[emacs] How do I control how Emacs makes backup files?

Emacs puts backup files named foo~ everywhere and I don't like having to remember to delete them. Also, if I edit a file that has a hard link somewhere else in the file system, the hard link points to the backup when I'm done editing, and that's confusing and awful. How can I either eliminate these backup files, or have them go somewhere other than the same directory?

The accepted answer is good, but it can be greatly improved by additionally backing up on every save and backing up versioned files.

First, basic settings as described in the accepted answer:

(setq version-control t     ;; Use version numbers for backups.
      kept-new-versions 10  ;; Number of newest versions to keep.
      kept-old-versions 0   ;; Number of oldest versions to keep.
      delete-old-versions t ;; Don't ask to delete excess backup versions.
      backup-by-copying t)  ;; Copy all files, don't rename them.

Next, also backup versioned files, which Emacs does not do by default (you don't commit on every save, right?):

(setq vc-make-backup-files t)

Finally, make a backup on each save, not just the first. We make two kinds of backups:

1. per-session backups: once on the first save of the buffer in each Emacs session. These simulate Emac's default backup behavior.

2. per-save backups: once on every save. Emacs does not do this by default, but it's very useful if you leave Emacs running for a long time.

The backups go in different places and Emacs creates the backup dirs automatically if they don't exist:

;; Default and per-save backups go here:
(setq backup-directory-alist '(("" . "~/.emacs.d/backup/per-save")))

(defun force-backup-of-buffer ()
  ;; Make a special "per session" backup at the first save of each
  ;; emacs session.
  (when (not buffer-backed-up)
    ;; Override the default parameters for per-session backups.
    (let ((backup-directory-alist '(("" . "~/.emacs.d/backup/per-session")))
          (kept-new-versions 3))
      (backup-buffer)))
  ;; Make a "per save" backup on each save.  The first save results in
  ;; both a per-session and a per-save backup, to keep the numbering
  ;; of per-save backups consistent.
  (let ((buffer-backed-up nil))
    (backup-buffer)))

(add-hook 'before-save-hook  'force-backup-of-buffer)

I became very interested in this topic after I wrote $< instead of $@ in my Makefile, about three hours after my previous commit :P

The above is based on an Emacs Wiki page I heavily edited.

You can disable them altogether by

(setq make-backup-files nil)

Emacs backup/auto-save files can be very helpful. But these features are confusing.

Backup files

Backup files have tildes (~ or ~9~) at the end and shall be written to the user home directory. When make-backup-files is non-nil Emacs automatically creates a backup of the original file the first time the file is saved from a buffer. If you're editing a new file Emacs will create a backup the second time you save the file.

No matter how many times you save the file the backup remains unchanged. If you kill the buffer and then visit the file again, or the next time you start a new Emacs session, a new backup file will be made. The new backup reflects the file's content after reopened, or at the start of editing sessions. But an existing backup is never touched again. Therefore I find it useful to created numbered backups (see the configuration below).

To create backups explicitly use save-buffer (C-x C-s) with prefix arguments.

diff-backup and dired-diff-backup compares a file with its backup or vice versa. But there is no function to restore backup files. For example, under Windows, to restore a backup file

C:\Users\USERNAME\.emacs.d\backups\!drive_c!Users!USERNAME!.emacs.el.~7~

it has to be manually copied as

C:\Users\USERNAME\.emacs.el

Auto-save files

Auto-save files use hashmarks (#) and shall be written locally within the project directory (along with the actual files). The reason is that auto-save files are just temporary files that Emacs creates until a file is saved again (like with hurrying obedience).

• Before the user presses C-x C-s (save-buffer) to save a file Emacs auto-saves files - based on counting keystrokes (auto-save-interval) or when you stop typing (auto-save-timeout).
• Emacs also auto-saves whenever it crashes, including killing the Emacs job with a shell command.

When the user saves the file, the auto-saved version is deleted. But when the user exits the file without saving it, Emacs or the X session crashes, the auto-saved files still exist.

Use revert-buffer or recover-file to restore auto-save files. Note that Emacs records interrupted sessions for later recovery in files named ~/.emacs.d/auto-save-list. The recover-session function will use this information.

The preferred method to recover from an auto-saved filed is M-x revert-buffer RET. Emacs will ask either "Buffer has been auto-saved recently. Revert from auto-save file?" or "Revert buffer from file FILENAME?". In case of the latter there is no auto-save file. For example, because you have saved before typing another auto-save-intervall keystrokes, in which case Emacs had deleted the auto-save file.

Auto-save is nowadays disabled by default because it can slow down editing when connected to a slow machine, and because many files contain sensitive data.

Configuration

Here is a configuration that IMHO works best:

(defvar --backup-directory (concat user-emacs-directory "backups"))
(if (not (file-exists-p --backup-directory))
        (make-directory --backup-directory t))
(setq backup-directory-alist `(("." . ,--backup-directory)))
(setq make-backup-files t               ; backup of a file the first time it is saved.
      backup-by-copying t               ; don't clobber symlinks
      version-control t                 ; version numbers for backup files
      delete-old-versions t             ; delete excess backup files silently
      delete-by-moving-to-trash t
      kept-old-versions 6               ; oldest versions to keep when a new numbered backup is made (default: 2)
      kept-new-versions 9               ; newest versions to keep when a new numbered backup is made (default: 2)
      auto-save-default t               ; auto-save every buffer that visits a file
      auto-save-timeout 20              ; number of seconds idle time before auto-save (default: 30)
      auto-save-interval 200            ; number of keystrokes between auto-saves (default: 300)
      )

Sensitive data

Another problem is that you don't want to have Emacs spread copies of files with sensitive data. Use this mode on a per-file basis. As this is a minor mode, for my purposes I renamed it sensitive-minor-mode.

To enable it for all .vcf and .gpg files, in your .emacs use something like:

(setq auto-mode-alist
      (append
       (list
        '("\\.\\(vcf\\|gpg\\)$" . sensitive-minor-mode)
        )
       auto-mode-alist))

Alternatively, to protect only some files, like some .txt files, use a line like

// -*-mode:asciidoc; mode:sensitive-minor; fill-column:132-*-

in the file.

Another way of configuring backup options is via the Customize interface. Enter:

M-x customize-group

And then at the Customize group: prompt enter backup.

If you scroll to the bottom of the buffer you'll see Backup Directory Alist. Click Show Value and set the first entry of the list as follows:

Regexp matching filename: .*
Backup directory name: /path/to/your/backup/dir

Alternatively, you can turn backups off my setting Make Backup Files to off.

If you don't want Emacs to automatically edit your .emacs file you'll want to set up a customisations file.

You can disable them altogether by

(setq make-backup-files nil)