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diff --git a/lispref/backups.texi b/lispref/backups.texi new file mode 100644 index 00000000000..57108105e0b --- /dev/null +++ b/lispref/backups.texi @@ -0,0 +1,621 @@ +@c -*-texinfo-*- +@c This is part of the GNU Emacs Lisp Reference Manual. +@c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. +@c See the file elisp.texi for copying conditions. +@setfilename ../info/backups +@node Backups and Auto-Saving, Buffers, Files, Top +@chapter 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. + +@menu +* Backup Files:: How backup files are made; how their names are chosen. +* Auto-Saving:: How auto-save files are made; how their names are chosen. +* Reverting:: @code{revert-buffer}, and how to customize what it does. +@end menu + +@node Backup Files, Auto-Saving, Backups and Auto-Saving, Backups and Auto-Saving +@section Backup Files +@cindex backup file + + A @dfn{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. + +@menu +* Making Backups:: How Emacs makes backup files, and when. +* Rename or Copy:: Two alternatives: renaming the old file or copying it. +* Numbered Backups:: Keeping multiple backups for each source file. +* Backup Names:: How backup file names are computed; customization. +@end menu + +@node Making Backups, Rename or Copy, Backup Files, Backup Files +@subsection Making Backup Files + +@defun backup-buffer + This function makes a backup of the file visited by the current +buffer, if appropriate. It is called by @code{save-buffer} before +saving the buffer the first time. +@end defun + +@defvar 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-@code{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). This is a +permanent local; @code{kill-local-variables} does not alter it. +@end defvar + +@defopt make-backup-files + This variable determines whether or not to make backup files. If it +is non-@code{nil}, then Emacs creates a backup of each file when it is +saved for the first time. + + The following example shows how to change the @code{make-backup-files} +variable only in the @file{RMAIL} buffer and not elsewhere. Setting it +@code{nil} stops Emacs from making backups of the @file{RMAIL} file, +which may save disk space. (You would put this code in your +@file{.emacs} file.) + +@smallexample +@group +(add-hook 'rmail-mode-hook + (function (lambda () + (make-local-variable + 'make-backup-files) + (setq make-backup-files nil)))) +@end group +@end smallexample +@end defopt + +@defvar backup-enable-predicate filename +This variable's value is a function to be called on certain occasions to +decide whether a there should be backup files for file name +@var{filename}. If it returns @code{nil}, backups are disabled. +Otherwise, the other variables in this section say whether and how to +make backups. + +The default value is this: + +@example +(lambda (name) + (or (< (length name) 5) + (not (string-equal "/tmp/" + (substring name 0 5))))) +@end example +@end defvar + +@defvar backup-inhibited +If this variable is non-@code{nil}, backups are inhibited. It records +the result of testing @code{backup-enable-predicate} on the visited file +name. It can also coherently be used by other mechanisms that inhibit +backups based on which file is visited. Major modes should not set this +variable. +@end defvar + +@node Rename or Copy, Numbered Backups, Making Backups, Backup Files +@subsection Backup by Renaming or by Copying? +@cindex backup files, how to make them + + There are two ways that Emacs can make a backup file: + +@itemize @bullet +@item +Emacs can rename the original file so that it becomes a backup file, and +then write the buffer being saved into a new file. After this +procedure, any other names (i.e., hard links) of the original file now +refer to the backup file. The new file is owned by the user doing the +editing, and its group is the default for new files written by the user +in that directory. + +@item +Emacs can copy the original file into a backup file, and then overwrite +the original file with new contents. After this procedure, any other +names (i.e., hard links) of the original file still refer to the current +version of the file. The file's owner and group will be unchanged. +@end itemize + + The first method, renaming, is the default. + + The variable @code{backup-by-copying}, if non-@code{nil}, says to use +the second method, which is to copy the original file and overwrite it +with the new buffer contents. The variable @code{file-precious-flag}, +if non-@code{nil}, also has this effect (as a sideline of its main +significance). @xref{Saving Buffers}. + +@defvar backup-by-copying +If this variable is non-@code{nil}, Emacs always