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26.1.4 Naming Backup Files

The functions in this section are documented mainly because you can customize the naming conventions for backup files by redefining them. If you change one, you probably need to change the rest.

— Function: backup-file-name-p filename

This function returns a non-nil value if filename is a possible name for a backup file. It just checks the name, not whether a file with the name filename exists.

          (backup-file-name-p "foo")
               ⇒ nil
          (backup-file-name-p "foo~")
               ⇒ 3

The standard definition of this function is as follows:

          (defun backup-file-name-p (file)
            "Return non-nil if FILE is a backup file \
          name (numeric or not)..."
            (string-match "~\\'" file))

Thus, the function returns a non-nil value if the file name ends with a ‘~’. (We use a backslash to split the documentation string's first line into two lines in the text, but produce just one line in the string itself.)

This simple expression is placed in a separate function to make it easy to redefine for customization.

— Function: make-backup-file-name filename

This function returns a string that is the name to use for a non-numbered backup file for file filename. On Unix, this is just filename with a tilde appended.

The standard definition of this function, on most operating systems, is as follows:

          (defun make-backup-file-name (file)
            "Create the non-numeric backup file name for FILE..."
            (concat file "~"))

You can change the backup-file naming convention by redefining this function. The following example redefines make-backup-file-name to prepend a ‘.’ in addition to appending a tilde:

          (defun make-backup-file-name (filename)
            (expand-file-name
              (concat "." (file-name-nondirectory filename) "~")
              (file-name-directory filename)))
          
          (make-backup-file-name "backups.texi")
               ⇒ ".backups.texi~"

Some parts of Emacs, including some Dired commands, assume that backup file names end with ‘~’. If you do not follow that convention, it will not cause serious problems, but these commands may give less-than-desirable results.

— Function: find-backup-file-name filename

This function computes the file name for a new backup file for filename. It may also propose certain existing backup files for deletion. find-backup-file-name returns a list whose car is the name for the new backup file and whose cdr is a list of backup files whose deletion is proposed. The value can also be nil, which means not to make a backup.

Two variables, kept-old-versions and kept-new-versions, determine which backup versions should be kept. This function keeps those versions by excluding them from the cdr of the value. See Numbered Backups.

In this example, the value says that ~rms/foo.~5~ is the name to use for the new backup file, and ~rms/foo.~3~ is an “excess” version that the caller should consider deleting now.

          (find-backup-file-name "~rms/foo")
               ⇒ ("~rms/foo.~5~" "~rms/foo.~3~")
— Function: file-newest-backup filename

This function returns the name of the most recent backup file for filename, or nil if that file has no backup files.

Some file comparison commands use this function so that they can automatically compare a file with its most recent backup.