What Are CHMOD 644 and 755? File Permissions Guide
On Linux servers, file permissions determine who can read, modify or execute a file. This guide explains how to interpret 644 and 755 safely.
What is CHMOD?
CHMOD is the command and permission model used to change file and directory permissions on Unix and Linux systems. Permissions are set separately for the owner, the associated group and other users.
The numeric field in an FTP or SFTP client is a compact representation of read, write and execute permissions. A change may be rejected when the server does not allow it or your account does not own the file.
How are CHMOD numbers calculated?
Each permission has a numeric value, and required permissions are added together.
The three digits represent the owner, group and other users. A value of 6 is 4+2, or read and write; 7 is 4+2+1 and includes all three permissions.
| Value | Permission | Meaning |
|---|---|---|
| 4 | Read (r) | Read a file or list directory contents |
| 2 | Write (w) | Modify a file or create/delete entries in a directory |
| 1 | Execute (x) | Run a file or enter a directory |
| 0 | No permission | The relevant action is not allowed |
What does CHMOD 644 mean?
644 is common for regular text, CSS, JavaScript and similar files on web servers. The owner can edit the file while the group and other users can only read it.
It is not automatically correct for every system. An application, web-server user or group setup may require a different permission model.
| User | Value | Permission |
|---|---|---|
| Owner | 6 | Read + write |
| Group | 4 | Read only |
| Others | 4 | Read only |
What does CHMOD 755 mean?
755 is common for directories. It lets the owner create and change content while allowing other users to enter the directory and read its contents.
For a directory, execute permission mainly means the ability to traverse or enter it. This is why 755 is frequently seen on directories below a web root.
| User | Value | Permission |
|---|---|---|
| Owner | 7 | Read + write + execute |
| Group | 5 | Read + execute |
| Others | 5 | Read + execute |
Why is 777 risky?
777 gives the owner, group and every other user permission to read, write and execute. Applying it as a quick fix can allow another user or compromised process on the server to modify the file.
When an application needs write access, identify the correct owner, group and specific directory first. Fix ownership and service-user configuration instead of hiding the problem with broad permissions.
Avoid 777 on web roots, configuration files and executables. Follow the ownership and permission model recommended by your hosting provider.
Checklist before changing permissions
- 1Record the current permission
Save the numeric value or selected boxes so you can restore them.
- 2Distinguish files from directories
Directories may require execute permission for traversal, while regular files often do not.
- 3Apply the narrowest permission
Grant only the required ability to the required user or group.
- 4Test the application
Check the web page, upload operation or relevant service. Restore the earlier value if it breaks.
- 5Be careful with recursive changes
Applying one value to every child can mix up the different needs of files and directories.
Put the guide into practice.
Review the related Varkuna product or open its detailed user guide.