makes backup files by +copying. +@end defvar + + The following two variables, when non-@code{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. + +@defvar backup-by-copying-when-linked +If this variable is non-@code{nil}, Emacs makes backups by copying for +files with multiple names (hard links). + +This variable is significant only if @code{backup-by-copying} is +@code{nil}, since copying is always used when that variable is +non-@code{nil}. +@end defvar + +@defvar backup-by-copying-when-mismatch +If this variable is non-@code{nil}, Emacs makes backups by copying in cases +where renaming would change either the owner or the group of the file. + +The value has no effect when renaming would not alter 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 @code{backup-by-copying} is +@code{nil}, since copying is always used when that variable is +non-@code{nil}. +@end defvar + +@node Numbered Backups, Backup Names, Rename or Copy, Backup Files +@subsection Making and Deleting Numbered Backup Files + + If a file's name is @file{foo}, the names of its numbered backup +versions are @file{foo.~@var{v}~}, for various integers @var{v}, like +this: @file{foo.~1~}, @file{foo.~2~}, @file{foo.~3~}, @dots{}, +@file{foo.~259~}, and so on. + +@defopt version-control +This variable controls whether to make a single non-numbered backup +file or multiple numbered backups. + +@table @asis +@item @code{nil} +Make numbered backups if the visited file already has numbered backups; +otherwise, do not. + +@item @code{never} +Do not make numbered backups. + +@item @var{anything else} +Do make numbered backups. +@end table +@end defopt + + The use of numbered backups ultimately leads to a large number of +backup versions, which must then be deleted. Emacs can do this +automatically. + +@defopt 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. +@end defopt + +@defopt 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. +@end defopt + + 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 are kept +as old versions and those numbered 5 and 7 are kept as new versions; +backup version 3 is deleted. The function @code{find-backup-file-name} +(@pxref{Backup Names}) is responsible for determining which backup +versions to delete, but does not delete them itself. + +@defopt trim-versions-without-asking +If this variable is non-@code{nil}, then saving a file deletes excess +backup versions silently. Otherwise, it asks the user whether to delete +them. +@end defopt + +@defopt dired-kept-versions +This variable specifies how many of the newest backup versions to keep +in the Dired command @kbd{.} (@code{dired-clean-directory}). That's the +same thing @code{kept-new-versions} does when you make a new backup +file. The default value is 2. +@end defopt + +@node Backup Names, , Numbered Backups, Backup Files +@subsection Naming Backup Files + + The functions in this section are documented mainly because you can +customize the naming conventions for backup files by redefining them. +If you change one, you probably need to change the rest. + +@defun backup-file-name-p filename +This function returns a non-@code{nil} value if @var{filename} is a +possible name for a backup file. A file with the name @var{filename} +need not exist; the function just checks the name. + +@smallexample +@group +(backup-file-name-p "foo") + @result{} nil +@end group +@group +(backup-file-name-p "foo~") + @result{} 3 +@end group +@end smallexample + +The standard definition of this function is as follows: + +@smallexample +@group +(defun backup-file-name-p (file) + "Return non-nil if FILE is a backup file \ +name (numeric or not)..." + (string-match "~$" file)) +@end group +@end smallexample + +@noindent +Thus, the function returns a non-@code{nil} value if the file name ends +with a @samp{~}. (We use a backslash to split the documentation +string's first line into two lines in the text, but produce just one +line in the string itself.) + +This simple expression is placed in a separate function to make it easy +to redefine for customization. +@end defun + +@defun make-backup-file-name filename +This function returns a string which is the name to use for a +non-numbered backup file for file @var{filename}. On Unix, this is just +@var{filename} with a tilde appended. + +The standard definition of this function is as follows: + +@smallexample +@group +(defun make-backup-file-name (file) + "Create the non-numeric backup file name for FILE. +@dots{}" + (concat file "~")) +@end group +@end smallexample + +You can change the backup file naming convention by redefining this +function. The following example redefines @code{make-backup-file-name} +to prepend a @samp{.