chmod Calculator

What is chmod?

chmod (change mode) is a Unix command that sets file and directory permissions. Every file on a Unix-like system has three permission groups -- owner, group, and others -- and each group can be granted read, write, and execute access independently.

Numeric notation

Permissions are represented as a three-digit octal number. Each digit is the sum of its component permissions: read (4), write (2), and execute (1). For example, 755 means the owner has full permissions (7 = 4+2+1), while group and others can read and execute but not write (5 = 4+1).

Symbolic notation

Symbolic notation uses the letters r, w, and x for read, write, and execute. A hyphen (-) indicates the permission is not set. The nine-character string is read in groups of three: owner, group, others. So rwxr-xr-x means the owner can do everything, and everyone else can read and execute.

Common permission patterns

644 is standard for files: the owner can read and write, everyone else can only read. 755 is standard for directories and executables: the owner has full access, others can read and traverse. 600 restricts access to the owner alone, commonly used for SSH keys and sensitive configuration files. 400 is read-only for the owner, useful for protecting files from accidental modification.

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