Backups and Auto-Saving
***********************

Backup files and auto-save files are two methods by which Emacs tries to protect the user from the consequences of crashes or of the user's own errors.  Auto-saving preserves the text from earlier in the current editing session; backup files preserve file contents prior to the current session.

Backup Files
============

A backup file is a copy of the old contents of a file you are editing.  Emacs makes a backup file the first time you save a buffer into its visited file.  Normally, this means that the backup file contains the contents of the file as it was before the current editing session.  The contents of the backup file normally remain unchanged once it exists.

Backups are usually made by renaming the visited file to a new name. Optionally, you can specify that backup files should be made by copying the visited file.  This choice makes a difference for files with multiple names; it also can affect whether the edited file remains owned by the original owner or becomes owned by the user editing it.

By default, Emacs makes a single backup file for each file edited. You can alternatively request numbered backups; then each new backup file gets a new name.  You can delete old numbered backups when you don't want them any more, or Emacs can delete them automatically.

Making Backup Files
-------------------

 -- 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.

 -- Variable: buffer-backed-up
     This buffer-local variable indicates whether this buffer's file has been backed up on account of this buffer.  If it is non-nil, then the backup file has been written.  Otherwise, the file should be backed up when it is next saved (if backup files are enabled).

 -- Variable: make-backup-files
     This variable determines whether or not backup files will be created. If it is non-nil, then Emacs will create a backup of each file when it is saved for the first time.

     The following example shows how to change the make-backup-files variable only in the RMAIL buffer and not elsewhere.  Setting it nil stops Emacs from making backups of the RMAIL file, which may save disk space.  (You would put this code in your .emacs file.)

          (setq rmail-mode-hook
                (function (lambda ()
                            (make-local-variable 'make-backup-files)
                            (setq make-backup-files nil)))

Backup by Renaming or by Copying?
---------------------------------

There are two ways that Emacs can make a backup file:

  * Emacs can rename the original file so that it becomes a backup file, and then write the buffer being saved into a new file.  In this case, any other names (i.e., hard links) of the original file will now refer to the backup file.  The new file will be owned by the user doing the editing, and its group will be the default for the user or the directory.

  * Emacs can copy the original file into a backup file, and then overwrite the original file with new contents.  In this case, any other names (i.e., hard links) of the original file will still refer to the current version of the file.  The file's owner and group will be unchanged.

The first method, renaming, is the default.

The variable backup-by-copying, if non-nil, says to use the second method, which is to copy the original file and overwrite it with the new buffer contents.  The variable file-precious-flag, if non-nil, also has this effect (as a sideline of its main significance).  See Saving Buffers.

The variables backup-by-copying-when-linked and backup-by-copying-when-mismatch, if non-nil, cause the second method to be used in certain special cases.  They have no effect on the treatment of files that don't fall into the special cases.

 -- Variable: backup-by-copying
     This variable controls whether to make backup files by copying.  If it is non-nil, then Emacs always copies the current contents of the file into the backup file before writing the buffer to be saved to the file.  (In many circumstances, this has the same effect as file-precious-flag.)

 -- Variable: backup-by-copying-when-linked
     This variable controls whether to make backups by copying for files with multiple names (hard links).  If it is non-nil, then Emacs will use copying to create backups for those files.

     This variable is significant only if backup-by-copying is nil, since copying is always used when that variable is non-nil.

 -- Variable: backup-by-copying-when-mismatch
     This variable controls whether to make backups by copying when renaming would cause either the owner or the group of the file to change.  If it is non-nil then Emacs will create backups by copying in such cases.

     The value has no effect when renaming would not result in changing the owner or group of the file; that is, for files which are owned by the user and whose group matches the default for a new file created there by the user.

     This variable is significant only if backup-by-copying is nil, since copying is always used when that variable is non-nil.

Making and Deleting Numbered Backup Files
-----------------------------------------

If a file's name is foo, the names of its numbered backup versions are foo.~V~, for various integers V, like this: foo.~1~, foo.~2~, foo.~3~, ..., foo.~259~, and so on.

 -- User Option: version-control
     This variable controls whether to make a single non-numbered backup file or multiple numbered backups.

     nil
          Make numbered backups if the visited file already has numbered backups; otherwise, do not.

     never
          Do not make numbered backups.

     ANYTHING ELSE
          Do make numbered backups.

The use of numbered backups ultimately leads to a large number of backup versions, which must then be deleted.  Emacs can do this automatically.

