2 @c This is part of the GNU Emacs Lisp Reference Manual.
3 @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995 Free Software Foundation, Inc.
4 @c See the file elisp.texi for copying conditions.
5 @setfilename ../info/backups
6 @node Backups and Auto-Saving, Buffers, Files, Top
7 @chapter Backups and Auto-Saving
9 Backup files and auto-save files are two methods by which Emacs tries
10 to protect the user from the consequences of crashes or of the user's
11 own errors. Auto-saving preserves the text from earlier in the current
12 editing session; backup files preserve file contents prior to the
16 * Backup Files:: How backup files are made; how their names are chosen.
17 * Auto-Saving:: How auto-save files are made; how their names are chosen.
18 * Reverting:: @code{revert-buffer}, and how to customize what it does.
25 A @dfn{backup file} is a copy of the old contents of a file you are
26 editing. Emacs makes a backup file the first time you save a buffer
27 into its visited file. Normally, this means that the backup file
28 contains the contents of the file as it was before the current editing
29 session. The contents of the backup file normally remain unchanged once
32 Backups are usually made by renaming the visited file to a new name.
33 Optionally, you can specify that backup files should be made by copying
34 the visited file. This choice makes a difference for files with
35 multiple names; it also can affect whether the edited file remains owned
36 by the original owner or becomes owned by the user editing it.
38 By default, Emacs makes a single backup file for each file edited.
39 You can alternatively request numbered backups; then each new backup
40 file gets a new name. You can delete old numbered backups when you
41 don't want them any more, or Emacs can delete them automatically.
44 * Making Backups:: How Emacs makes backup files, and when.
45 * Rename or Copy:: Two alternatives: renaming the old file or copying it.
46 * Numbered Backups:: Keeping multiple backups for each source file.
47 * Backup Names:: How backup file names are computed; customization.
51 @subsection Making Backup Files
54 This function makes a backup of the file visited by the current
55 buffer, if appropriate. It is called by @code{save-buffer} before
56 saving the buffer the first time.
59 @defvar buffer-backed-up
60 This buffer-local variable indicates whether this buffer's file has
61 been backed up on account of this buffer. If it is non-@code{nil}, then
62 the backup file has been written. Otherwise, the file should be backed
63 up when it is next saved (if backups are enabled). This is a
64 permanent local; @code{kill-local-variables} does not alter it.
67 @defopt make-backup-files
68 This variable determines whether or not to make backup files. If it
69 is non-@code{nil}, then Emacs creates a backup of each file when it is
70 saved for the first time---provided that @code{backup-inhibited}
71 is @code{nil} (see below).
73 The following example shows how to change the @code{make-backup-files}
74 variable only in the Rmail buffers and not elsewhere. Setting it
75 @code{nil} stops Emacs from making backups of these files, which may
76 save disk space. (You would put this code in your @file{.emacs} file.)
80 (add-hook 'rmail-mode-hook
84 (setq make-backup-files nil))))
89 @defvar backup-enable-predicate
90 This variable's value is a function to be called on certain occasions to
91 decide whether a file should have backup files. The function receives
92 one argument, a file name to consider. If the function returns
93 @code{nil}, backups are disabled for that file. Otherwise, the other
94 variables in this section say whether and how to make backups.
96 The default value is this:
100 (or (< (length name) 5)
101 (not (string-equal "/tmp/"
102 (substring name 0 5)))))
106 @defvar backup-inhibited
107 If this variable is non-@code{nil}, backups are inhibited. It records
108 the result of testing @code{backup-enable-predicate} on the visited file
109 name. It can also coherently be used by other mechanisms that inhibit
110 backups based on which file is visited. For example, VC sets this
111 variable non-@code{nil} to prevent making backups for files managed
112 with a version control system.
114 This is a permanent local, so that changing the major mode does not lose
115 its value. Major modes should not set this variable---they should set
116 @code{make-backup-files} instead.
120 @subsection Backup by Renaming or by Copying?
