A directory name is a string that must name a directory if it names any file at all. A directory is actually a kind of file, and it has a file name (called the directory file name), which is related to the directory name but is typically not identical. (This is not quite the same as the usual POSIX terminology.) These two names for the same entity are related by a syntactic transformation. On GNU and other POSIX-like systems, this is simple: to obtain a directory name, append a ‘/’ to a directory file name that does not already end in ‘/’. On MS-DOS the relationship is more complicated.
The difference between a directory name and a directory file name is
subtle but crucial. When an Emacs variable or function argument is
described as being a directory name, a directory file name is not
file-name-directory returns a string, that is
always a directory name.
The following two functions convert between directory names and directory file names. They do nothing special with environment variable substitutions such as ‘$HOME’, and the constructs ‘~’, ‘.’ and ‘..’.
This function returns a string representing filename in a form that the operating system will interpret as the name of a directory (a directory name). On most systems, this means appending a slash to the string (if it does not already end in one).
(file-name-as-directory "~rms/lewis") ⇒ "~rms/lewis/"
This function returns non-
nil if filename ends with a
directory separator character. This is the forward slash ‘/’ on
GNU and other POSIX-like systems; MS-Windows and MS-DOS recognize both
the forward slash and the backslash ‘\’ as directory separators.
This function returns a string representing dirname in a form that the operating system will interpret as the name of a file (a directory file name). On most systems, this means removing the final directory separators from the string, unless the string consists entirely of directory separators.
(directory-file-name "~lewis/") ⇒ "~lewis"
Concatenate components to directory, inserting a slash before the components if directory or the preceding component didn’t end with a slash.
(file-name-concat "/tmp" "foo") ⇒ "/tmp/foo"
A directory or components that are
nil or the empty
string are ignored—they are filtered out first and do not affect the
results in any way.
This is almost the same as using
concat, but dirname (and
the non-final components) may or may not end with slash characters,
and this function will not double those characters.
To convert a directory name to its abbreviation, use this function:
This function returns an abbreviated form of filename. It
applies the abbreviations specified in
(see File Aliases in The GNU Emacs Manual),
then substitutes ‘~’ for the user’s home directory if the
argument names a file in the home directory or one of its
subdirectories. If the home directory is a root directory, it is not
replaced with ‘~’, because this does not make the result shorter
on many systems.
You can use this function for directory names and for file names, because it recognizes abbreviations even as part of the name.