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 GNU and other POSIX-like systems, after any leading ‘~’ has been expanded, an absolute file name starts with 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
nil otherwise. A file name is considered to be
absolute if its first component is ‘~’, or is ‘~user’
where user is a valid login name. In the following examples,
assume that there is a user named ‘rms’ but no user named
(file-name-absolute-p "~rms/foo") ⇒ t
(file-name-absolute-p "~nosuchuser/foo") ⇒ nil
(file-name-absolute-p "rms/foo") ⇒ nil
(file-name-absolute-p "/user/rms/foo") ⇒ t
Given a possibly relative file name, you can expand any
leading ‘~’ and convert the result to an
absolute name using
expand-file-name (see File Name Expansion). This function converts absolute file names to relative
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"