Chmod Calculator

chmod (change mode) is a Unix/Linux command that changes file and directory permissions. It controls who can read, write, or execute a file. Permissions are set for three classes: the file owner, the group, and all other users.

Each permission type has a numeric value: read (r) = 4, write (w) = 2, execute (x) = 1. You sum these values for each class (owner, group, others) to get a three-digit octal number. For example, 755 means owner has rwx (7), group has r-x (5), and others have r-x (5).

644 (rw-r--r--) allows the owner to read and write, while group and others can only read. 755 (rwxr-xr-x) additionally grants execute permission to all classes and write permission to the owner. 644 is typical for regular files, while 755 is typical for directories and executable scripts.

Symbolic notation shows permissions as a 9-character string split into three groups of three: owner, group, others. Each group shows r (read), w (write), x (execute), or - (not set). So rwxr-xr-x means the owner can read/write/execute, and group/others can read and execute but not write.

Common values: 755 for executable files and directories, 644 for regular files, 600 for private files (owner-only read/write), 777 for full access to everyone (use with caution), and 400 for read-only by owner. Web servers often need 755 for directories and 644 for files.