 -- User Option: kept-new-versions
     The value of this variable is the number of oldest versions to keep when a new numbered backup is made.  The newly made backup is included in the count.  The default value is 2.

 -- User Option: kept-old-versions
     The value of this variable is the number of oldest versions to keep when a new numbered backup is made.  The default value is 2.

 -- User Option: dired-kept-versions
     This variable plays a role in Dired's dired-clean-directory (.) command like that played by kept-old-versions when a backup file is made.  The default value is 2.

If there are backups numbered 1, 2, 3, 5, and 7, and both of these variables have the value 2, then the backups numbered 1 and 2 will be kept as old versions and those numbered 5 and 7 will be kept as new versions; backup version 3 will be deleted.  The function find-backup-file-name is responsible for determining which backup versions to delete, but does not delete them itself.

 -- User Option: trim-versions-without-asking
     If this variable is non-nil, then excess backup versions are deleted silently.  Otherwise, the user is asked whether to delete them.

Naming Backup Files
-------------------

The functions in this section are documented mainly because you can customize the naming conventions for backup files by redefining them.

 -- Function: backup-file-name-p filename
     This function returns a non-nil value if FILENAME is a possible name for a backup file.  A file with the name FILENAME need not exist; the function just checks the name.

          (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 ~

     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 which 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 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.  In the following example, make-backup-file-name is redefined to prepend a . as well as to append a tilde.

          (defun make-backup-file-name (filename)
            (concat "." filename "~"))

          (make-backup-file-name "backups.texi")
               => ".backups.texi~"

     If you do redefine make-backup-file-name, be sure to redefine backup-file-name-p and find-backup-file-name as well.

 -- 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.

     Two variables called kept-old-versions and kept-new-versions determine which old backup versions will be kept (by excluding them from the list of backup files ripe for deletion).  See Numbered Backups.

     In this example, ~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~")

Auto-Saving
===========

Emacs periodically saves all files that you are visiting; this is called auto-saving.  Auto-saving prevents you from losing more than a limited amount of work if the system crashes.  By default, auto-saves happen every 300 keystrokes.  See Auto-Saving: Protection Against Disasters (The GNU Emacs Manual), for information on auto-save for users.  Here we describe the functions used to implement auto-saving and the variables that control them.

 -- Variable: buffer-auto-save-file-name
     This buffer-local variable is the name of the file used for auto-saving the current buffer.  It is nil if the buffer should not be auto-saved.

          buffer-auto-save-file-name
          => "/xcssun/users/rms/lewis/#files.texi#"

 -- Command: auto-save-mode arg
     When used interactively without an argument, this command is a toggle switch: it turns on auto-saving of the current buffer if it is off, and vice-versa.  With an argument ARG, the command turns auto-saving on if the value of ARG is t, a nonempty list, or a positive integer.  Otherwise, it turns auto-saving off.

 -- Function: auto-save-file-name-p filename
     This function returns a non-nil value if FILENAME is a string that could be the name of an auto-save file.  It works based on knowledge of the naming convention for auto-save files: a name that begins and ends with hash marks (#) is a possible auto-save file name.  The argument FILENAME should not contain a directory part.

          (make-auto-save-file-name)
               => "/xcssun/users/rms/lewis/#files.texi#"
          (auto-save-file-name-p "#files.texi#")
               => 0
          (auto-save-file-name-p "files.texi")
               => nil

     The standard definition of this function is as follows:

          (defun auto-save-file-name-p (filename)
            "Return non-nil if FILENAME can be yielded by..."
            (string-match "^#.*#$" filename))

     This function exists so that you can customize it if you wish to change the naming convention for auto-save files.  If you redefine it, be sure to redefine make-auto-save-file-name correspondingly.

 -- Function: make-auto-save-file-name
     This function returns the file name to use for auto-saving the current buffer.  This is just the file name with hash marks (#) appended and prepended to it.  This function does not look at the variable auto-save-visited-file-name; that should be checked before this function is called.

          (make-auto-save-file-name)
               => "/xcssun/users/rms/lewis/#backup.texi#"

     The standard definition of this function is as follows:

          (defun make-auto-save-file-name ()
            "Return file name to use for auto-saves of current buffer..."
            (if buffer-file-name
                (concat (file-name-directory buffer-file-name)
                        "#"
                        (file-name-nondirectory buffer-file-name)
                        "#")
              (expand-file-name (concat "#%" (buffer-name) "#"))))

     This exists as a separate function so that you can redefine it to customize the naming convention for auto-save files.  Be sure to change auto-save-file-name-p in a corresponding way.

