2 @c This is part of the SXEmacs Lisp Reference Manual.
3 @c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc.
4 @c Copyright (C) 2005 Sebastian Freundt <hroptatyr@sxemacs.org>
5 @c See the file lispref.texi for copying conditions.
6 @setfilename ../../info/backups.info
7 @node Backups and Auto-Saving, Buffers, Files, Top
8 @chapter Backups and Auto-Saving
10 Backup files and auto-save files are two methods by which SXEmacs tries
11 to protect the user from the consequences of crashes or of the user's
12 own errors. Auto-saving preserves the text from earlier in the current
13 editing session; backup files preserve file contents prior to the
17 * Backup Files:: How backup files are made; how their names are chosen.
18 * Auto-Saving:: How auto-save files are made; how their names are chosen.
19 * 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. SXEmacs makes a backup file the first time you save a buffer
29 into its visited file. Normally, this means that the backup file
30 contains the contents of the file as it was before the current editing
31 session. The contents of the backup file normally remain unchanged once
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, SXEmacs 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 SXEmacs can delete them automatically.
46 * Making Backups:: How SXEmacs 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.
54 @subsection Making Backup Files
57 This function makes a backup of the file visited by the current
58 buffer, if appropriate. It is called by @code{save-buffer} before
59 saving the buffer the first time.
62 @defvar buffer-backed-up
63 This buffer-local variable indicates whether this buffer's file has
64 been backed up on account of this buffer. If it is non-@code{nil}, then
65 the backup file has been written. Otherwise, the file should be backed
66 up when it is next saved (if backups are enabled). This is a
67 permanent local; @code{kill-local-variables} does not alter it.
70 @defopt make-backup-files
71 This variable determines whether or not to make backup files. If it
72 is non-@code{nil}, then SXEmacs creates a backup of each file when it is
73 saved for the first time---provided that @code{backup-inhibited}
74 is @code{nil} (see below).
76 The following example shows how to change the @code{make-backup-files}
77 variable only in the @file{RMAIL} buffer and not elsewhere. Setting it
78 @code{nil} stops SXEmacs from making backups of the @file{RMAIL} file,
79 which may save disk space. (You would put this code in your
84 (add-hook 'rmail-mode-hook
88 (setq make-backup-files nil))))
93 @defvar backup-enable-predicate
94 This variable's value is a function to be called on certain occasions to
95 decide whether a file should have backup files. The function receives
96 one argument, a file name to consider. If the function returns
97 @code{nil}, backups are disabled for that file. Otherwise, the other
98 variables in this section say whether and how to make backups.
100 The default value is this:
104 (or (< (length name) 5)
105 (not (string-equal "/tmp/"
106 (substring name 0 5)))))
110 @defvar backup-inhibited
111 If this variable is non-@code{nil}, backups are inhibited. It records
112 the result of testing @code{backup-enable-predicate} on the visited file
113 name. It can also coherently be used by other mechanisms that inhibit
114 backups based on which file is visited. For example, VC sets this
115 variable non-@code{nil} to prevent making backups for files managed
116 with a version control system.
118 This is a permanent local, so that changing the major mode does not lose
119 its value. Major modes should not set this variable---they should set
120 @code{make-backup-files} instead.
125 @subsection Backup by Renaming or by Copying?
126 @cindex backup files, how to make them
128 There are two ways that SXEmacs can make a backup file:
132 SXEmacs can rename the original file so that it becomes a backup file,
133 and then write the buffer being saved into a new file. After this
134 procedure, any other names (i.e., hard links) of the original file now
135 refer to the backup file. The new file is owned by the user doing the
136 editing, and its group is the default for new files written by the user
140 SXEmacs can copy the original file into a backup file, and then
141 overwrite the original file with new contents. After this procedure,
142 any other names (i.e., hard links) of the original file still refer to
143 the current version of the file. The file's owner and group will be
147 The first method, renaming, is the default.
149 The variable @code{backup-by-copying}, if non-@code{nil}, says to use
150 the second method, which is to copy the original file and overwrite it
151 with the new buffer contents. The variable @code{file-precious-flag},
152 if non-@code{nil}, also has this effect (as a sideline of its main
153 significance). @xref{Saving Buffers}.
