# Linux - File Permissions

Linux file permissions play a crucial role in maintaining the security and integrity of the operating system. Understanding how to manage file permissions allows you to control access to files and directories, ensuring only authorized users can read, write, or execute certain files. In this blog, we’ll dive deep into Linux file permissions, explaining how they work, how to change them, and best practices for managing them.

#### <mark>1. </mark> **<mark>Linux File Permission Structure</mark>**

In Linux, each file and directory is associated with three types of users and three types of permissions. Let’s break these down:

**User Types:**

* **Owner**: The user who created the file or directory.
    
* **Group**: A set of users who share the same group permissions.
    
* **Others**: All other users on the system who are neither the owner nor in the group.
    

**Permission Types:**

* **Read (**`r`): Grants permission to read or view the contents of a file.
    
* **Write (**`w`): Grants permission to modify or delete the file.
    
* **Execute (**`x`): Grants permission to run a file if it’s a script or program.
    

You can view the permissions of a file by using the `ls -l` command:

```plaintext
ls -l file_name
```

![](https://cdn.hashnode.com/res/hashnode/image/upload/v1726685855539/c237fd6c-b0f6-4e30-827f-b2cd19fe2390.png align="center")

The output looks like this:

```plaintext
-rw-rw-r--
```

Let’s break this down:

The file permission string `-rw-rw-r--` can be broken down as follows:

1. **First character (**`-`): This indicates the file type. The options include:
    
    * `-` for a regular file
        
    * `d` for a directory
        
    * `l` for a symbolic link
        
    
    In this case, it is a regular file.
    
2. **Next three characters (**`rw-`): These represent the **Owner** (user) permissions:
    
    * `r` (read) means the owner can read the file.
        
    * `w` (write) means the owner can modify the file.
        
    * `-` (no execute) means the owner cannot execute the file as a program.
        
    
    So, the owner has read and write permissions but no execute permissions.
    
3. **Next three characters (**`rw-`): These represent the **Group** permissions:
    
    * `r` (read) means users in the group can read the file.
        
    * `w` (write) means users in the group can modify the file.
        
    * `-` (no execute) means group users cannot execute the file as a program.
        
    
    So, the group has read and write permissions but no execute permissions.
    
4. **Last three characters (**`r--`): These represent the **Others** (everyone else) permissions:
    
    * `r` (read) means other users can read the file.
        
    * `-` (no write) means they cannot modify the file.
        
    * `-` (no execute) means they cannot execute the file.
        
    
    So, others have read-only access.
    

### Summary:

* **Owner**: Read and write permissions
    
* **Group**: Read and write permissions
    
* **Others**: Read-only permission
    

#### <mark>2. </mark> **<mark>Changing File Permissions: </mark>** `chmod` Command

To change the permissions of a file or directory, you use the `chmod` command. There are two ways to set permissions: symbolic and numeric.

##### a. **Symbolic Mode**

In symbolic mode, you modify permissions using letters:

* `u` for user/owner
    
* `g` for group
    
* `o` for others
    
* `a` for all (user, group, and others)
    

You can add (+), remove (-), or set (=) specific permissions:

```plaintext
chmod u+x file_name     # Adds execute permission for the owner
chmod g-w file_name     # Removes write permission for the group
chmod a=r file_name     # Sets read-only permission for everyone
```

![](https://cdn.hashnode.com/res/hashnode/image/upload/v1726687835480/11afb548-328e-44e5-aef9-423fdbb9b8d4.png align="center")

##### b. **Numeric Mode**

In numeric mode, permissions are represented by a three-digit octal number, where each digit represents the permission for the user, group, and others respectively. Each permission has a numerical value:

* **Read (r)** = 4
    
* **Write (w)** = 2
    
* **Execute (x)** = 1
    

For example, if you want to give the owner read, write, and execute permissions, the group read and execute, and others read-only, the permissions would be `rwxr-xr--`, which is equivalent to `754`:

```plaintext
chmod 754 file_name
```

![](https://cdn.hashnode.com/res/hashnode/image/upload/v1726686387675/7a565b69-d565-4b27-b7f9-e5e5c3841d18.png align="center")

Here’s how the permissions work in numeric form:

| Permission | Numeric Value |
| --- | --- |
| `---` | 0 |
| `--x` | 1 |
| `-w-` | 2 |
| `-wx` | 3 |
| `r--` | 4 |
| `r-x` | 5 |
| `rw-` | 6 |
| `rwx` | 7 |

#### <mark>3. </mark> **<mark>Changing Ownership: </mark>** `chown` Command

The `chown` command changes the owner and group of a file or directory. For example, `chown ubuntu:dev filename` changes the owner to `ubuntu` and the group to `dev`.

```plaintext
sudo chown user:group filename
```

![](https://cdn.hashnode.com/res/hashnode/image/upload/v1726687348590/17b47e76-f23a-4781-8cb1-1fb57a7e403d.png align="center")

#### <mark>4. </mark> **<mark>Changing Group Ownership</mark>:** `chgrp` Command

The `chgrp` command changes the group ownership of a file or directory.

```plaintext
sudo chgrp group filename
```

![](https://cdn.hashnode.com/res/hashnode/image/upload/v1726687494326/cea97eb2-bdac-46cc-b1c0-4066dfaf1140.png align="center")

#### **Conclusion**

Understanding and managing Linux file permissions is essential for maintaining the security and proper functioning of a Linux system. With `chmod`, `chown` you can fine-tune who can access, modify, or execute files and directories. Mastering these tools ensures your Linux environment stays safe and efficient.