Chmod Calculator

Unix file permissions control who can read, write, and execute files. Every file has three permission groups: owner (the user who created it), group (users in the file's group), and others (everyone else). Each group can have read (r), write (w), and execute (x) permissions.

Permissions are represented in two ways: symbolic notation like rwxr-xr-x (9 characters, 3 per group) and numeric/octal notation like 755. In octal, each digit is the sum of: read=4, write=2, execute=1. So 7 (4+2+1) means full access, 5 (4+1) means read+execute, and 0 means no access.

Common permission sets: 755 for executable files and directories (owner full, others read+execute), 644 for regular files (owner read+write, others read-only), 600 for private files (owner only), and 777 for full access (avoid in production — security risk). The chmod command changes permissions: chmod 755 filename.