121 @cindex backup files, how to make them
123 There are two ways that Emacs can make a backup file:
127 Emacs can rename the original file so that it becomes a backup file, and
128 then write the buffer being saved into a new file. After this
129 procedure, any other names (i.e., hard links) of the original file now
130 refer to the backup file. The new file is owned by the user doing the
131 editing, and its group is the default for new files written by the user
135 Emacs can copy the original file into a backup file, and then overwrite
136 the original file with new contents. After this procedure, any other
137 names (i.e., hard links) of the original file continue to refer to the
138 current (updated) version of the file. The file's owner and group will
142 The first method, renaming, is the default.
144 The variable @code{backup-by-copying}, if non-@code{nil}, says to use
145 the second method, which is to copy the original file and overwrite it
146 with the new buffer contents. The variable @code{file-precious-flag},
147 if non-@code{nil}, also has this effect (as a sideline of its main
148 significance). @xref{Saving Buffers}.
150 @defvar backup-by-copying
151 If this variable is non-@code{nil}, Emacs always makes backup files by
155 The following two variables, when non-@code{nil}, cause the second
156 method to be used in certain special cases. They have no effect on the
157 treatment of files that don't fall into the special cases.
159 @defvar backup-by-copying-when-linked
160 If this variable is non-@code{nil}, Emacs makes backups by copying for
161 files with multiple names (hard links).
163 This variable is significant only if @code{backup-by-copying} is
164 @code{nil}, since copying is always used when that variable is
168 @defvar backup-by-copying-when-mismatch
169 If this variable is non-@code{nil}, Emacs makes backups by copying in cases
170 where renaming would change either the owner or the group of the file.
172 The value has no effect when renaming would not alter the owner or
173 group of the file; that is, for files which are owned by the user and
174 whose group matches the default for a new file created there by the
177 This variable is significant only if @code{backup-by-copying} is
178 @code{nil}, since copying is always used when that variable is
182 @node Numbered Backups
183 @subsection Making and Deleting Numbered Backup Files
185 If a file's name is @file{foo}, the names of its numbered backup
186 versions are @file{foo.~@var{v}~}, for various integers @var{v}, like
187 this: @file{foo.~1~}, @file{foo.~2~}, @file{foo.~3~}, @dots{},
188 @file{foo.~259~}, and so on.
190 @defopt version-control
191 This variable controls whether to make a single non-numbered backup
192 file or multiple numbered backups.
196 Make numbered backups if the visited file already has numbered backups;
200 Do not make numbered backups.
202 @item @var{anything else}
203 Make numbered backups.
207 The use of numbered backups ultimately leads to a large number of
208 backup versions, which must then be deleted. Emacs can do this
209 automatically or it can ask the user whether to delete them.
211 @defopt kept-new-versions
212 The value of this variable is the number of newest versions to keep
213 when a new numbered backup is made. The newly made backup is included
214 in the count. The default value is 2.
217 @defopt kept-old-versions
218 The value of this variable is the number of oldest versions to keep
219 when a new numbered backup is made. The default value is 2.
222 If there are backups numbered 1, 2, 3, 5, and 7, and both of these
223 variables have the value 2, then the backups numbered 1 and 2 are kept
224 as old versions and those numbered 5 and 7 are kept as new versions;
225 backup version 3 is excess. The function @code{find-backup-file-name}
226 (@pxref{Backup Names}) is responsible for determining which backup
227 versions to delete, but does not delete them itself.
229 @tindex delete-old-versions
230 @defopt delete-old-versions
231 If this variable is non-@code{nil}, then saving a file deletes excess
232 backup versions silently. Otherwise, it asks the user whether to delete
236 @defopt dired-kept-versions
237 This variable specifies how many of the newest backup versions to keep
238 in the Dired command @kbd{.} (@code{dired-clean-directory}). That's the
239 same thing @code{kept-new-versions} specifies when you make a new backup
240 file. The default value is 2.
244 @subsection Naming Backup Files
246 The functions in this section are documented mainly because you can
247 customize the naming conventions for backup files by redefining them.
248 If you change one, you probably need to change the rest.
250 @defun backup-file-name-p filename
251 This function returns a non-@code{nil} value if @var{filename} is a
252 possible name for a backup file. A file with the name @var{filename}
253 need not exist; the function just checks the name.