155 @defvar backup-by-copying
156 If this variable is non-@code{nil}, SXEmacs always makes backup files by
160 The following two variables, when non-@code{nil}, cause the second
161 method to be used in certain special cases. They have no effect on the
162 treatment of files that don't fall into the special cases.
164 @defvar backup-by-copying-when-linked
165 If this variable is non-@code{nil}, SXEmacs makes backups by copying for
166 files with multiple names (hard links).
168 This variable is significant only if @code{backup-by-copying} is
169 @code{nil}, since copying is always used when that variable is
173 @defvar backup-by-copying-when-mismatch
174 If this variable is non-@code{nil}, SXEmacs makes backups by copying in
175 cases where renaming would change either the owner or the group of the
178 The value has no effect when renaming would not alter the owner or
179 group of the file; that is, for files which are owned by the user and
180 whose group matches the default for a new file created there by the
183 This variable is significant only if @code{backup-by-copying} is
184 @code{nil}, since copying is always used when that variable is
189 @node Numbered Backups
190 @subsection Making and Deleting Numbered Backup Files
192 If a file's name is @file{foo}, the names of its numbered backup
193 versions are @file{foo.~@var{v}~}, for various integers @var{v}, like
194 this: @file{foo.~1~}, @file{foo.~2~}, @file{foo.~3~}, @dots{},
195 @file{foo.~259~}, and so on.
197 @defopt version-control
198 This variable controls whether to make a single non-numbered backup
199 file or multiple numbered backups.
203 Make numbered backups if the visited file already has numbered backups;
207 Do not make numbered backups.
209 @item @var{anything else}
210 Make numbered backups.
214 The use of numbered backups ultimately leads to a large number of
215 backup versions, which must then be deleted. SXEmacs can do this
216 automatically or it can ask the user whether to delete them.
218 @defopt kept-new-versions
219 The value of this variable is the number of newest versions to keep
220 when a new numbered backup is made. The newly made backup is included
221 in the count. The default value is 2.
224 @defopt kept-old-versions
225 The value of this variable is the number of oldest versions to keep
226 when a new numbered backup is made. The default value is 2.
229 If there are backups numbered 1, 2, 3, 5, and 7, and both of these
230 variables have the value 2, then the backups numbered 1 and 2 are kept
231 as old versions and those numbered 5 and 7 are kept as new versions;
232 backup version 3 is excess. The function @code{find-backup-file-name}
233 (@pxref{Backup Names}) is responsible for determining which backup
234 versions to delete, but does not delete them itself.
236 @defopt delete-old-versions
237 If this variable is non-@code{nil}, then saving a file deletes excess
238 backup versions silently. Otherwise, it asks the user whether to delete
242 @defopt dired-kept-versions
243 This variable specifies how many of the newest backup versions to keep
244 in the Dired command @kbd{.} (@code{dired-clean-directory}). That's the
245 same thing @code{kept-new-versions} specifies when you make a new backup
246 file. The default value is 2.
250 @subsection Naming Backup Files
252 The functions in this section are documented mainly because you can
253 customize the naming conventions for backup files by redefining them.
254 If you change one, you probably need to change the rest.
256 @defun backup-file-name-p filename
257 This function returns a non-@code{nil} value if @var{filename} is a
258 possible name for a backup file. A file with the name @var{filename}
259 need not exist; the function just checks the name.
263 (backup-file-name-p "foo")
267 (backup-file-name-p "foo~")
272 The standard definition of this function is as follows:
276 (defun backup-file-name-p (file)
277 "Return non-nil if FILE is a backup file \
278 name (numeric or not)..."
279 (string-match "~$" file))
284 Thus, the function returns a non-@code{nil} value if the file name ends
285 with a @samp{~}. (We use a backslash to split the documentation
286 string's first line into two lines in the text, but produce just one
287 line in the string itself.)
289 This simple expression is placed in a separate function to make it easy
290 to redefine for customization.
293 @defun make-backup-file-name filename
294 This function returns a string that is the name to use for a
295 non-numbered backup file for file @var{filename}. On Unix, this is just
296 @var{filename} with a tilde appended.
298 The standard definition of this function is as follows:
302 (defun make-backup-file-name (file)
303 "Create the non-numeric backup file name for FILE.
