Chmod Calculator

Linux and Unix file permissions control who can read, write, and execute files and directories. Permissions are assigned to three categories: the file owner, the group, and all other users. Understanding chmod is essential for system administration, web server configuration, and security hardening.

Numeric (Octal) Notation

The numeric notation uses three digits, one for each category (owner, group, other). Each digit is the sum of its permission values: 4 for read, 2 for write, and 1 for execute. For example, 755 means owner has full access (7 = 4+2+1), while group and others can read and execute (5 = 4+1).

Symbolic Notation

The symbolic notation uses nine characters in three groups of three: rwxr-xr-x. Each group represents owner, group, and other permissions. The letters r, w, and x indicate read, write, and execute. A dash - indicates the permission is not set.

Common Permission Values

755 is the standard for executable files and directories. 644 is typical for regular files. 600 is used for private files like SSH keys. 777 grants full access to everyone and should generally be avoided for security reasons.

Directory Permissions

For directories, the execute permission has a special meaning: it allows users to access (traverse) the directory. Without execute permission on a directory, users cannot cd into it or access any files within, even if they have read permission on those files.