257 (backup-file-name-p "foo")
261 (backup-file-name-p "foo~")
266 The standard definition of this function is as follows:
270 (defun backup-file-name-p (file)
271 "Return non-nil if FILE is a backup file \
272 name (numeric or not)..."
273 (string-match "~$" file))
278 Thus, the function returns a non-@code{nil} value if the file name ends
279 with a @samp{~}. (We use a backslash to split the documentation
280 string's first line into two lines in the text, but produce just one
281 line in the string itself.)
283 This simple expression is placed in a separate function to make it easy
284 to redefine for customization.
287 @defun make-backup-file-name filename
288 This function returns a string that is the name to use for a
289 non-numbered backup file for file @var{filename}. On Unix, this is just
290 @var{filename} with a tilde appended.
292 The standard definition of this function, on most operating systems, is
297 (defun make-backup-file-name (file)
298 "Create the non-numeric backup file name for FILE@enddots{}"
303 You can change the backup-file naming convention by redefining this
304 function. The following example redefines @code{make-backup-file-name}
305 to prepend a @samp{.} in addition to appending a tilde:
309 (defun make-backup-file-name (filename)
311 (concat "." (file-name-nondirectory filename) "~")
312 (file-name-directory filename)))
316 (make-backup-file-name "backups.texi")
317 @result{} ".backups.texi~"
321 Some parts of Emacs, including some Dired commands, assume that backup
322 file names end with @samp{~}. If you do not follow that convention, it
323 will not cause serious problems, but these commands may give
324 less-than-desirable results.
327 @defun find-backup-file-name filename
328 This function computes the file name for a new backup file for
329 @var{filename}. It may also propose certain existing backup files for
330 deletion. @code{find-backup-file-name} returns a list whose @sc{car} is
331 the name for the new backup file and whose @sc{cdr} is a list of backup
332 files whose deletion is proposed.
334 Two variables, @code{kept-old-versions} and @code{kept-new-versions},
335 determine which backup versions should be kept. This function keeps
336 those versions by excluding them from the @sc{cdr} of the value.
337 @xref{Numbered Backups}.
339 In this example, the value says that @file{~rms/foo.~5~} is the name
340 to use for the new backup file, and @file{~rms/foo.~3~} is an ``excess''
341 version that the caller should consider deleting now.
345 (find-backup-file-name "~rms/foo")
346 @result{} ("~rms/foo.~5~" "~rms/foo.~3~")
352 @defun file-newest-backup filename
353 This function returns the name of the most recent backup file for
354 @var{filename}, or @code{nil} if that file has no backup files.
356 Some file comparison commands use this function so that they can
357 automatically compare a file with its most recent backup.
364 Emacs periodically saves all files that you are visiting; this is
365 called @dfn{auto-saving}. Auto-saving prevents you from losing more
366 than a limited amount of work if the system crashes. By default,
367 auto-saves happen every 300 keystrokes, or after around 30 seconds of
368 idle time. @xref{Auto-Save, Auto-Save, Auto-Saving: Protection Against
369 Disasters, emacs, The GNU Emacs Manual}, for information on auto-save
370 for users. Here we describe the functions used to implement auto-saving
371 and the variables that control them.
373 @defvar buffer-auto-save-file-name
374 This buffer-local variable is the name of the file used for
375 auto-saving the current buffer. It is @code{nil} if the buffer
376 should not be auto-saved.
380 buffer-auto-save-file-name
381 => "/xcssun/users/rms/lewis/#files.texi#"
386 @deffn Command auto-save-mode arg
387 When used interactively without an argument, this command is a toggle
388 switch: it turns on auto-saving of the current buffer if it is off, and
389 vice-versa. With an argument @var{arg}, the command turns auto-saving
390 on if the value of @var{arg} is @code{t}, a nonempty list, or a positive
391 integer. Otherwise, it turns auto-saving off.
394 @defun auto-save-file-name-p filename
395 This function returns a non-@code{nil} value if @var{filename} is a
396 string that could be the name of an auto-save file. It works based on
397 knowledge of the naming convention for auto-save files: a name that
398 begins and ends with hash marks (@samp{#}) is a possible auto-save file
399 name. The argument @var{filename} should not contain a directory part.
