IYH see https://thelinuxcode.com/mastering-owners-permissions-access-control-in-linux-a-beginners-guide/

See also 3 minutes overview f1-mini LLM:

How does a Linux system's permission and access control system work?

Here's a detailed overview of how a Linux system's permission and access control system works:

**Key Concepts:**

1. **Users**: Individuals or entities interacting with the system.

2. **Groups**: Collections of users with similar permissions or interests.

3. **Ownership**: Each file/directory has an owner (user) and a group.

4. **Permissions**: Rules defining what actions can be performed on a file/directory.

5. **Access Control**: The system enforcing permissions to regulate access.

**Permission Structure:**

Linux uses a permission structure based on three types of access for each of three categories of users:

| **Category** | **Description** |

| --- | --- |

| **User (u)** | The owner of the file/directory |

| **Group (g)** | The group associated with the file/directory |

| **Others (o)** | All other users on the system |

**Types of Access (Permissions):**

For each category (User, Group, Others), there are three types of permissions:

| **Permission** | **Abbreviation** | **Description** |

| --- | --- | --- |

| **Read** | `r` | Ability to view contents |

| **Write** | `w` | Ability to modify or delete contents |

| **Execute** | `x` | Ability to run the file (if executable) or traverse the directory |

**Permission Notation:**

Permissions are often represented in two formats:

1. **Symbolic Notation**:

* `rwx` for each category (e.g., `rwxr-x---` means `rwx` for User, `r-x` for Group, `---` for Others)

2. **Octal (Numeric) Notation**:

* Each permission set is represented by a number (0-7), calculated by assigning `r=4`, `w=2`, `x=1`, and summing the values for each category.

* Example: `755` translates to `rwx` (7) for User, `r-x` (5) for Group, and `r-x` (5) for Others.

**How Permissions are Evaluated:**

1. **User Permissions**: If you own the file/directory, your access is determined by the **User** permissions.

2. **Group Permissions**: If you're not the owner but are in the associated group, your access is determined by the **Group** permissions.

3. **Others Permissions**: If you don't own the file/directory and aren't in its group, your access is determined by the **Others** permissions.

**Additional Access Control Mechanisms:**

1. **SetUID (SUID), SetGID (SGID), and Sticky Bit**:

* **SUID**: Executes a file with the owner's permissions.

* **SGID**: Executes a file with the group's permissions or sets the group ID for new files in a directory.

* **Sticky Bit**: Prevents users from deleting files they don't own in a directory.

2. **Access Control Lists (ACLs)**:

* Provide more fine-grained permissions by allowing administrators to set permissions for specific users or groups beyond the standard owner, group, and others.

3. **Linux Capabilities**:

* A more secure alternative to SUID, allowing for the assignment of specific root privileges to executables.

4. **SELinux (Security-Enhanced Linux) and AppArmor**:

* Mandatory Access Control (MAC) systems that provide an additional layer of security by enforcing a set of rules based on the sensitivity level of resources (e.g., files, network ports).

**Managing Permissions:**

- **`chmod`**: Changes permissions of files and directories.

- **`chown`**: Changes the owner of files and directories.

- **`chgrp`**: Changes the group of files and directories.

- **`setfacl` and `getfacl`**: Manage Access Control Lists (ACLs).

A Linux system's permission and access control system is built on three primary permission categories: read (r), write (w), and execute (x), applied to three user classes: owner, group, and others. Each file or directory has these permissions managed via the chmod and chown commands. In my experience, while setting up secure environments for IT projects, I assigned minimal permissions to files to prevent unauthorized access, ensuring only specific users could modify or execute critical scripts. Linux also supports Access Control Lists (ACLs) for fine-grained permission control, which I’ve utilized to define custom rules for specific user needs in collaborative environments.