On UNIX/Linux-based systems, chmod is used to control access rights for files and folders. Using the chmod command, permissions can be quickly and easily granted or revoked for individual folders or files.
This permissions system is based on three levels: owner, group and others.
| Category | Definition / Property |
| Owner | Read, write and execute permissions (for the user who owns the file). |
| Group | Read, write and execute permissions for members of the group to which the file belongs. (Users can be grouped within Linux to enable shared access.) |
| Others | Read, write and execute permissions for all users who have access to the system (e.g. web servers, etc.). |
The chmod permission system uses a weighting of permissions.
| Owner | Group | Others | |
| Read | 4 | 4 | 4 |
| Post | 2 | 2 | 2 |
| Execute | 1 | 1 | 1 |
The chmod value is calculated by adding individual values (e.g. chmod 644). For example, if a PHP script is being executed that requires access rights to other users’ scripts in order to modify the file (e.g. editing an .htaccess file via the WordPress backend), the file must have at least read and write permissions at the group level.
Here are a few examples of the most common chmod permissions for Linux operating systems for folder and/or file permissions:
chmod 644:
Owner = read, write
; Group = read
; Others = read
chmod 755
Owner = read, write, execute
Group = read, execute
Others = read, execute
chmod 777
:
Owner = read, write, execute
; Group = read, write, execute
; Others = read, write, execute