chmod Calculator — Linux File Permission Converter

公式

How this is calculated

Linux and Unix file permissions are governed by three groups — owner (user), group and others — each holding three independent permission bits: read (r, value 4), write (w, value 2) and execute (x, value 1). Adding the values of the active bits produces a single octal digit 0–7 for each group. The three digits are concatenated to form the familiar chmod code, such as 755 or 644.

The nine-character symbolic notation is decoded from those three digits. Each group of three characters uses the same rule: if bit 2 is set the first character is "r", otherwise "-"; bit 1 gives "w" or "-"; bit 0 gives "x" or "-". Concatenating owner, group and others (prefixed with a file-type character, "-" for regular files) gives the full notation such as -rwxr-xr-x for 755.

Common presets: 755 (rwxr-xr-x) suits executables and directories — the owner can read, write and execute; everyone else can only read and execute. 644 (rw-r--r--) is the default for regular files — owner reads and writes, others read only. 600 (rw-------) is used for sensitive files like SSH private keys. Avoid 777 on production servers because it grants write access to every user on the system.