2 @c This is part of the GNU Emacs Lisp Reference Manual.
3 @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1999, 2001, 2002, 2003,
4 @c 2004, 2005, 2006, 2007, 2008 Free Software Foundation, Inc.
5 @c See the file elisp.texi for copying conditions.
6 @setfilename ../../info/backups
7 @node Backups and Auto-Saving, Buffers, Files, Top
8 @chapter Backups and Auto-Saving
9 @cindex backups and auto-saving
11 Backup files and auto-save files are two methods by which Emacs tries
12 to protect the user from the consequences of crashes or of the user's
13 own errors. Auto-saving preserves the text from earlier in the current
14 editing session; backup files preserve file contents prior to the
18 * Backup Files:: How backup files are made; how their names are chosen.
19 * Auto-Saving:: How auto-save files are made; how their names are chosen.
20 * Reverting:: @code{revert-buffer}, and how to customize what it does.
27 A @dfn{backup file} is a copy of the old contents of a file you are
28 editing. Emacs makes a backup file the first time you save a buffer
29 into its visited file. Thus, normally, the backup file contains the
30 contents of the file as it was before the current editing session.
31 The contents of the backup file normally remain unchanged once it
34 Backups are usually made by renaming the visited file to a new name.
35 Optionally, you can specify that backup files should be made by copying
36 the visited file. This choice makes a difference for files with
37 multiple names; it also can affect whether the edited file remains owned
38 by the original owner or becomes owned by the user editing it.
40 By default, Emacs makes a single backup file for each file edited.
41 You can alternatively request numbered backups; then each new backup
42 file gets a new name. You can delete old numbered backups when you
43 don't want them any more, or Emacs can delete them automatically.
46 * Making Backups:: How Emacs makes backup files, and when.
47 * Rename or Copy:: Two alternatives: renaming the old file or copying it.
48 * Numbered Backups:: Keeping multiple backups for each source file.
49 * Backup Names:: How backup file names are computed; customization.
53 @subsection Making Backup Files
56 This function makes a backup of the file visited by the current
57 buffer, if appropriate. It is called by @code{save-buffer} before
58 saving the buffer the first time.
60 If a backup was made by renaming, the return value is a cons cell of
61 the form (@var{modes} . @var{backupname}), where @var{modes} are the
62 mode bits of the original file, as returned by @code{file-modes}
63 (@pxref{File Attributes,, Other Information about Files}), and
64 @var{backupname} is the name of the backup. In all other cases, that
65 is, if a backup was made by copying or if no backup was made, this
66 function returns @code{nil}.
69 @defvar buffer-backed-up
70 This buffer-local variable says whether this buffer's file has
71 been backed up on account of this buffer. If it is non-@code{nil},
72 the backup file has been written. Otherwise, the file should be backed
73 up when it is next saved (if backups are enabled). This is a
74 permanent local; @code{kill-all-local-variables} does not alter@tie{}it.
77 @defopt make-backup-files
78 This variable determines whether or not to make backup files. If it
79 is non-@code{nil}, then Emacs creates a backup of each file when it is
80 saved for the first time---provided that @code{backup-inhibited}
81 is @code{nil} (see below).
83 The following example shows how to change the @code{make-backup-files}
84 variable only in the Rmail buffers and not elsewhere. Setting it
85 @code{nil} stops Emacs from making backups of these files, which may
86 save disk space. (You would put this code in your init file.)
90 (add-hook 'rmail-mode-hook
94 (setq make-backup-files nil))))
99 @defvar backup-enable-predicate
100 This variable's value is a function to be called on certain occasions to
101 decide whether a file should have backup files. The function receives
102 one argument, an absolute file name to consider. If the function returns
103 @code{nil}, backups are disabled for that file. Otherwise, the other
104 variables in this section say whether and how to make backups.
106 @findex normal-backup-enable-predicate
107 The default value is @code{normal-backup-enable-predicate}, which checks
108 for files in @code{temporary-file-directory} and
109 @code{small-temporary-file-directory}.
112 @defvar backup-inhibited
113 If this variable is non-@code{nil}, backups are inhibited. It records
114 the result of testing @code{backup-enable-predicate} on the visited file
115 name. It can also coherently be used by other mechanisms that inhibit
116 backups based on which file is visited. For example, VC sets this
117 variable non-@code{nil} to prevent making backups for files managed
118 with a version control system.
