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696 lines
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696 lines
25 KiB
Plaintext
@c -*-texinfo-*-
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@c This is part of the GNU Emacs Lisp Reference Manual.
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@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995 Free Software Foundation, Inc.
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@c See the file elisp.texi for copying conditions.
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@setfilename ../info/backups
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@node Backups and Auto-Saving, Buffers, Files, Top
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@chapter Backups and Auto-Saving
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Backup files and auto-save files are two methods by which Emacs tries
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to protect the user from the consequences of crashes or of the user's
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own errors. Auto-saving preserves the text from earlier in the current
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editing session; backup files preserve file contents prior to the
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current session.
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@menu
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* Backup Files:: How backup files are made; how their names are chosen.
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* Auto-Saving:: How auto-save files are made; how their names are chosen.
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* Reverting:: @code{revert-buffer}, and how to customize what it does.
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@end menu
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@node Backup Files
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@section Backup Files
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@cindex backup file
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A @dfn{backup file} is a copy of the old contents of a file you are
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editing. Emacs makes a backup file the first time you save a buffer
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into its visited file. Normally, this means that the backup file
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contains the contents of the file as it was before the current editing
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session. The contents of the backup file normally remain unchanged once
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it exists.
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Backups are usually made by renaming the visited file to a new name.
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Optionally, you can specify that backup files should be made by copying
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the visited file. This choice makes a difference for files with
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multiple names; it also can affect whether the edited file remains owned
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by the original owner or becomes owned by the user editing it.
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By default, Emacs makes a single backup file for each file edited.
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You can alternatively request numbered backups; then each new backup
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file gets a new name. You can delete old numbered backups when you
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don't want them any more, or Emacs can delete them automatically.
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@menu
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* Making Backups:: How Emacs makes backup files, and when.
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* Rename or Copy:: Two alternatives: renaming the old file or copying it.
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* Numbered Backups:: Keeping multiple backups for each source file.
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* Backup Names:: How backup file names are computed; customization.
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@end menu
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@node Making Backups
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@subsection Making Backup Files
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@defun backup-buffer
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This function makes a backup of the file visited by the current
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buffer, if appropriate. It is called by @code{save-buffer} before
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saving the buffer the first time.
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@end defun
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@defvar buffer-backed-up
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This buffer-local variable indicates whether this buffer's file has
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been backed up on account of this buffer. If it is non-@code{nil}, then
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the backup file has been written. Otherwise, the file should be backed
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up when it is next saved (if backups are enabled). This is a
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permanent local; @code{kill-all-local-variables} does not alter it.
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@end defvar
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@defopt make-backup-files
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This variable determines whether or not to make backup files. If it
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is non-@code{nil}, then Emacs creates a backup of each file when it is
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saved for the first time---provided that @code{backup-inhibited}
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is @code{nil} (see below).
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The following example shows how to change the @code{make-backup-files}
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variable only in the Rmail buffers and not elsewhere. Setting it
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@code{nil} stops Emacs from making backups of these files, which may
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save disk space. (You would put this code in your init file.)
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@smallexample
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@group
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(add-hook 'rmail-mode-hook
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(function (lambda ()
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(make-local-variable
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'make-backup-files)
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(setq make-backup-files nil))))
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@end group
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@end smallexample
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@end defopt
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@defvar backup-enable-predicate
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This variable's value is a function to be called on certain occasions to
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decide whether a file should have backup files. The function receives
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one argument, a file name to consider. If the function returns
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@code{nil}, backups are disabled for that file. Otherwise, the other
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variables in this section say whether and how to make backups.
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The default value is this:
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@example
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(lambda (name)
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(or (< (length name) 5)
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(not (string-equal "/tmp/"
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(substring name 0 5)))))
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@end example
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@end defvar
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@defvar backup-inhibited
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If this variable is non-@code{nil}, backups are inhibited. It records
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the result of testing @code{backup-enable-predicate} on the visited file
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name. It can also coherently be used by other mechanisms that inhibit
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backups based on which file is visited. For example, VC sets this
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variable non-@code{nil} to prevent making backups for files managed
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with a version control system.
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This is a permanent local, so that changing the major mode does not lose
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its value. Major modes should not set this variable---they should set
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@code{make-backup-files} instead.
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@end defvar
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@node Rename or Copy
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@subsection Backup by Renaming or by Copying?
