Info Catalog elisp: Backup Files elisp: Rename or Copy

elisp: Making Backups

 
 25.1.1 Making Backup Files
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  -- Function: backup-buffer
      This function makes a backup of the file visited by the current
      buffer, if appropriate.  It is called by ‘save-buffer’ before
      saving the buffer the first time.
 
      If a backup was made by renaming, the return value is a cons cell
      of the form (MODES EXTRA-ALIST BACKUPNAME), where MODES are the
      mode bits of the original file, as returned by ‘file-modes’ (
      Testing Accessibility), EXTRA-ALIST is an alist describing the
      original file’s extended attributes, as returned by
      ‘file-extended-attributes’ (Extended Attributes), and
      BACKUPNAME is the name of the backup.
 
      In all other cases (i.e., if a backup was made by copying or if no
      backup was made), this function returns ‘nil’.
 
  -- Variable: buffer-backed-up
      This buffer-local variable says whether this buffer’s file has been
      backed up on account of this buffer.  If it is non-‘nil’, the
      backup file has been written.  Otherwise, the file should be backed
      up when it is next saved (if backups are enabled).  This is a
      permanent local; ‘kill-all-local-variables’ does not alter it.
 
  -- User Option: make-backup-files
      This variable determines whether or not to make backup files.  If
      it is non-‘nil’, then Emacs creates a backup of each file when it
      is saved for the first time—provided that ‘backup-inhibited’ is
      ‘nil’ (see below).
 
      The following example shows how to change the ‘make-backup-files’
      variable only in the Rmail buffers and not elsewhere.  Setting it
      ‘nil’ stops Emacs from making backups of these files, which may
      save disk space.  (You would put this code in your init file.)
 
           (add-hook 'rmail-mode-hook
                     (lambda () (setq-local make-backup-files nil)))
 
  -- Variable: backup-enable-predicate
      This variable’s value is a function to be called on certain
      occasions to decide whether a file should have backup files.  The
      function receives one argument, an absolute file name to consider.
      If the function returns ‘nil’, backups are disabled for that file.
      Otherwise, the other variables in this section say whether and how
      to make backups.
 
      The default value is ‘normal-backup-enable-predicate’, which checks
      for files in ‘temporary-file-directory’ and
      ‘small-temporary-file-directory’.
 
  -- Variable: backup-inhibited
      If this variable is non-‘nil’, backups are inhibited.  It records
      the result of testing ‘backup-enable-predicate’ on the visited file
      name.  It can also coherently be used by other mechanisms that
      inhibit backups based on which file is visited.  For example, VC
      sets this variable non-‘nil’ to prevent making backups for files
      managed with a version control system.
 
      This is a permanent local, so that changing the major mode does not
      lose its value.  Major modes should not set this variable—they
      should set ‘make-backup-files’ instead.
 
  -- User Option: backup-directory-alist
      This variable’s value is an alist of filename patterns and backup
      directory names.  Each element looks like
           (REGEXP . DIRECTORY)
 
      Backups of files with names matching REGEXP will be made in
      DIRECTORY.  DIRECTORY may be relative or absolute.  If it is
      absolute, so that all matching files are backed up into the same
      directory, the file names in this directory will be the full name
      of the file backed up with all directory separators changed to ‘!’
      to prevent clashes.  This will not work correctly if your
      filesystem truncates the resulting name.
 
      For the common case of all backups going into one directory, the
      alist should contain a single element pairing ‘"."’ with the
      appropriate directory name.
 
      If this variable is ‘nil’ (the default), or it fails to match a
      filename, the backup is made in the original file’s directory.
 
      On MS-DOS filesystems without long names this variable is always
      ignored.
 
  -- User Option: make-backup-file-name-function
      This variable’s value is a function to use for making backup file
      names.  The function ‘make-backup-file-name’ calls it.  
      Naming Backup Files Backup Names.
 
      This could be buffer-local to do something special for specific
      files.  If you change it, you may need to change
      ‘backup-file-name-p’ and ‘file-name-sans-versions’ too.
 

Info Catalog elisp: Backup Files elisp: Rename or Copy