120 This is a permanent local, so that changing the major mode does not lose
121 its value. Major modes should not set this variable---they should set
122 @code{make-backup-files} instead.
125 @defvar backup-directory-alist
126 This variable's value is an alist of filename patterns and backup
127 directory names. Each element looks like
129 (@var{regexp} . @var{directory})
133 Backups of files with names matching @var{regexp} will be made in
134 @var{directory}. @var{directory} may be relative or absolute. If it is
135 absolute, so that all matching files are backed up into the same
136 directory, the file names in this directory will be the full name of the
137 file backed up with all directory separators changed to @samp{!} to
138 prevent clashes. This will not work correctly if your filesystem
139 truncates the resulting name.
141 For the common case of all backups going into one directory, the alist
142 should contain a single element pairing @samp{"."} with the appropriate
145 If this variable is @code{nil}, or it fails to match a filename, the
146 backup is made in the original file's directory.
148 On MS-DOS filesystems without long names this variable is always
152 @defvar make-backup-file-name-function
153 This variable's value is a function to use for making backups instead
154 of the default @code{make-backup-file-name}. A value of @code{nil}
155 gives the default @code{make-backup-file-name} behavior.
156 @xref{Backup Names,, Naming Backup Files}.
158 This could be buffer-local to do something special for specific
159 files. If you define it, you may need to change
160 @code{backup-file-name-p} and @code{file-name-sans-versions} too.
165 @subsection Backup by Renaming or by Copying?
166 @cindex backup files, rename or copy
168 There are two ways that Emacs can make a backup file:
172 Emacs can rename the original file so that it becomes a backup file, and
173 then write the buffer being saved into a new file. After this
174 procedure, any other names (i.e., hard links) of the original file now
175 refer to the backup file. The new file is owned by the user doing the
176 editing, and its group is the default for new files written by the user
180 Emacs can copy the original file into a backup file, and then overwrite
181 the original file with new contents. After this procedure, any other
182 names (i.e., hard links) of the original file continue to refer to the
183 current (updated) version of the file. The file's owner and group will
187 The first method, renaming, is the default.
189 The variable @code{backup-by-copying}, if non-@code{nil}, says to use
190 the second method, which is to copy the original file and overwrite it
191 with the new buffer contents. The variable @code{file-precious-flag},
192 if non-@code{nil}, also has this effect (as a sideline of its main
193 significance). @xref{Saving Buffers}.
195 @defopt backup-by-copying
196 If this variable is non-@code{nil}, Emacs always makes backup files by
200 The following three variables, when non-@code{nil}, cause the second
201 method to be used in certain special cases. They have no effect on the
202 treatment of files that don't fall into the special cases.
204 @defopt backup-by-copying-when-linked
205 If this variable is non-@code{nil}, Emacs makes backups by copying for
206 files with multiple names (hard links).
208 This variable is significant only if @code{backup-by-copying} is
209 @code{nil}, since copying is always used when that variable is
213 @defopt backup-by-copying-when-mismatch
214 If this variable is non-@code{nil}, Emacs makes backups by copying in cases
215 where renaming would change either the owner or the group of the file.
217 The value has no effect when renaming would not alter the owner or
218 group of the file; that is, for files which are owned by the user and
219 whose group matches the default for a new file created there by the
222 This variable is significant only if @code{backup-by-copying} is
223 @code{nil}, since copying is always used when that variable is
227 @defopt backup-by-copying-when-privileged-mismatch
228 This variable, if non-@code{nil}, specifies the same behavior as
229 @code{backup-by-copying-when-mismatch}, but only for certain user-id
230 values: namely, those less than or equal to a certain number. You set
231 this variable to that number.
233 Thus, if you set @code{backup-by-copying-when-privileged-mismatch}
234 to 0, backup by copying is done for the superuser only,
235 when necessary to prevent a change in the owner of the file.