309 You can change the backup-file naming convention by redefining this
310 function. The following example redefines @code{make-backup-file-name}
311 to prepend a @samp{.} in addition to appending a tilde:
315 (defun make-backup-file-name (filename)
316 (concat "." filename "~"))
320 (make-backup-file-name "backups.texi")
321 @result{} ".backups.texi~"
326 @defun find-backup-file-name filename
327 This function computes the file name for a new backup file for
328 @var{filename}. It may also propose certain existing backup files for
329 deletion. @code{find-backup-file-name} returns a list whose @sc{car} is
330 the name for the new backup file and whose @sc{cdr} is a list of backup
331 files whose deletion is proposed.
333 Two variables, @code{kept-old-versions} and @code{kept-new-versions},
334 determine which backup versions should be kept. This function keeps
335 those versions by excluding them from the @sc{cdr} of the value.
336 @xref{Numbered Backups}.
338 In this example, the value says that @file{~rms/foo.~5~} is the name
339 to use for the new backup file, and @file{~rms/foo.~3~} is an ``excess''
340 version that the caller should consider deleting now.
344 (find-backup-file-name "~rms/foo")
345 @result{} ("~rms/foo.~5~" "~rms/foo.~3~")
351 @defun file-newest-backup filename
352 This function returns the name of the most recent backup file for
353 @var{filename}, or @code{nil} if that file has no backup files.
355 Some file comparison commands use this function so that they can
356 automatically compare a file with its most recent backup.
364 SXEmacs 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, sxemacs, The SXEmacs User's Manual}, for information on
370 auto-save for users. Here we describe the functions used to implement
371 auto-saving 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 &optional filename
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 \
458 (file-name-directory buffer-file-name)
460 (file-name-nondirectory buffer-file-name)
463 (concat "#%" (buffer-name) "#"))))
467 This exists as a separate function so that you can redefine it to
468 customize the naming convention for auto-save files. Be sure to
469 change @code{auto-save-file-name-p} in a corresponding way.
472 @defvar auto-save-visited-file-name
473 If this variable is non-@code{nil}, SXEmacs auto-saves buffers in
474 the files they are visiting. That is, the auto-save is done in the same
475 file that you are editing. Normally, this variable is @code{nil}, so
476 auto-save files have distinct names that are created by
477 @code{make-auto-save-file-name}.
479 When you change the value of this variable, the value does not take
480 effect until the next time auto-save mode is reenabled in any given
481 buffer. If auto-save mode is already enabled, auto-saves continue to go
482 in the same file name until @code{auto-save-mode} is called again.
485 @defun recent-auto-save-p
486 This function returns @code{t} if the current buffer has been
487 auto-saved since the last time it was read in or saved.
490 @defun set-buffer-auto-saved
491 This function marks the current buffer as auto-saved. The buffer will
492 not be auto-saved again until the buffer text is changed again. The
493 function returns @code{nil}.
496 @defopt auto-save-interval
497 The value of this variable is the number of characters that SXEmacs
498 reads from the keyboard between auto-saves. Each time this many more
499 characters are read, auto-saving is done for all buffers in which it is
503 @defopt auto-save-timeout
504 The value of this variable is the number of seconds of idle time that
505 should cause auto-saving. Each time the user pauses for this long,
506 SXEmacs auto-saves any buffers that need it.
508 Note: Actually, the specified timeout is multiplied by a factor
509 depending on the size of the current buffer.
512 @defvar auto-save-hook
513 This normal hook is run whenever an auto-save is about to happen.
516 @defopt auto-save-default
517 If this variable is non-@code{nil}, buffers that are visiting files
518 have auto-saving enabled by default. Otherwise, they do not.
521 @deffn Command do-auto-save &optional no-message current-only
522 This function auto-saves all buffers that need to be auto-saved. It
523 saves all buffers for which auto-saving is enabled and that have been
524 changed since the previous auto-save.
526 Normally, if any buffers are auto-saved, a message that says
527 @samp{Auto-saving...} is displayed in the echo area while auto-saving is
528 going on. However, if @var{no-message} is non-@code{nil}, the message
531 If @var{current-only} is non-@code{nil}, only the current buffer
535 @defun delete-auto-save-file-if-necessary
536 This function deletes the current buffer's auto-save file if
537 @code{delete-auto-save-files} is non-@code{nil}. It is called every
538 time a buffer is saved.