 -- Variable: auto-save-visited-file-name
     If this variable is non-nil, Emacs will auto-save buffers in the files they are visiting.  That is, the auto-save is done in the same file which you are editing.  Normally, this variable is nil, so auto-save files have distinct names that are created by make-auto-save-file-name.

     When you change the value of this variable, the value does not take effect until the next time auto-save mode is reenabled in any given buffer.  If auto-save mode is already enabled, auto-saves continue to go in the same file name until auto-save-mode is called again.

 -- Function: recent-auto-save-p
     This function returns t if the current buffer has been auto-saved since the last time it was read in or saved.

 -- Function: set-buffer-auto-saved
     This function marks the current buffer as auto-saved.  The buffer will not be auto-saved again until the buffer text is changed again.  The function returns nil.

 -- User Option: auto-save-interval
     The value of this variable is the number of characters that Emacs reads from the keyboard between auto-saves.  Each time this many more characters are read, auto-saving is done for all buffers in which it is enabled.

 -- User Option: auto-save-default
     If this variable is non-nil, buffers that are visiting files have auto-saving enabled by default.  Otherwise, they do not.

 -- Command: do-auto-save &optional no-message
     This function auto-saves all buffers that need to be auto-saved. This is all buffers for which auto-saving is enabled and that have been changed since the last time they were auto-saved.

     Normally, if any buffers are auto-saved, a message Auto-saving... is displayed in the echo area while auto-saving is going on.  However, if NO-MESSAGE is non-nil, the message is inhibited.

 -- Function: delete-auto-save-file-if-necessary
     This function deletes the auto-save file for the current buffer if variable delete-auto-save-files is non-nil.  It is called every time a buffer is saved.

 -- Variable: delete-auto-save-files
     This variable is used by the function delete-auto-save-file-if-necessary.  If it is non-nil, Emacs will delete auto-save files when a true save is done (in the visited file).  This saves on disk space and unclutters your directory.

 -- Function: rename-auto-save-file
     This function adjusts the current buffer's auto-save file name if the visited file name has changed.  It also renames an existing auto-save file.  If the visited file name has not changed, this function does nothing.

Reverting
=========

If you have made extensive changes to a file and then change your mind about them, you can get rid of them by reading in the previous version of the file with the revert-buffer command.  See Reverting a Buffer (The GNU Emacs Manual).

 -- Command: revert-buffer &optional no-auto-save-offer-p noconfirm
     This command replaces the buffer text with the text of the visited file on disk.  This action undoes all changes since the file was visited or saved.

     When the value of the NO-AUTO-SAVE-OFFER-P argument is nil, and the latest auto-save file is more recent than the visited file, revert-buffer asks the user whether to use that instead.  Otherwise, it always uses the latest backup file.  This argument is the numeric prefix argument when the function is called interactively.

     When the value of the NOCONFIRM argument is non-nil, revert-buffer does not ask for confirmation for the reversion action.  This means that the buffer contents are deleted and replaced by the text from the file on the disk, with no further opportunities for the user to prevent it.

     Since reverting works by deleting the entire text of the buffer and inserting the file contents, all the buffer's markers are relocated to point at the beginning of the buffer.  This is not ``correct'', but then, there is no way to determine what would be correct.  It is not possible to determine, from the text before and after, which characters after reversion correspond to which characters before.

     If the value of the revert-buffer-function variable is non-nil, it is called as a function with no arguments to do the work.

 -- Variable: revert-buffer-function
     The value of this variable is the function to use to revert this buffer; but if the value of this variable is nil, then the revert-buffer function carries out its default action.  Modes such as Dired mode, in which the text being edited does not consist of a file's contents but can be regenerated in some other fashion, give this variable a buffer-local value that is a function to regenerate the contents.

 -- Command: recover-file filename
     This function visits FILENAME, but gets the contents from its last auto-save file.  This is useful after the system has crashed, to resume editing the same file without losing all the work done in the previous session.

     An error is signaled if there is no auto-save file for FILENAME, or if FILENAME is newer than its auto-save file.  If FILENAME does not exist, but its auto-save file does, then the auto-save file is read as usual.  This last situation may occur if you visited a nonexistent file and never actually saved it.