403 (make-auto-save-file-name)
404 @result{} "/xcssun/users/rms/lewis/#files.texi#"
407 (auto-save-file-name-p "#files.texi#")
411 (auto-save-file-name-p "files.texi")
416 The standard definition of this function is as follows:
420 (defun auto-save-file-name-p (filename)
421 "Return non-nil if FILENAME can be yielded by..."
422 (string-match "^#.*#$" filename))
426 This function exists so that you can customize it if you wish to
427 change the naming convention for auto-save files. If you redefine it,
428 be sure to redefine the function @code{make-auto-save-file-name}
432 @defun make-auto-save-file-name
433 This function returns the file name to use for auto-saving the current
434 buffer. This is just the file name with hash marks (@samp{#}) appended
435 and prepended to it. This function does not look at the variable
436 @code{auto-save-visited-file-name} (described below); you should check
437 that before calling this function.
441 (make-auto-save-file-name)
442 @result{} "/xcssun/users/rms/lewis/#backup.texi#"
446 The standard definition of this function is as follows:
450 (defun make-auto-save-file-name ()
451 "Return file name to use for auto-saves \
452 of current buffer@enddots{}"
457 (file-name-directory buffer-file-name)
459 (file-name-nondirectory buffer-file-name)
462 (concat "#%" (buffer-name) "#"))))
466 This exists as a separate function so that you can redefine it to
467 customize the naming convention for auto-save files. Be sure to
468 change @code{auto-save-file-name-p} in a corresponding way.
471 @defvar auto-save-visited-file-name
472 If this variable is non-@code{nil}, Emacs auto-saves buffers in
473 the files they are visiting. That is, the auto-save is done in the same
474 file that you are editing. Normally, this variable is @code{nil}, so
475 auto-save files have distinct names that are created by
476 @code{make-auto-save-file-name}.
478 When you change the value of this variable, the value does not take
479 effect until the next time auto-save mode is reenabled in any given
480 buffer. If auto-save mode is already enabled, auto-saves continue to go
481 in the same file name until @code{auto-save-mode} is called again.
484 @defun recent-auto-save-p
485 This function returns @code{t} if the current buffer has been
486 auto-saved since the last time it was read in or saved.
489 @defun set-buffer-auto-saved
490 This function marks the current buffer as auto-saved. The buffer will
491 not be auto-saved again until the buffer text is changed again. The
492 function returns @code{nil}.
495 @defopt auto-save-interval
496 The value of this variable is the number of characters that Emacs
497 reads from the keyboard between auto-saves. Each time this many more
498 characters are read, auto-saving is done for all buffers in which it is
502 @defopt auto-save-timeout
503 The value of this variable is the number of seconds of idle time that
504 should cause auto-saving. Each time the user pauses for this long,
505 Emacs auto-saves any buffers that need it. (Actually, the specified
506 timeout is multiplied by a factor depending on the size of the current
510 @defvar auto-save-hook
511 This normal hook is run whenever an auto-save is about to happen.
514 @defopt auto-save-default
515 If this variable is non-@code{nil}, buffers that are visiting files
516 have auto-saving enabled by default. Otherwise, they do not.
519 @deffn Command do-auto-save &optional no-message current-only
520 This function auto-saves all buffers that need to be auto-saved. It
521 saves all buffers for which auto-saving is enabled and that have been
522 changed since the previous auto-save.
524 Normally, if any buffers are auto-saved, a message that says
525 @samp{Auto-saving...} is displayed in the echo area while auto-saving is
526 going on. However, if @var{no-message} is non-@code{nil}, the message
529 If @var{current-only} is non-@code{nil}, only the current buffer
533 @defun delete-auto-save-file-if-necessary
534 This function deletes the current buffer's auto-save file if
535 @code{delete-auto-save-files} is non-@code{nil}. It is called every
536 time a buffer is saved.
539 @defvar delete-auto-save-files
540 This variable is used by the function
541 @code{delete-auto-save-file-if-necessary}. If it is non-@code{nil},
542 Emacs deletes auto-save files when a true save is done (in the visited
543 file). This saves disk space and unclutters your directory.