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@cindex backup files, how to make them
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There are two ways that Emacs can make a backup file:
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@itemize @bullet
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@item
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Emacs can rename the original file so that it becomes a backup file, and
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then write the buffer being saved into a new file. After this
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procedure, any other names (i.e., hard links) of the original file now
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refer to the backup file. The new file is owned by the user doing the
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editing, and its group is the default for new files written by the user
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in that directory.
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@item
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Emacs can copy the original file into a backup file, and then overwrite
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the original file with new contents. After this procedure, any other
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names (i.e., hard links) of the original file continue to refer to the
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current (updated) version of the file. The file's owner and group will
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be unchanged.
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@end itemize
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The first method, renaming, is the default.
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The variable @code{backup-by-copying}, if non-@code{nil}, says to use
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the second method, which is to copy the original file and overwrite it
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with the new buffer contents. The variable @code{file-precious-flag},
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if non-@code{nil}, also has this effect (as a sideline of its main
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significance). @xref{Saving Buffers}.
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@defvar backup-by-copying
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If this variable is non-@code{nil}, Emacs always makes backup files by
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copying.
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@end defvar
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The following two variables, when non-@code{nil}, cause the second
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method to be used in certain special cases. They have no effect on the
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treatment of files that don't fall into the special cases.
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@defvar backup-by-copying-when-linked
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If this variable is non-@code{nil}, Emacs makes backups by copying for
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files with multiple names (hard links).
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This variable is significant only if @code{backup-by-copying} is
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@code{nil}, since copying is always used when that variable is
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non-@code{nil}.
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@end defvar
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@defvar backup-by-copying-when-mismatch
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If this variable is non-@code{nil}, Emacs makes backups by copying in cases
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where renaming would change either the owner or the group of the file.
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The value has no effect when renaming would not alter the owner or
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group of the file; that is, for files which are owned by the user and
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whose group matches the default for a new file created there by the
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user.
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This variable is significant only if @code{backup-by-copying} is
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@code{nil}, since copying is always used when that variable is
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non-@code{nil}.
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@end defvar
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@defvar backup-by-copying-when-privileged-mismatch
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This variable, if non-@code{nil}, specifies the same behavior as
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@code{backup-by-copying-when-mismatch}, but only for certain user-id
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values: namely, those less than or equal to a certain number. You set
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this variable to that number.
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Thus, if you set @code{backup-by-copying-when-privileged-mismatch}
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to 0, backup by copying is done for the superuser only,
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when necessary to prevent a change in the owner of the file.
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The default is 200.
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@end defvar
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@node Numbered Backups
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@subsection Making and Deleting Numbered Backup Files
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If a file's name is @file{foo}, the names of its numbered backup
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versions are @file{foo.~@var{v}~}, for various integers @var{v}, like
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this: @file{foo.~1~}, @file{foo.~2~}, @file{foo.~3~}, @dots{},
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@file{foo.~259~}, and so on.
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@defopt version-control
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This variable controls whether to make a single non-numbered backup
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file or multiple numbered backups.
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@table @asis
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@item @code{nil}
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Make numbered backups if the visited file already has numbered backups;
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otherwise, do not.
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@item @code{never}
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Do not make numbered backups.
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@item @var{anything else}
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Make numbered backups.
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@end table
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@end defopt
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The use of numbered backups ultimately leads to a large number of
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backup versions, which must then be deleted. Emacs can do this
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automatically or it can ask the user whether to delete them.
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@defopt kept-new-versions
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The value of this variable is the number of newest versions to keep
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when a new numbered backup is made. The newly made backup is included
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in the count. The default value is 2.
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@end defopt
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@defopt kept-old-versions
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The value of this variable is the number of oldest versions to keep
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when a new numbered backup is made. The default value is 2.
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@end defopt
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If there are backups numbered 1, 2, 3, 5, and 7, and both of these
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variables have the value 2, then the backups numbered 1 and 2 are kept
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as old versions and those numbered 5 and 7 are kept as new versions;
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backup version 3 is excess. The function @code{find-backup-file-name}
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(@pxref{Backup Names}) is responsible for determining which backup
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versions to delete, but does not delete them itself.
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@tindex delete-old-versions
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@defopt delete-old-versions
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If this variable is @code{t}, then saving a file deletes excess
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backup versions silently. If it is @code{nil}, that means
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to ask for confirmation before deleting excess backups.
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Otherwise, they are not deleted at all.