541 @defvar delete-auto-save-files
542 This variable is used by the function
543 @code{delete-auto-save-file-if-necessary}. If it is non-@code{nil},
544 Emacs deletes auto-save files when a true save is done (in the visited
545 file). This saves disk space and unclutters your directory.
548 @defun rename-auto-save-file
549 This function adjusts the current buffer's auto-save file name if the
550 visited file name has changed. It also renames an existing auto-save
551 file. If the visited file name has not changed, this function does
555 @defvar buffer-saved-size
556 The value of this buffer-local variable is the length of the current
557 buffer as of the last time it was read in, saved, or auto-saved. This is
558 used to detect a substantial decrease in size, and turn off auto-saving
561 If it is -1, that means auto-saving is temporarily shut off in this
562 buffer due to a substantial deletion. Explicitly saving the buffer
563 stores a positive value in this variable, thus reenabling auto-saving.
564 Turning auto-save mode off or on also alters this variable.
567 @defvar auto-save-list-file-name
568 This variable (if non-@code{nil}) specifies a file for recording the
569 names of all the auto-save files. Each time SXEmacs does auto-saving, it
570 writes two lines into this file for each buffer that has auto-saving
571 enabled. The first line gives the name of the visited file (it's empty
572 if the buffer has none), and the second gives the name of the auto-save
575 If SXEmacs exits normally, it deletes this file. If SXEmacs crashes, you
576 can look in the file to find all the auto-save files that might contain
577 work that was otherwise lost. The @code{recover-session} command uses
580 The default name for this file is in your home directory and starts with
581 @samp{.saves-}. It also contains the SXEmacs process @sc{id} and the
589 If you have made extensive changes to a file and then change your mind
590 about them, you can get rid of them by reading in the previous version
591 of the file with the @code{revert-buffer} command. @xref{Reverting, ,
592 Reverting a Buffer, sxemacs, The SXEmacs User's Manual}.
594 @deffn Command revert-buffer &optional check-auto-save noconfirm preserve-modes
595 This command replaces the buffer text with the text of the visited
596 file on disk. This action undoes all changes since the file was visited
599 If the argument @var{check-auto-save} is non-@code{nil}, and the
600 latest auto-save file is more recent than the visited file,
601 @code{revert-buffer} asks the user whether to use that instead.
602 Otherwise, it always uses the text of the visited file itself.
603 Interactively, @var{check-auto-save} is set if there is a numeric prefix
606 Normally, @code{revert-buffer} asks for confirmation before it changes
607 the buffer; but if the argument @var{noconfirm} is non-@code{nil},
608 @code{revert-buffer} does not ask for confirmation.
610 Optional third argument @var{preserve-modes} non-@code{nil} means don't
611 alter the files modes. Normally we reinitialize them using
614 Reverting tries to preserve marker positions in the buffer by using the
615 replacement feature of @code{insert-file-contents}. If the buffer
616 contents and the file contents are identical before the revert
617 operation, reverting preserves all the markers. If they are not
618 identical, reverting does change the buffer; then it preserves the
619 markers in the unchanged text (if any) at the beginning and end of the
620 buffer. Preserving any additional markers would be problematical.
623 You can customize how @code{revert-buffer} does its work by setting
624 these variables---typically, as buffer-local variables.
626 @defvar revert-buffer-function
627 The value of this variable is the function to use to revert this buffer.
628 If non-@code{nil}, it is called as a function with no arguments to do
629 the work of reverting. If the value is @code{nil}, reverting works the
632 Modes such as Dired mode, in which the text being edited does not
633 consist of a file's contents but can be regenerated in some other
634 fashion, give this variable a buffer-local value that is a function to
635 regenerate the contents.
638 @defvar revert-buffer-insert-file-contents-function
639 The value of this variable, if non-@code{nil}, is the function to use to
640 insert the updated contents when reverting this buffer. The function
641 receives two arguments: first the file name to use; second, @code{t} if
642 the user has asked to read the auto-save file.
645 @defvar before-revert-hook
646 This normal hook is run by @code{revert-buffer} before actually
647 inserting the modified contents---but only if
648 @code{revert-buffer-function} is @code{nil}.
650 Font Lock mode uses this hook to record that the buffer contents are no
654 @defvar after-revert-hook
655 This normal hook is run by @code{revert-buffer} after actually inserting
656 the modified contents---but only if @code{revert-buffer-function} is
659 Font Lock mode uses this hook to recompute the fonts for the updated