Relative File Names - GNU Emacs Lisp Reference Manual

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25.8.3 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 and GNU/Linux, an absolute file name starts with a slash or a tilde (‘~’), and a relative one does not. On MS-DOS and MS-Windows, an absolute file name starts with a slash or a backslash, or with a drive specification ‘x:/’, where x is the drive letter. 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
     