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25.6.2 Distinguishing Kinds of Files

This section describes how to distinguish various kinds of files, such as directories, symbolic links, and ordinary files.

Symbolic links are ordinarily followed wherever they appear. For example, to interpret the file name a/b/c, any of a, a/b, and a/b/c can be symbolic links that are followed, possibly recursively if the link targets are themselves symbolic links. However, a few functions do not follow symbolic links at the end of a file name (a/b/c in this example). Such a function is said to not follow symbolic links.

— Function: file-symlink-p filename

If the file filename is a symbolic link, this function does not follow it and instead returns its link target as a string. (The link target string is not necessarily the full absolute file name of the target; determining the full file name that the link points to is nontrivial, see below.)

If the file filename is not a symbolic link, or does not exist, file-symlink-p returns nil.

Here are a few examples of using this function: 

          (file-symlink-p "not-a-symlink")
               ⇒ nil
          (file-symlink-p "sym-link")
               ⇒ "not-a-symlink"
          (file-symlink-p "sym-link2")
               ⇒ "sym-link"
          (file-symlink-p "/bin")
               ⇒ "/pub/bin"

Note that in the third example, the function returned sym-link, but did not proceed to resolve it, although that file is itself a symbolic link. That is because this function does not follow symbolic links—the process of following the symbolic links does not apply to the last component of the file name.

The string that this function returns is what is recorded in the symbolic link; it may or may not include any leading directories. This function does not expand the link target to produce a fully-qualified file name, and in particular does not use the leading directories, if any, of the filename argument if the link target is not an absolute file name. Here's an example: 

          (file-symlink-p "/foo/bar/baz")
               ⇒ "some-file"

Here, although /foo/bar/baz was given as a fully-qualified file name, the result is not, and in fact does not have any leading directories at all. And since some-file might itself be a symbolic link, you cannot simply prepend leading directories to it, nor even naively use expand-file-name (see File Name Expansion) to produce its absolute file name.

For this reason, this function is seldom useful if you need to determine more than just the fact that a file is or isn't a symbolic link. If you actually need the file name of the link target, use file-chase-links or file-truename, described in Truenames.

— Function: file-directory-p filename

This function returns t if filename is the name of an existing directory, nil otherwise. This function follows symbolic links. 

          (file-directory-p "~rms")
               ⇒ t
          (file-directory-p "~rms/lewis/files.texi")
               ⇒ nil
          (file-directory-p "~rms/lewis/no-such-file")
               ⇒ nil
          (file-directory-p "$HOME")
               ⇒ nil
          (file-directory-p
           (substitute-in-file-name "$HOME"))
               ⇒ t
— Function: file-regular-p filename

This function returns t if the file filename exists and is a regular file (not a directory, named pipe, terminal, or other I/O device). This function follows symbolic links.