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@end defopt
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@defopt dired-kept-versions
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This variable specifies how many of the newest backup versions to keep
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in the Dired command @kbd{.} (@code{dired-clean-directory}). That's the
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same thing @code{kept-new-versions} specifies when you make a new backup
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file. The default value is 2.
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@end defopt
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@node Backup Names
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@subsection Naming Backup Files
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The functions in this section are documented mainly because you can
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customize the naming conventions for backup files by redefining them.
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If you change one, you probably need to change the rest.
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@defun backup-file-name-p filename
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This function returns a non-@code{nil} value if @var{filename} is a
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possible name for a backup file. A file with the name @var{filename}
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need not exist; the function just checks the name.
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@smallexample
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@group
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(backup-file-name-p "foo")
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@result{} nil
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@end group
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@group
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(backup-file-name-p "foo~")
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@result{} 3
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@end group
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@end smallexample
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The standard definition of this function is as follows:
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@smallexample
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@group
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(defun backup-file-name-p (file)
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"Return non-nil if FILE is a backup file \
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name (numeric or not)..."
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(string-match "~\\'" file))
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@end group
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@end smallexample
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@noindent
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Thus, the function returns a non-@code{nil} value if the file name ends
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with a @samp{~}. (We use a backslash to split the documentation
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string's first line into two lines in the text, but produce just one
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line in the string itself.)
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This simple expression is placed in a separate function to make it easy
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to redefine for customization.
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@end defun
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@defun make-backup-file-name filename
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This function returns a string that is the name to use for a
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non-numbered backup file for file @var{filename}. On Unix, this is just
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@var{filename} with a tilde appended.
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The standard definition of this function, on most operating systems, is
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as follows:
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@smallexample
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@group
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(defun make-backup-file-name (file)
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"Create the non-numeric backup file name for FILE..."
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(concat file "~"))
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@end group
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@end smallexample
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You can change the backup-file naming convention by redefining this
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function. The following example redefines @code{make-backup-file-name}
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to prepend a @samp{.} in addition to appending a tilde:
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@smallexample
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@group
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(defun make-backup-file-name (filename)
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(expand-file-name
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(concat "." (file-name-nondirectory filename) "~")
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(file-name-directory filename)))
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@end group
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@group
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(make-backup-file-name "backups.texi")
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@result{} ".backups.texi~"
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@end group
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@end smallexample
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Some parts of Emacs, including some Dired commands, assume that backup
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file names end with @samp{~}. If you do not follow that convention, it
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will not cause serious problems, but these commands may give
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less-than-desirable results.
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@end defun
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@defun find-backup-file-name filename
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This function computes the file name for a new backup file for
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@var{filename}. It may also propose certain existing backup files for
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deletion. @code{find-backup-file-name} returns a list whose @sc{car} is
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the name for the new backup file and whose @sc{cdr} is a list of backup
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files whose deletion is proposed.
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Two variables, @code{kept-old-versions} and @code{kept-new-versions},
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determine which backup versions should be kept. This function keeps
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those versions by excluding them from the @sc{cdr} of the value.
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@xref{Numbered Backups}.
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In this example, the value says that @file{~rms/foo.~5~} is the name
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to use for the new backup file, and @file{~rms/foo.~3~} is an ``excess''
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version that the caller should consider deleting now.
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@smallexample
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@group
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(find-backup-file-name "~rms/foo")
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@result{} ("~rms/foo.~5~" "~rms/foo.~3~")
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@end group
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@end smallexample
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@end defun
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@c Emacs 19 feature
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@defun file-newest-backup filename
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This function returns the name of the most recent backup file for
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@var{filename}, or @code{nil} if that file has no backup files.
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Some file comparison commands use this function so that they can
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automatically compare a file with its most recent backup.
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@end defun
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@node Auto-Saving
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@section Auto-Saving
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@cindex auto-saving
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Emacs periodically saves all files that you are visiting; this is
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called @dfn{auto-saving}. Auto-saving prevents you from losing more
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than a limited amount of work if the system crashes. By default,
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auto-saves happen every 300 keystrokes, or after around 30 seconds of
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idle time. @xref{Auto-Save, Auto-Save, Auto-Saving: Protection Against
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Disasters, emacs, The GNU Emacs Manual}, for information on auto-save
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for users. Here we describe the functions used to implement auto-saving
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and the variables that control them.