546 @defun rename-auto-save-file
547 This function adjusts the current buffer's auto-save file name if the
548 visited file name has changed. It also renames an existing auto-save
549 file. If the visited file name has not changed, this function does
553 @defvar buffer-saved-size
554 The value of this buffer-local variable is the length of the current
555 buffer as of the last time it was read in, saved, or auto-saved. This is
556 used to detect a substantial decrease in size, and turn off auto-saving
559 If it is @minus{}1, that means auto-saving is temporarily shut off in this
560 buffer due to a substantial deletion. Explicitly saving the buffer
561 stores a positive value in this variable, thus reenabling auto-saving.
562 Turning auto-save mode off or on also alters this variable.
565 @defvar auto-save-list-file-name
566 This variable (if non-@code{nil}) specifies a file for recording the
567 names of all the auto-save files. Each time Emacs does auto-saving, it
568 writes two lines into this file for each buffer that has auto-saving
569 enabled. The first line gives the name of the visited file (it's empty
570 if the buffer has none), and the second gives the name of the auto-save
573 If Emacs exits normally, it deletes this file. If Emacs crashes, you
574 can look in the file to find all the auto-save files that might contain
575 work that was otherwise lost. The @code{recover-session} command uses
578 The default name for this file is in your home directory and starts with
579 @samp{.saves-}. It also contains the Emacs process @sc{id} and the host
586 If you have made extensive changes to a file and then change your mind
587 about them, you can get rid of them by reading in the previous version
588 of the file with the @code{revert-buffer} command. @xref{Reverting, ,
589 Reverting a Buffer, emacs, The GNU Emacs Manual}.
591 @deffn Command revert-buffer &optional ignore-auto noconfirm
592 This command replaces the buffer text with the text of the visited
593 file on disk. This action undoes all changes since the file was visited
596 By default, if the latest auto-save file is more recent than the visited
597 file, @code{revert-buffer} asks the user whether to use that instead.
598 But if the argument @var{ignore-auto} is non-@code{nil}, then only the
599 the visited file itself is used. Interactively, @var{ignore-auto} is
600 @code{t} unless there is a numeric prefix argument; thus, the
601 interactive default is to check the auto-save file.
603 Normally, @code{revert-buffer} asks for confirmation before it changes
604 the buffer; but if the argument @var{noconfirm} is non-@code{nil},
605 @code{revert-buffer} does not ask for confirmation.
607 Reverting tries to preserve marker positions in the buffer by using the
608 replacement feature of @code{insert-file-contents}. If the buffer
609 contents and the file contents are identical before the revert
610 operation, reverting preserves all the markers. If they are not
611 identical, reverting does change the buffer; then it preserves the
612 markers in the unchanged text (if any) at the beginning and end of the
613 buffer. Preserving any additional markers would be problematical.
616 You can customize how @code{revert-buffer} does its work by setting
617 these variables---typically, as buffer-local variables.
619 @defvar revert-without-query
620 This variable holds a list of files that should be reverted without
621 query. The value is a list of regular expressions. If a file name
622 matches one of these regular expressions, then @code{revert-buffer}
623 reverts the file without asking the user for confirmation, if the file
624 has changed on disk and the buffer is not modified.
627 @defvar revert-buffer-function
628 The value of this variable is the function to use to revert this buffer.
629 If non-@code{nil}, it is called as a function with no arguments to do
630 the work of reverting. If the value is @code{nil}, reverting works the
633 Modes such as Dired mode, in which the text being edited does not
634 consist of a file's contents but can be regenerated in some other
635 fashion, give this variable a buffer-local value that is a function to
636 regenerate the contents.
639 @defvar revert-buffer-insert-file-contents-function
640 The value of this variable, if non-@code{nil}, is the function to use to
641 insert the updated contents when reverting this buffer. The function
642 receives two arguments: first the file name to use; second, @code{t} if
643 the user has asked to read the auto-save file.
646 @defvar before-revert-hook
647 This normal hook is run by @code{revert-buffer} before actually
648 inserting the modified contents---but only if
649 @code{revert-buffer-function} is @code{nil}.
652 @defvar after-revert-hook
653 This normal hook is run by @code{revert-buffer} after actually inserting
654 the modified contents---but only if @code{revert-buffer-function} is