read-file-modes

read-file-modes is a compiled Lisp function in `files.el'.

(read-file-modes &optional PROMPT ORIG-FILE)

Read file modes in octal or symbolic notation and return its numeric value.
PROMPT is used as the prompt, default to `File modes (octal or symbolic): '.
ORIG-FILE is the name of a file on whose mode bits to base returned
permissions if what user types requests to add, remove, or set permissions
based on existing mode bits, as in "og+rX-w".