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#### 24.8.3 Directory Names

A directory name is the name of a directory. A directory is actually a kind of file, so it has a file name (called the directory file name, which is related to the directory name but not identical to it. (This is not quite the same as the usual Unix terminology.) These two different names for the same entity are related by a syntactic transformation. On GNU and Unix systems, this is simple: a directory name ends in a slash, whereas the directory file name lacks that slash. On MS-DOS the relationship is more complicated.

The difference between directory name and 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 acceptable. When `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 ‘`..`’.

Function: file-name-as-directory filename

  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/"

Function: directory-name-p filename

  This function returns non-`nil` if filename ends with a directory separator character. This is the forward slash ‘`/`’ on Unix and GNU systems; MS-Windows and MS-DOS recognize both the forward slash and the backslash ‘`\`’ as directory separators.

Function: directory-file-name dirname

  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 slash (or backslash) from the string.

      (directory-file-name "~lewis/")
           ⇒ "~lewis"

Given a directory name, you can combine it with a relative file name using `concat`:

    (concat dirname relfile)

Be sure to verify that the file name is relative before doing that. If you use an absolute file name, the results could be syntactically invalid or refer to the wrong file.

If you want to use a directory file name in making such a combination, you must first convert it to a directory name using `file-name-as-directory`:

    (concat (file-name-as-directory dirfile) relfile)

Don’t try concatenating a slash by hand, as in

    ;;; Wrong!
    (concat dirfile "/" relfile)

because this is not portable. Always use `file-name-as-directory`.

To avoid the issues mentioned above, or if the dirname value might be nil (for example, from an element of `load-path`), use:

    (expand-file-name relfile dirname)

To convert a directory name to its abbreviation, use this function:

Function: abbreviate-file-name filename

  This function returns an abbreviated form of filename. It applies the abbreviations specified in `directory-abbrev-alist` (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.

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