240 @node Numbered Backups
241 @subsection Making and Deleting Numbered Backup Files
243 If a file's name is @file{foo}, the names of its numbered backup
244 versions are @file{foo.~@var{v}~}, for various integers @var{v}, like
245 this: @file{foo.~1~}, @file{foo.~2~}, @file{foo.~3~}, @dots{},
246 @file{foo.~259~}, and so on.
248 @defopt version-control
249 This variable controls whether to make a single non-numbered backup
250 file or multiple numbered backups.
254 Make numbered backups if the visited file already has numbered backups;
255 otherwise, do not. This is the default.
258 Do not make numbered backups.
260 @item @var{anything else}
261 Make numbered backups.
265 The use of numbered backups ultimately leads to a large number of
266 backup versions, which must then be deleted. Emacs can do this
267 automatically or it can ask the user whether to delete them.
269 @defopt kept-new-versions
270 The value of this variable is the number of newest versions to keep
271 when a new numbered backup is made. The newly made backup is included
272 in the count. The default value is@tie{}2.
275 @defopt kept-old-versions
276 The value of this variable is the number of oldest versions to keep
277 when a new numbered backup is made. The default value is@tie{}2.
280 If there are backups numbered 1, 2, 3, 5, and 7, and both of these
281 variables have the value 2, then the backups numbered 1 and 2 are kept
282 as old versions and those numbered 5 and 7 are kept as new versions;
283 backup version 3 is excess. The function @code{find-backup-file-name}
284 (@pxref{Backup Names}) is responsible for determining which backup
285 versions to delete, but does not delete them itself.
287 @defopt delete-old-versions
288 If this variable is @code{t}, then saving a file deletes excess
289 backup versions silently. If it is @code{nil}, that means
290 to ask for confirmation before deleting excess backups.
291 Otherwise, they are not deleted at all.
294 @defopt dired-kept-versions
295 This variable specifies how many of the newest backup versions to keep
296 in the Dired command @kbd{.} (@code{dired-clean-directory}). That's the
297 same thing @code{kept-new-versions} specifies when you make a new backup
298 file. The default is@tie{}2.
302 @subsection Naming Backup Files
304 The functions in this section are documented mainly because you can
305 customize the naming conventions for backup files by redefining them.
306 If you change one, you probably need to change the rest.
308 @defun backup-file-name-p filename
309 This function returns a non-@code{nil} value if @var{filename} is a
310 possible name for a backup file. It just checks the name, not whether
311 a file with the name @var{filename} exists.
315 (backup-file-name-p "foo")
319 (backup-file-name-p "foo~")
324 The standard definition of this function is as follows:
328 (defun backup-file-name-p (file)
329 "Return non-nil if FILE is a backup file \
330 name (numeric or not)..."
331 (string-match "~\\'" file))
336 Thus, the function returns a non-@code{nil} value if the file name ends
337 with a @samp{~}. (We use a backslash to split the documentation
338 string's first line into two lines in the text, but produce just one
339 line in the string itself.)
341 This simple expression is placed in a separate function to make it easy
342 to redefine for customization.
345 @defun make-backup-file-name filename
346 This function returns a string that is the name to use for a
347 non-numbered backup file for file @var{filename}. On Unix, this is just
348 @var{filename} with a tilde appended.
350 The standard definition of this function, on most operating systems, is
355 (defun make-backup-file-name (file)
356 "Create the non-numeric backup file name for FILE..."
361 You can change the backup-file naming convention by redefining this
362 function. The following example redefines @code{make-backup-file-name}
363 to prepend a @samp{.} in addition to appending a tilde:
367 (defun make-backup-file-name (filename)
369 (concat "." (file-name-nondirectory filename) "~")
370 (file-name-directory filename)))
374 (make-backup-file-name "backups.texi")
375 @result{} ".backups.texi~"
379 Some parts of Emacs, including some Dired commands, assume that backup
380 file names end with @samp{~}. If you do not follow that convention, it
381 will not cause serious problems, but these commands may give
382 less-than-desirable results.
385 @defun find-backup-file-name filename
386 This function computes the file name for a new backup file for
387 @var{filename}. It may also propose certain existing backup files for
388 deletion. @code{find-backup-file-name} returns a list whose @sc{car} is
389 the name for the new backup file and whose @sc{cdr} is a list of backup
390 files whose deletion is proposed. The value can also be @code{nil},
391 which means not to make a backup.
