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25.6.3 Truenames

The truename of a file is the name that you get by following symbolic links at all levels until none remain, then simplifying away ‘.’ and ‘..’ appearing as name components. This results in a sort of canonical name for the file. A file does not always have a unique truename; the number of distinct truenames a file has is equal to the number of hard links to the file. However, truenames are useful because they eliminate symbolic links as a cause of name variation.

function file-truename filename​

This function returns the truename of the file filename. If the argument is not an absolute file name, this function first expands it against default-directory.

This function does not expand environment variables. Only substitute-in-file-name does that. See Definition of substitute-in-file-name.

If you may need to follow symbolic links preceding ‘..’ appearing as a name component, call file-truename without prior direct or indirect calls to expand-file-name. Otherwise, the file name component immediately preceding ‘..’ will be simplified away before file-truename is called. To eliminate the need for a call to expand-file-name, file-truename handles ‘~’ in the same way that expand-file-name does.

If the target of a symbolic links has remote file name syntax, file-truename returns it quoted. See Functions that Expand Filenames.

This function follows symbolic links, starting with filename, until it finds a file name which is not the name of a symbolic link. Then it returns that file name. This function does not follow symbolic links at the level of parent directories.

If you specify a number for limit, then after chasing through that many links, the function just returns what it has even if that is still a symbolic link.

To illustrate the difference between file-chase-links and file-truename, suppose that /usr/foo is a symbolic link to the directory /home/foo, and /home/foo/hello is an ordinary file (or at least, not a symbolic link) or nonexistent. Then we would have:

(file-chase-links "/usr/foo/hello")
     ;; This does not follow the links in the parent directories.
     ⇒ "/usr/foo/hello"
(file-truename "/usr/foo/hello")
     ;; Assuming that /home is not a symbolic link.
     ⇒ "/home/foo/hello"

function file-equal-p file1 file2​

This function returns t if the files file1 and file2 name the same file. This is similar to comparing their truenames, except that remote file names are also handled in an appropriate manner. If file1 or file2 does not exist, the return value is unspecified.

function file-name-case-insensitive-p filename​

Sometimes file names or their parts need to be compared as strings, in which case it’s important to know whether the underlying filesystem is case-insensitive. This function returns t if file filename is on a case-insensitive filesystem. It always returns t on MS-DOS and MS-Windows. On Cygwin and macOS, filesystems may or may not be case-insensitive, and the function tries to determine case-sensitivity by a runtime test. If the test is inconclusive, the function returns t on Cygwin and nil on macOS.

Currently this function always returns nil on platforms other than MS-DOS, MS-Windows, Cygwin, and macOS. It does not detect case-insensitivity of mounted filesystems, such as Samba shares or NFS-mounted Windows volumes. On remote hosts, it assumes t for the ‘smb’ method. For all other connection methods, runtime tests are performed.

function file-in-directory-p file dir​

This function returns t if file is a file in directory dir, or in a subdirectory of dir. It also returns t if file and dir are the same directory. It compares the truenames of the two directories. If dir does not name an existing directory, the return value is nil.

function vc-responsible-backend file​

This function determines the responsible VC backend of the given file. For example, if emacs.c is a file tracked by Git, (vc-responsible-backend "emacs.c") returns ‘Git’. Note that if file is a symbolic link, vc-responsible-backend will not resolve it—the backend of the symbolic link file itself is reported. To get the backend VC of the file to which file refers, wrap file with a symbolic link resolving function such as file-chase-links:

(vc-responsible-backend (file-chase-links "emacs.c"))