25.8.3 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, 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's name as a file lacks that slash. On MS-DOS and VMS, the relationship is more complicated.

The difference between a directory name and its name as a file is subtle but crucial. When an Emacs variable or function argument is described as being a directory name, a file name of a directory 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 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. On most systems, this means appending a slash to the string (if it does not already end in one). On VMS, the function converts a string of the form ‘[X]Y.DIR.1’ to the form ‘[X.Y]’.

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

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. On most systems, this means removing the final slash (or backslash) from the string. On VMS, the function converts a string of the form ‘[X.Y]’ to ‘[X]Y.DIR.1’.

(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.

Directory name abbreviations are useful for directories that are normally accessed through symbolic links. Sometimes the users recognize primarily the link's name as “the name” of the directory, and find it annoying to see the directory's “real” name. If you define the link name as an abbreviation for the “real” name, Emacs shows users the abbreviation instead.

Variable: directory-abbrev-alist

The variable directory-abbrev-alist contains an alist of abbreviations to use for file directories. Each element has the form (from . to), and says to replace from with to when it appears in a directory name. The from string is actually a regular expression; it should always start with ‘^’. The to string should be an ordinary absolute directory name. Do not use ‘~’ to stand for a home directory in that string. The function abbreviate-file-name performs these substitutions.

You can set this variable in ‘site-init.el’ to describe the abbreviations appropriate for your site.

Here's an example, from a system on which file system ‘/home/fsf’ and so on are normally accessed through symbolic links named ‘/fsf’ and so on.

(("^/home/fsf" . "/fsf") ("^/home/gp" . "/gp") ("^/home/gd" . "/gd"))

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

Function: abbreviate-file-name filename

This function applies abbreviations from directory-abbrev-alist to its argument, and substitutes ‘~’ for the user's home directory. You can use it for directory names and for file names, because it recognizes abbreviations even as part of the name.

Ce document a été généré par Eric Reinbold le 13 Octobre 2007 en utilisant texi2html 1.78.