393 Two variables, @code{kept-old-versions} and @code{kept-new-versions},
394 determine which backup versions should be kept. This function keeps
395 those versions by excluding them from the @sc{cdr} of the value.
396 @xref{Numbered Backups}.
398 In this example, the value says that @file{~rms/foo.~5~} is the name
399 to use for the new backup file, and @file{~rms/foo.~3~} is an ``excess''
400 version that the caller should consider deleting now.
404 (find-backup-file-name "~rms/foo")
405 @result{} ("~rms/foo.~5~" "~rms/foo.~3~")
411 @defun file-newest-backup filename
412 This function returns the name of the most recent backup file for
413 @var{filename}, or @code{nil} if that file has no backup files.
415 Some file comparison commands use this function so that they can
416 automatically compare a file with its most recent backup.
421 @c @cindex auto-saving Lots of symbols starting with auto-save here.
423 Emacs periodically saves all files that you are visiting; this is
424 called @dfn{auto-saving}. Auto-saving prevents you from losing more
425 than a limited amount of work if the system crashes. By default,
426 auto-saves happen every 300 keystrokes, or after around 30 seconds of
427 idle time. @xref{Auto Save, Auto Save, Auto-Saving: Protection Against
428 Disasters, emacs, The GNU Emacs Manual}, for information on auto-save
429 for users. Here we describe the functions used to implement auto-saving
430 and the variables that control them.
432 @defvar buffer-auto-save-file-name
433 This buffer-local variable is the name of the file used for
434 auto-saving the current buffer. It is @code{nil} if the buffer
435 should not be auto-saved.
439 buffer-auto-save-file-name
440 @result{} "/xcssun/users/rms/lewis/#backups.texi#"
445 @deffn Command auto-save-mode arg
446 When used interactively without an argument, this command is a toggle
447 switch: it turns on auto-saving of the current buffer if it is off, and
448 vice versa. With an argument @var{arg}, the command turns auto-saving
449 on if the value of @var{arg} is @code{t}, a nonempty list, or a positive
450 integer. Otherwise, it turns auto-saving off.
453 @defun auto-save-file-name-p filename
454 This function returns a non-@code{nil} value if @var{filename} is a
455 string that could be the name of an auto-save file. It assumes
456 the usual naming convention for auto-save files: a name that
457 begins and ends with hash marks (@samp{#}) is a possible auto-save file
458 name. The argument @var{filename} should not contain a directory part.
462 (make-auto-save-file-name)
463 @result{} "/xcssun/users/rms/lewis/#backups.texi#"
466 (auto-save-file-name-p "#backups.texi#")
470 (auto-save-file-name-p "backups.texi")
475 The standard definition of this function is as follows:
479 (defun auto-save-file-name-p (filename)
480 "Return non-nil if FILENAME can be yielded by..."
481 (string-match "^#.*#$" filename))
485 This function exists so that you can customize it if you wish to
486 change the naming convention for auto-save files. If you redefine it,
487 be sure to redefine the function @code{make-auto-save-file-name}
491 @defun make-auto-save-file-name
492 This function returns the file name to use for auto-saving the current
493 buffer. This is just the file name with hash marks (@samp{#}) prepended
494 and appended to it. This function does not look at the variable
495 @code{auto-save-visited-file-name} (described below); callers of this
496 function should check that variable first.
500 (make-auto-save-file-name)
501 @result{} "/xcssun/users/rms/lewis/#backups.texi#"
505 Here is a simplified version of the standard definition of this
510 (defun make-auto-save-file-name ()
511 "Return file name to use for auto-saves \
517 (file-name-directory buffer-file-name)
519 (file-name-nondirectory buffer-file-name)
522 (concat "#%" (buffer-name) "#"))))
526 This exists as a separate function so that you can redefine it to
527 customize the naming convention for auto-save files. Be sure to
528 change @code{auto-save-file-name-p} in a corresponding way.
531 @defopt auto-save-visited-file-name
532 If this variable is non-@code{nil}, Emacs auto-saves buffers in
533 the files they are visiting. That is, the auto-save is done in the same
534 file that you are editing. Normally, this variable is @code{nil}, so
535 auto-save files have distinct names that are created by
536 @code{make-auto-save-file-name}.
