Directory Names

A directory name is the name of a directory. A directory is a kind of file, and 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 Unix, this is simple: a directory name ends in a slash, whereas the directory's name as a file lacks that slash. On 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.

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. In Unix, 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 Unix, this means removing the final slash 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"

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 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 dirname

This function applies abbreviations from directory-abbrev-alist to its argument, and substitutes `~' for the user's home directory.

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