GNU Emacs Lisp Reference Manual

22.8.3: Absolute and Relative File Names

All the directories in the file system form a tree starting at the root directory. A file name can specify all the directory names starting from the root of the tree; then it is called an absolute file name. Or it can specify the position of the file in the tree relative to a default directory; then it is called a relative file name. On Unix, an absolute file name starts with a slash or a tilde (~), and a relative one does not. The rules on VMS are complicated.

Function: file-name-absolute-p filename

This function returns t if file filename is an absolute file name, nil otherwise. On VMS, this function understands both Unix syntax and VMS syntax.

(file-name-absolute-p "~rms/foo")
     => t
(file-name-absolute-p "rms/foo")
     => nil
(file-name-absolute-p "/user/rms/foo")
     => t