to sign in to a computer.

Authorization determines what that authenticated user is allowed to do.

For example, after a user successfully signs in, the operating system may determine that the user can read a particular document but cannot delete or modify it.

Therefore:

Authentication = Who are you?

Authorization = What are you allowed to do?

File permissions are primarily related to authorization.

Conclusion

Computer file permissions and access control provide a structured way of protecting files, folders, and other computer resources. They determine which users or groups can access resources and which operations they can perform. Permissions such as read, write, and execute form the foundation of many operating-system security models, while ownership, groups, and access-control rules provide more detailed control.

Understanding file permissions is important for computer users, system administrators, developers, and cybersecurity professionals because properly configured access rights help protect information, maintain data integrity, prevent unauthorized changes, and ensure that users receive only the access necessary for their work.