4.3b Octal notation: 755, 644, 600 — what every combination means¶

🔍 Context Introduction¶

When working with AI infrastructure, you will frequently need to control who can read, write, or execute files and directories. Linux uses a simple but powerful permission system based on octal (base-8) notation. Three numbers — like 755, 644, and 600 — are the most common permission sets you will encounter. Understanding what each digit means is essential for securing AI models, datasets, configuration files, and scripts.

⚙️ How Octal Permissions Work¶

Each octal digit represents a set of three permissions for a specific user group:

• Owner (the user who owns the file)
• Group (users in the file's group)
• Others (everyone else)

Each permission type has a numeric value:

To calculate a permission digit, add the values of the permissions you want to grant.

🧮 Breaking Down the Three Most Common Combinations¶

755 — Standard for Executable Scripts and Directories¶

• Owner: 7 (read + write + execute = 4 + 2 + 1)
• Group: 5 (read + execute = 4 + 1)
• Others: 5 (read + execute = 4 + 1)

What it means: The owner has full control. Everyone else can read and execute the file or directory but cannot modify it.

When to use: For AI training scripts, model inference scripts, and directories containing datasets that multiple users need to access.

644 — Standard for Configuration Files and Text Files¶

• Owner: 6 (read + write = 4 + 2)
• Group: 4 (read only)
• Others: 4 (read only)

What it means: The owner can read and modify the file. Everyone else can only read it.

When to use: For AI model configuration files (YAML, JSON, TOML), log files, documentation, and dataset metadata files.

600 — Private Files for Sensitive Data¶

• Owner: 6 (read + write = 4 + 2)
• Group: 0 (no permissions)
• Others: 0 (no permissions)

What it means: Only the owner can read or write the file. No one else has any access.

When to use: For API keys, database passwords, private SSH keys, and any sensitive AI model weights or proprietary training data.

📊 Visual Representation: Octal Permission Value Breakdown (755 Example)¶

This diagram illustrates how an octal permission code (e.g., 755) maps to User, Group, and Other privileges.

📊 Quick Reference Table¶

🕵️ How to Read an Octal Permission String¶

When you see a permission string like rwxr-xr-x, it maps directly to octal:

• First three characters (rwx) = owner = 7
• Next three characters (r-x) = group = 5
• Last three characters (r-x) = others = 5

So rwxr-xr-x equals 755.

🛠️ Practical Examples for AI Infrastructure¶

For a training script that everyone needs to run but only you should edit: - Use 755

For a model configuration file that others need to read but not modify: - Use 644

For a file containing your cloud provider API credentials: - Use 600

For a shared dataset directory where users need to browse and read files: - Use 755 on the directory itself - Use 644 on the files inside

✅ Key Takeaways for New Engineers¶

• The first digit always controls the owner, the second controls the group, and the third controls others.
• 755 is the most permissive safe default — it gives full control to the owner and read+execute to everyone else.
• 644 is the standard for files that should not be executed.
• 600 is for secrets — never use anything more permissive for sensitive AI infrastructure files.
• Always apply the principle of least privilege: grant only the permissions necessary for the task.

📝 Final Note¶

As you manage AI infrastructure, you will see these three octal combinations constantly. Memorize them:

• 755 = owner can do everything, others can read and execute
• 644 = owner can read and write, others can only read
• 600 = only the owner can read or write

This simple knowledge will help you keep your AI models, datasets, and infrastructure secure from the start.