538 When you change the value of this variable, the new value does not take
539 effect in an existing buffer until the next time auto-save mode is
540 reenabled in it. If auto-save mode is already enabled, auto-saves
541 continue to go in the same file name until @code{auto-save-mode} is
545 @defun recent-auto-save-p
546 This function returns @code{t} if the current buffer has been
547 auto-saved since the last time it was read in or saved.
550 @defun set-buffer-auto-saved
551 This function marks the current buffer as auto-saved. The buffer will
552 not be auto-saved again until the buffer text is changed again. The
553 function returns @code{nil}.
556 @defopt auto-save-interval
557 The value of this variable specifies how often to do auto-saving, in
558 terms of number of input events. Each time this many additional input
559 events are read, Emacs does auto-saving for all buffers in which that is
560 enabled. Setting this to zero disables autosaving based on the
561 number of characters typed.
564 @defopt auto-save-timeout
565 The value of this variable is the number of seconds of idle time that
566 should cause auto-saving. Each time the user pauses for this long,
567 Emacs does auto-saving for all buffers in which that is enabled. (If
568 the current buffer is large, the specified timeout is multiplied by a
569 factor that increases as the size increases; for a million-byte
570 buffer, the factor is almost 4.)
572 If the value is zero or @code{nil}, then auto-saving is not done as a
573 result of idleness, only after a certain number of input events as
574 specified by @code{auto-save-interval}.
577 @defvar auto-save-hook
578 This normal hook is run whenever an auto-save is about to happen.
581 @defopt auto-save-default
582 If this variable is non-@code{nil}, buffers that are visiting files
583 have auto-saving enabled by default. Otherwise, they do not.
586 @deffn Command do-auto-save &optional no-message current-only
587 This function auto-saves all buffers that need to be auto-saved. It
588 saves all buffers for which auto-saving is enabled and that have been
589 changed since the previous auto-save.
591 If any buffers are auto-saved, @code{do-auto-save} normally displays a
592 message saying @samp{Auto-saving...} in the echo area while
593 auto-saving is going on. However, if @var{no-message} is
594 non-@code{nil}, the message is inhibited.
596 If @var{current-only} is non-@code{nil}, only the current buffer
600 @defun delete-auto-save-file-if-necessary &optional force
601 This function deletes the current buffer's auto-save file if
602 @code{delete-auto-save-files} is non-@code{nil}. It is called every
603 time a buffer is saved.
605 Unless @var{force} is non-@code{nil}, this function only deletes the
606 file if it was written by the current Emacs session since the last
610 @defopt delete-auto-save-files
611 This variable is used by the function
612 @code{delete-auto-save-file-if-necessary}. If it is non-@code{nil},
613 Emacs deletes auto-save files when a true save is done (in the visited
614 file). This saves disk space and unclutters your directory.
617 @defun rename-auto-save-file
618 This function adjusts the current buffer's auto-save file name if the
619 visited file name has changed. It also renames an existing auto-save
620 file, if it was made in the current Emacs session. If the visited
621 file name has not changed, this function does nothing.
624 @defvar buffer-saved-size
625 The value of this buffer-local variable is the length of the current
626 buffer, when it was last read in, saved, or auto-saved. This is
627 used to detect a substantial decrease in size, and turn off auto-saving
630 If it is @minus{}1, that means auto-saving is temporarily shut off in
631 this buffer due to a substantial decrease in size. Explicitly saving
632 the buffer stores a positive value in this variable, thus reenabling
633 auto-saving. Turning auto-save mode off or on also updates this
634 variable, so that the substantial decrease in size is forgotten.
637 @defvar auto-save-list-file-name
638 This variable (if non-@code{nil}) specifies a file for recording the
639 names of all the auto-save files. Each time Emacs does auto-saving, it
640 writes two lines into this file for each buffer that has auto-saving
641 enabled. The first line gives the name of the visited file (it's empty
642 if the buffer has none), and the second gives the name of the auto-save
645 When Emacs exits normally, it deletes this file; if Emacs crashes, you
646 can look in the file to find all the auto-save files that might contain
647 work that was otherwise lost. The @code{recover-session} command uses
648 this file to find them.
