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 ‘/’ or a ‘~’ (see abbreviate-file-name), 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.
This function returns t if file filename is an absolute
(file-name-absolute-p "~rms/foo") ⇒ t
(file-name-absolute-p "rms/foo") ⇒ nil
(file-name-absolute-p "/user/rms/foo") ⇒ t
Given a possibly relative file name, you can convert it to an
absolute name using
expand-file-name (see File Name Expansion). This function converts absolute file names to relative
file-relative-namefilename &optional directory
This function tries to return a relative name that is equivalent to
filename, assuming the result will be interpreted relative to
directory (an absolute directory name or directory file name).
If directory is omitted or
nil, it defaults to the
current buffer’s default directory.
On some operating systems, an absolute file name begins with a device
name. On such systems, filename has no relative equivalent based
on directory if they start with two different device names. In
file-relative-name returns filename in absolute
(file-relative-name "/foo/bar" "/foo/") ⇒ "bar" (file-relative-name "/foo/bar" "/hack/") ⇒ "../foo/bar"