Linux chmod calculator

Understanding Linux File Permissions: A Technical Guide

The “chmod” (Change Mode) command serves as the basis of Linux’s Security System. Although our calculator found in the second image does the math automatically for you, it is important to understand the math behind how to perform any Linux System Administration action.

The Math Behind the chmod Calculator

File permission types are calculated based upon an octal (base 8) numeration system. Each type of permission has its own assigned integer value:

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

To get the numeric value of a user class, simply add these values together; i.e.. Read (4) + Execute (1) = 5.

Who are the Three Users?

Every Linux file has 3 levels of ownership:

1. User (Owner): The person who has created the file.
2. Group: The group of users who have been granted rights to access the file.
3. Others (Public): All other users on the system.

Common Permission Sets You Should Know

Using our tool, you can generate these common configurations that you may often see when hosting a website or creating Ansible Server Configuration playbooks:

• 755 (drwxr-xr-x): The standard directory permission type, where the owner can do everything, and where the other users (group and public) can read and browse the directory.
• 644 (-rw-r–r–): The standard permission for most file types (i.e., HTML, CSS); the owner has full rights (read and write) while the other users (group and public) only have read rights.
• 400 (-r——–): The highest level of security.The only person who is permitted to open this type of document is its owner (a good example of this are SSH private key files).

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Security Warning: Why you should avoid chmod 777

Novice users of Linux often find themselves using ‘chmod 777’ as a method of fixing “permission denied” errors. In doing this, you are granting EVERYONE on that server, unrestricted access to read, write and execute any of your files! This is a significant security risk for production servers. Instead of relying on default file permissions of 777, we recommend that you use our chmod permission calculator to determine what the minimum permission level is for your file.

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Frequently Asked Questions (FAQ)

Q: What’s the difference between symbolic and numeric file permissions?

A: Numeric permissions (such as 755) are much quicker to type for power users, while symbolic permissions (u+rwx,g+rx,o+rx) tend to be more human-readable for beginner users.

Q: How do I set permissions for all files within a directory?

A: You can set permissions for all files in a directory and its sub-directories by using the recursive option with chmod, like this: chmod -R 755 followed by the path to your directory. It’s important to pay attention when using this command because it will apply the same permission setting to all files and directories within that directory.

Q: Why can’t I set permissions on a file that I do not own?

A: In a Linux environment, only the owner of a file or a user who has been assigned “sudo” (root) level access, can change the permission settings of a file.