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@defvar buffer-auto-save-file-name
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This buffer-local variable is the name of the file used for
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auto-saving the current buffer. It is @code{nil} if the buffer
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should not be auto-saved.
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@example
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@group
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buffer-auto-save-file-name
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@result{} "/xcssun/users/rms/lewis/#backups.texi#"
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@end group
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@end example
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@end defvar
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@deffn Command auto-save-mode arg
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When used interactively without an argument, this command is a toggle
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switch: it turns on auto-saving of the current buffer if it is off, and
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vice versa. With an argument @var{arg}, the command turns auto-saving
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on if the value of @var{arg} is @code{t}, a nonempty list, or a positive
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integer. Otherwise, it turns auto-saving off.
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@end deffn
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@defun auto-save-file-name-p filename
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This function returns a non-@code{nil} value if @var{filename} is a
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string that could be the name of an auto-save file. It assumes
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the usual naming convention for auto-save files: a name that
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begins and ends with hash marks (@samp{#}) is a possible auto-save file
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name. The argument @var{filename} should not contain a directory part.
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@example
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@group
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(make-auto-save-file-name)
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@result{} "/xcssun/users/rms/lewis/#backups.texi#"
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@end group
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@group
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(auto-save-file-name-p "#backups.texi#")
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@result{} 0
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@end group
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@group
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(auto-save-file-name-p "backups.texi")
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@result{} nil
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@end group
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@end example
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The standard definition of this function is as follows:
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@example
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@group
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(defun auto-save-file-name-p (filename)
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"Return non-nil if FILENAME can be yielded by..."
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(string-match "^#.*#$" filename))
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@end group
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@end example
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This function exists so that you can customize it if you wish to
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change the naming convention for auto-save files. If you redefine it,
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be sure to redefine the function @code{make-auto-save-file-name}
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correspondingly.
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@end defun
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@defun make-auto-save-file-name
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This function returns the file name to use for auto-saving the current
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buffer. This is just the file name with hash marks (@samp{#}) prepended
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and appended to it. This function does not look at the variable
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@code{auto-save-visited-file-name} (described below); callers of this
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function should check that variable first.
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@example
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@group
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(make-auto-save-file-name)
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@result{} "/xcssun/users/rms/lewis/#backups.texi#"
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@end group
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@end example
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The standard definition of this function is as follows:
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@example
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@group
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(defun make-auto-save-file-name ()
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"Return file name to use for auto-saves \
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of current buffer.."
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(if buffer-file-name
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@end group
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@group
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(concat
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(file-name-directory buffer-file-name)
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"#"
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(file-name-nondirectory buffer-file-name)
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"#")
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(expand-file-name
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(concat "#%" (buffer-name) "#"))))
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@end group
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@end example
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This exists as a separate function so that you can redefine it to
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customize the naming convention for auto-save files. Be sure to
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change @code{auto-save-file-name-p} in a corresponding way.
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@end defun
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@defvar auto-save-visited-file-name
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If this variable is non-@code{nil}, Emacs auto-saves buffers in
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the files they are visiting. That is, the auto-save is done in the same
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file that you are editing. Normally, this variable is @code{nil}, so
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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 new value does not take
|
|
effect in an existing buffer until the next time auto-save mode is
|
|
reenabled in it. 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 specifies how often to do auto-saving, in
|
|
terms of number of input events. Each time this many additional input
|
|
events are read, Emacs does auto-saving for all buffers in which that 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 does auto-saving for all buffers in which that is enabled. (If
|
|
the current buffer is large, the specified timeout is multiplied by a
|
|
factor that depends increases as the size increases; for a million-byte
|
|
buffer, the factor is almost 4.)
|
|
|
|
If the value is zero or nil, then auto-saving is not done as a result
|
|
of idleness, only after a certain number of input events
|
|
as specified by @code{auto-save-interval}.
|
|
@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 current-only
|
|
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.
|
|
|
|
If @var{current-only} is non-@code{nil}, only the current buffer
|
|
is auto-saved.
|
|
@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
|
|
|
|
@defvar buffer-saved-size
|
|
The value of this buffer-local variable is the length of the current
|
|
buffer, when it was last read in, saved, or auto-saved. This is
|
|
used to detect a substantial decrease in size, and turn off auto-saving
|
|
in response.