} as well as appending a tilde: + +@smallexample +@group +(defun make-backup-file-name (filename) + (concat "." filename "~")) +@end group + +@group +(make-backup-file-name "backups.texi") + @result{} ".backups.texi~" +@end group +@end smallexample +@end defun + +@defun find-backup-file-name filename +This function computes the file name for a new backup file for +@var{filename}. It may also propose certain existing backup files for +deletion. @code{find-backup-file-name} returns a list whose @sc{car} is +the name for the new backup file and whose @sc{cdr} is a list of backup +files whose deletion is proposed. + +Two variables, @code{kept-old-versions} and @code{kept-new-versions}, +determine which backup versions should be kept. This function keeps +those versions by excluding them from the @sc{cdr} of the value. +@xref{Numbered Backups}. + +In this example, the value says that @file{~rms/foo.~5~} is the name +to use for the new backup file, and @file{~rms/foo.~3~} is an ``excess'' +version that the caller should consider deleting now. + +@smallexample +@group +(find-backup-file-name "~rms/foo") + @result{} ("~rms/foo.~5~" "~rms/foo.~3~") +@end group +@end smallexample +@end defun + +@c Emacs 19 feature +@defun file-newest-backup filename +This function returns the name of the most recent backup file for +@var{filename}, or @code{nil} that file has no backup files. + +Some file comparison commands use this function in order to compare +a file by default with its most recent backup. +@end defun + +@node Auto-Saving, Reverting, Backup Files, Backups and Auto-Saving +@section Auto-Saving +@cindex auto-saving + + Emacs periodically saves all files that you are visiting; this is +called @dfn{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, or after around 30 seconds of +idle time. @xref{Auto-Save, Auto-Save, Auto-Saving: Protection Against +Disasters, emacs, 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. + +@defvar buffer-auto-save-file-name +This buffer-local variable is the name of the file used for +auto-saving the current buffer. It is @code{nil} if the buffer +should not be auto-saved. + +@example +@group +buffer-auto-save-file-name +=> "/xcssun/users/rms/lewis/#files.texi#" +@end group +@end example +@end defvar + +@deffn 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 @var{arg}, the command turns auto-saving +on if the value of @var{arg} is @code{t}, a nonempty list, or a positive +integer. Otherwise, it turns auto-saving off. +@end deffn + +@defun auto-save-file-name-p filename +This function returns a non-@code{nil} value if @var{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 (@samp{#}) is a possible auto-save file +name. The argument @var{filename} should not contain a directory part. + +@example +@group +(make-auto-save-file-name) + @result{} "/xcssun/users/rms/lewis/#files.texi#" +@end group +@group +(auto-save-file-name-p "#files.texi#") + @result{} 0 +@end group +@group +(auto-save-file-name-p "files.texi") + @result{} nil +@end group +@end example + +The standard definition of this function is as follows: + +@example +@group +(defun auto-save-file-name-p (filename) + "Return non-nil if FILENAME can be yielded by..." + (string-match "^#.*#$" filename)) +@end group +@end example + +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 the function @code{make-auto-save-file-name} +correspondingly. +@end defun + +@defun 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 (@samp{#}) appended +and prepended to it. This function does not look at the variable +@code{auto-save-visited-file-name}; you should check that before calling +this function. + +@example +@group +(make-auto-save-file-name) + @result{} "/xcssun/users/rms/lewis/#backup.texi#" +@end group +@end example + +The standard definition of this function is as follows: + +@example +@group +(defun make-auto-save-file-name () + "Return file name to use for auto-saves \ +of current buffer. +@dots{}" + (if buffer-file-name +@end group +@group + (concat + (file-name-directory buffer-file-name) + "#" + (file-name-nondirectory buffer-file-name) + "#") + (expand-file-name + (concat "#%" (buffer-name) "#")))) +@end group +@end example + +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 @code{auto-save-file-name-p} in a corresponding way. +@end defun + +@defvar auto-save-visited-file-name +If this variable is non-@code{nil}, Emacs auto-saves 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 @code{nil}, so +auto-save files have distinct names that are created by +@code{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 @code{auto-save-mode} is called again. +@end defvar + +@defun recent-auto-save-p +This function returns @code{t} if the current buffer has