650 The default name for this file specifies your home directory and starts
651 with @samp{.saves-}. It also contains the Emacs process @acronym{ID} and the
655 @defvar auto-save-list-file-prefix
656 After Emacs reads your init file, it initializes
657 @code{auto-save-list-file-name} (if you have not already set it
658 non-@code{nil}) based on this prefix, adding the host name and process
659 ID. If you set this to @code{nil} in your init file, then Emacs does
660 not initialize @code{auto-save-list-file-name}.
666 If you have made extensive changes to a file and then change your mind
667 about them, you can get rid of them by reading in the previous version
668 of the file with the @code{revert-buffer} command. @xref{Reverting, ,
669 Reverting a Buffer, emacs, The GNU Emacs Manual}.
671 @deffn Command revert-buffer &optional ignore-auto noconfirm preserve-modes
672 This command replaces the buffer text with the text of the visited
673 file on disk. This action undoes all changes since the file was visited
676 By default, if the latest auto-save file is more recent than the visited
677 file, and the argument @var{ignore-auto} is @code{nil},
678 @code{revert-buffer} asks the user whether to use that auto-save
679 instead. When you invoke this command interactively, @var{ignore-auto}
680 is @code{t} if there is no numeric prefix argument; thus, the
681 interactive default is not to check the auto-save file.
683 Normally, @code{revert-buffer} asks for confirmation before it changes
684 the buffer; but if the argument @var{noconfirm} is non-@code{nil},
685 @code{revert-buffer} does not ask for confirmation.
687 Normally, this command reinitializes the buffer's major and minor modes
688 using @code{normal-mode}. But if @var{preserve-modes} is
689 non-@code{nil}, the modes remain unchanged.
691 Reverting tries to preserve marker positions in the buffer by using the
692 replacement feature of @code{insert-file-contents}. If the buffer
693 contents and the file contents are identical before the revert
694 operation, reverting preserves all the markers. If they are not
695 identical, reverting does change the buffer; in that case, it preserves
696 the markers in the unchanged text (if any) at the beginning and end of
697 the buffer. Preserving any additional markers would be problematical.
700 You can customize how @code{revert-buffer} does its work by setting
701 the variables described in the rest of this section.
703 @defopt revert-without-query
704 This variable holds a list of files that should be reverted without
705 query. The value is a list of regular expressions. If the visited file
706 name matches one of these regular expressions, and the file has changed
707 on disk but the buffer is not modified, then @code{revert-buffer}
708 reverts the file without asking the user for confirmation.
711 Some major modes customize @code{revert-buffer} by making
712 buffer-local bindings for these variables:
714 @defvar revert-buffer-function
715 @anchor{Definition of revert-buffer-function}
716 The value of this variable is the function to use to revert this
717 buffer. If non-@code{nil}, it should be a function with two optional
718 arguments to do the work of reverting. The two optional arguments,
719 @var{ignore-auto} and @var{noconfirm}, are the arguments that
720 @code{revert-buffer} received. If the value is @code{nil}, reverting
723 Modes such as Dired mode, in which the text being edited does not
724 consist of a file's contents but can be regenerated in some other
725 fashion, can give this variable a buffer-local value that is a function to
726 regenerate the contents.
729 @defvar revert-buffer-insert-file-contents-function
730 The value of this variable, if non-@code{nil}, specifies the function to use to
731 insert the updated contents when reverting this buffer. The function
732 receives two arguments: first the file name to use; second, @code{t} if
733 the user has asked to read the auto-save file.
735 The reason for a mode to set this variable instead of
736 @code{revert-buffer-function} is to avoid duplicating or replacing the
737 rest of what @code{revert-buffer} does: asking for confirmation,
738 clearing the undo list, deciding the proper major mode, and running the
742 @defvar before-revert-hook
743 This normal hook is run by @code{revert-buffer} before
744 inserting the modified contents---but only if
745 @code{revert-buffer-function} is @code{nil}.
748 @defvar after-revert-hook
749 This normal hook is run by @code{revert-buffer} after inserting
750 the modified contents---but only if @code{revert-buffer-function} is
755 arch-tag: 295a6321-e5ab-46d5-aef5-0bb4f447a67f