You've just uploaded a new file to your web server, maybe a PHP script or an image gallery. You try to access it in your browser, and instead of your new content, you're greeted with a stark, frustrating error: "403 Forbidden" or "Permission denied." It’s a classic Linux rite of passage, and the answer almost always lies in a single, cryptic command: `chmod 755`.
As someone who has managed Linux servers for over a decade, I've typed that command thousands of times. It's often treated like a magic incantation to fix access problems. But what does it actually *do*? Why is this specific number the go-to standard for so many web directories and scripts? This guide will break down the numbers, explain the logic, and give you the confidence to use file permissions not as a magic spell, but as a precise and powerful tool.
The Building Blocks of Linux Permissions
Before the numbers make sense, you need to know the three basic questions Linux asks about any file or directory:
- Who can access it? (The Users)
- What can they do? (The Permissions)
There are three types of "users": the Owner (you), the Group (a specific set of users), and Other (everyone else). And they can be granted three core "permissions": the ability to Read (r), Write (w), and Execute (x) a file.
Decoding the Numbers: What "755" Actually Means
The `chmod` command uses a numeric system where each permission is assigned a value: Read is 4, Write is 2, and Execute is 1. To create a permission set for a user, you simply add the numbers together. This is where `755` comes from.
- The **first digit (7)** is for the **Owner**. A `7` is `4 + 2 + 1`, which means the owner gets full control: Read, Write, and Execute (rwx).
- The **second digit (5)** is for the **Group**. A `5` is `4 + 1`, meaning the group can Read and Execute, but *cannot* write or modify the file (r-x).
- The **third digit (5)** is for **Other**. A `5` is also `4 + 1`, meaning everyone else can also Read and Execute, but cannot write (r-x).
So, `chmod 755` sets the file so that you (the owner) have full control, while everyone else can only read and run the file, but not change it. This simple combination is the key to its power and popularity.
Why 755 is the Standard for Web Servers
This permission set is the default for a reason: it perfectly balances security and accessibility for the web.
When applied to a **directory**, `755` means the web server (which runs as a different user) can enter the directory (`execute` permission) and list its files (`read` permission), but it can't add, delete, or modify files (`write` permission is missing). This is exactly what you want. It allows the server to serve your website to visitors without giving it—or anyone else—the ability to deface your site.
When applied to a **file**, like a PHP script, `755` allows the web server to execute the script and serve its output. Again, the server can't modify the script's code, which is a critical security measure.
When to Use Other Permissions (The Quick Guide)
While `755` is a fantastic default, it's not the only tool in the box. Using the right permission is the mark of a pro. Here are the other common settings:
| Permission | What It Means | Best For |
|---|---|---|
| 644 | Owner can read/write; everyone else can only read. | Standard for non-executable web files like HTML, CSS, and images. |
| 700 | Only the owner has full access. No one else can do anything. | Private scripts or directories that only you should ever access. |
| 600 | Only the owner can read and write. No one can execute. | Sensitive configuration or data files (e.g., `wp-config.php`). |
If you're ever unsure, a handy chmod calculator can help you translate these numbers back and forth.
Putting It to Use: The Commands
Applying the permission is straightforward. To change a single file:
chmod 755 my_script.shTo change a directory and everything inside it recursively, use the `-R` flag:
chmod -R 755 my_public_html/Final Thoughts: Beyond the Magic Number
Understanding `chmod 755` is a fundamental skill for anyone working with Linux. It's not a magic fix, but a deliberate choice that provides a secure yet functional way to set permissions for your web directories and executable files. By understanding what it means and why it's the standard, you can ensure that your files are both accessible to your users and safe from modification.