been +auto-saved since the last time it was read in or saved. +@end defun + +@defun 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 @code{nil}. +@end defun + +@defopt 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. +@end defopt + +@defopt auto-save-timeout +The value of this variable is the number of seconds of idle time that +should cause auto-saving. Each time the user pauses for this long, +Emacs auto-saves any buffers that need it. (Actually, the specified +timeout is multiplied by a factor depending on the size of the current +buffer.) +@end defopt + +@defvar auto-save-hook +This normal hook is run whenever an auto-save is about to happen. +@end defvar + +@defopt auto-save-default +If this variable is non-@code{nil}, buffers that are visiting files +have auto-saving enabled by default. Otherwise, they do not. +@end defopt + +@deffn Command do-auto-save &optional no-message +This function auto-saves all buffers that need to be auto-saved. It +saves all buffers for which auto-saving is enabled and that have been +changed since the previous auto-save. + +Normally, if any buffers are auto-saved, a message that says +@samp{Auto-saving...} is displayed in the echo area while auto-saving is +going on. However, if @var{no-message} is non-@code{nil}, the message +is inhibited. +@end deffn + +@defun delete-auto-save-file-if-necessary +This function deletes the current buffer's auto-save file if +@code{delete-auto-save-files} is non-@code{nil}. It is called every +time a buffer is saved. +@end defun + +@defvar delete-auto-save-files +This variable is used by the function +@code{delete-auto-save-file-if-necessary}. If it is non-@code{nil}, +Emacs deletes auto-save files when a true save is done (in the visited +file). This saves disk space and unclutters your directory. +@end defvar + +@defun 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. +@end defun + +@node Reverting, , Auto-Saving, Backups and Auto-Saving +@section 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 @code{revert-buffer} command. @xref{Reverting, , +Reverting a Buffer, emacs, The GNU Emacs Manual}. + +@deffn Command revert-buffer &optional check-auto-save 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. + +If the argument @var{check-auto-save} is non-@code{nil}, and the +latest auto-save file is more recent than the visited file, +@code{revert-buffer} asks the user whether to use that instead. +Otherwise, it always uses the text of the visited file itself. +Interactively, @var{check-auto-save} is set if there is a numeric prefix +argument. + +Normally, @code{revert-buffer} asks for confirmation before it changes +the buffer; but if the argument @var{noconfirm} is non-@code{nil}, +@code{revert-buffer} does not ask for confirmation. + +Reverting tries to preserve marker positions in the buffer by using the +replacement feature of @code{insert-file-contents}. If there is no +actual difference between the buffer and the file, before reversion, +this preserves all the markers. If reversion does change the buffer, +this preserves the markers in the unchanged text (if any) at the +beginning and end of the buffer. Preserving any additional markers +would be problematical. + +If the value of the @code{revert-buffer-function} variable is +non-@code{nil}, it is called as a function with no arguments to do the +work. +@end deffn + +@defvar 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 @code{nil}, then the +@code{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. +@end defvar + +@defvar revert-buffer-insert-file-contents-function +The value of this variable, if non-@code{nil}, is the function to use +to insert contents when reverting this buffer. The function receives +two arguments, first the file name to use, and second, @code{t} if the +user has asked to read the auto-save file. +@end defvar + +@defvar before-revert-hook +This normal hook is run by @code{revert-buffer} before actually +inserting the modified contents---but only if +@code{revert-buffer-function} is @code{nil}. + +Font Lock mode uses this hook to record that the buffer contents are no +longer fontified. +@end defvar + +@defvar after-revert-hook +This normal hook is run by @code{revert-buffer} after actually inserting +the modified contents---but only if @code{revert-buffer-function} is +@code{nil}. + +Font Lock mode uses this hook to recompute the fonts for the updated +buffer contents. +@end defvar + +@deffn Command recover-file filename +This function visits @var{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 @var{filename}, +or if @var{filename} is newer than its auto-save file. If +@var{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. +@end deffn + |