|
|
|
|
If it is @minus{}1, that means auto-saving is temporarily shut off in
|
|
this buffer due to a substantial decrease in size. Explicitly saving
|
|
the buffer stores a positive value in this variable, thus reenabling
|
|
auto-saving. Turning auto-save mode off or on also updates this
|
|
variable, so that the substantial decrease in size is forgotten.
|
|
@end defvar
|
|
|
|
@defvar auto-save-list-file-name
|
|
This variable (if non-@code{nil}) specifies a file for recording the
|
|
names of all the auto-save files. Each time Emacs does auto-saving, it
|
|
writes two lines into this file for each buffer that has auto-saving
|
|
enabled. The first line gives the name of the visited file (it's empty
|
|
if the buffer has none), and the second gives the name of the auto-save
|
|
file.
|
|
|
|
When Emacs exits normally, it deletes this file; if Emacs crashes, you
|
|
can look in the file to find all the auto-save files that might contain
|
|
work that was otherwise lost. The @code{recover-session} command uses
|
|
this file to find them.
|
|
|
|
The default name for this file specifies your home directory and starts
|
|
with @samp{.saves-}. It also contains the Emacs process @sc{id} and the
|
|
host name.
|
|
@end defvar
|
|
|
|
@defvar auto-save-list-file-prefix
|
|
@tindex auto-save-list-file-prefix
|
|
After Emacs reads your init file, it initializes
|
|
@code{auto-save-list-file-name} (if you have not already set it
|
|
non-@code{nil}) based on this prefix, adding the host name and process
|
|
ID. If you set this to @code{nil} in your init file, then Emacs does
|
|
not initialize @code{auto-save-list-file-name}.
|
|
@end defvar
|
|
|
|
@node Reverting
|
|
@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 ignore-auto 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.
|
|
|
|
By default, if the latest auto-save file is more recent than the visited
|
|
file, and the argument @var{ignore-auto} is @code{nil},
|
|
@code{revert-buffer} asks the user whether to use that auto-save
|
|
instead. When you invoke this command interactively, @var{ignore-auto}
|
|
is @code{t} if there is no numeric prefix argument; thus, the
|
|
interactive default is not to check the auto-save file.
|
|
|
|
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 the buffer
|
|
contents and the file contents are identical before the revert
|
|
operation, reverting preserves all the markers. If they are not
|
|
identical, reverting does change the buffer; in that case, it preserves
|
|
the markers in the unchanged text (if any) at the beginning and end of
|
|
the buffer. Preserving any additional markers would be problematical.
|
|
@end deffn
|
|
|
|
You can customize how @code{revert-buffer} does its work by setting
|
|
the variables described in the rest of this section.
|
|
|
|
@defvar revert-without-query
|
|
This variable holds a list of files that should be reverted without
|
|
query. The value is a list of regular expressions. If the visited file
|
|
name matches one of these regular expressions, and the file has changed
|
|
on disk but the buffer is not modified, then @code{revert-buffer}
|
|
reverts the file without asking the user for confirmation.
|
|
@end defvar
|
|
|
|
Some major modes customize @code{revert-buffer} by making
|
|
buffer-local bindings for these variables:
|
|
|
|
@defvar revert-buffer-function
|
|
The value of this variable is the function to use to revert this buffer.
|
|
If non-@code{nil}, it is called as a function with no arguments to do
|
|
the work of reverting. If the value is @code{nil}, reverting works the
|
|
usual way.
|
|
|
|
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, can 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}, specifies the function to use to
|
|
insert the updated contents when reverting this buffer. The function
|
|
receives two arguments: first the file name to use; second, @code{t} if
|
|
the user has asked to read the auto-save file.
|
|
|
|
The reason for a mode to set this variable instead of
|
|
@code{revert-buffer-function} is to avoid duplicating or replacing the
|
|
rest of what @code{revert-buffer} does: asking for confirmation,
|
|
clearing the undo list, deciding the proper major mode, and running the
|
|
hooks listed below.
|
|
@end defvar
|
|
|
|
@defvar before-revert-hook
|
|
This normal hook is run by @code{revert-buffer} before
|
|
inserting the modified contents---but only if
|
|
@code{revert-buffer-function} is @code{nil}.
|
|
@end defvar
|
|
|
|
@defvar after-revert-hook
|
|
This normal hook is run by @code{revert-buffer} after inserting
|
|
the modified contents---but only if @code{revert-buffer-function} is
|
|
@code{nil}.
|
|
@end defvar
|