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.
A 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.
save-buffer before
saving the buffer the first time.
nil, then
the backup file has been written. Otherwise, the file should be backed
up when it is next saved (if backups are enabled). This is a
permanent local; kill-local-variables does not alter it.
nil, then Emacs creates a backup of each file when it is
saved for the first time--provided that backup-inhibited
is nil (see below).
The following example shows how to change the make-backup-files
variable only in the Rmail buffers and not elsewhere. Setting it
nil stops Emacs from making backups of these files, which may
save disk space. (You would put this code in your `.emacs' file.)
(add-hook 'rmail-mode-hook
(function (lambda ()
(make-local-variable
'make-backup-files)
(setq make-backup-files nil))))
nil, backups are disabled for that file. Otherwise, the other
variables in this section say whether and how to make backups.
The default value is this:
(lambda (name)
(or (< (length name) 5)
(not (string-equal "/tmp/"
(substring name 0 5)))))
nil, backups are inhibited. It records
the result of testing 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. For example, VC sets this
variable non-nil to prevent making backups for files managed
with a version control system.
This is a permanent local, so that changing the major mode does not lose
its value. Major modes should not set this variable--they should set
make-backup-files instead.
There are two ways that Emacs can make a backup file:
The first method, renaming, is the default.
The variable backup-by-copying, if non-nil, says to use
the second method, which is to copy the original file and overwrite it
with the new buffer contents. The variable file-precious-flag,
if non-nil, also has this effect (as a sideline of its main
significance). See section Saving Buffers.
nil, Emacs always makes backup files by
copying.
The following two variables, when non-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.
nil, Emacs makes backups by copying for
files with multiple names (hard links).
This variable is significant only if backup-by-copying is
nil, since copying is always used when that variable is
non-nil.
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 backup-by-copying is
nil, since copying is always used when that variable is
non-nil.
If a file's name is `foo', the names of its numbered backup versions are `foo.~v~', for various integers v, like this: `foo.~1~', `foo.~2~', `foo.~3~', ..., `foo.~259~', and so on.
nil
never
The use of numbered backups ultimately leads to a large number of backup versions, which must then be deleted. Emacs can do this automatically or it can ask the user whether to delete them.
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 excess. The function find-backup-file-name
(see section Naming Backup Files) is responsible for determining which backup
versions to delete, but does not delete them itself.
nil, then saving a file deletes excess
backup versions silently. Otherwise, it asks the user whether to delete
them.
dired-clean-directory). That's the
same thing kept-new-versions specifies when you make a new backup
file. The default value is 2.
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.
nil value if filename is a
possible name for a backup file. A file with the name filename
need not exist; the function just checks the name.
(backup-file-name-p "foo")
=> nil
(backup-file-name-p "foo~")
=> 3
The standard definition of this function is as follows:
(defun backup-file-name-p (file) "Return non-nil if FILE is a backup file \ name (numeric or not)..." (string-match "~$" file))
Thus, the function returns a non-nil value if the file name ends
with a `~'. (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.
The standard definition of this function, on most operating systems, is as follows:
(defun make-backup-file-name (file)
"Create the non-numeric backup file name for FILE@enddots{}"
(concat file "~"))
You can change the backup-file naming convention by redefining this
function. The following example redefines make-backup-file-name
to prepend a `.' in addition to appending a tilde:
(defun make-backup-file-name (filename)
(expand-file-name
(concat "." (file-name-nondirectory filename) "~")
(file-name-directory filename)))
(make-backup-file-name "backups.texi")
=> ".backups.texi~"
Some parts of Emacs, including some Dired commands, assume that backup file names end with `~'. If you do not follow that convention, it will not cause serious problems, but these commands may give less-than-desirable results.
find-backup-file-name returns a list whose CAR is
the name for the new backup file and whose CDR is a list of backup
files whose deletion is proposed.
Two variables, kept-old-versions and kept-new-versions,
determine which backup versions should be kept. This function keeps
those versions by excluding them from the CDR of the value.
See section Making and Deleting Numbered Backup Files.
In this example, the value says that `~rms/foo.~5~' is the name to use for the new backup file, and `~rms/foo.~3~' is an "excess" version that the caller should consider deleting now.
(find-backup-file-name "~rms/foo")
=> ("~rms/foo.~5~" "~rms/foo.~3~")
nil if that file has no backup files.
Some file comparison commands use this function so that they can automatically compare a file with its most recent backup.
Emacs periodically saves all files that you are visiting; this is called 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. See section `Auto-Saving: Protection Against Disasters' in 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.
nil if the buffer
should not be auto-saved.
buffer-auto-save-file-name => "/xcssun/users/rms/lewis/#files.texi#"
t, a nonempty list, or a positive
integer. Otherwise, it turns auto-saving off.
nil value if 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 (`#') is a possible auto-save file
name. The argument filename should not contain a directory part.
(make-auto-save-file-name)
=> "/xcssun/users/rms/lewis/#files.texi#"
(auto-save-file-name-p "#files.texi#")
=> 0
(auto-save-file-name-p "files.texi")
=> nil
The standard definition of this function is as follows:
(defun auto-save-file-name-p (filename) "Return non-nil if FILENAME can be yielded by..." (string-match "^#.*#$" filename))
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 make-auto-save-file-name
correspondingly.
auto-save-visited-file-name (described below); you should check
that before calling this function.
(make-auto-save-file-name)
=> "/xcssun/users/rms/lewis/#backup.texi#"
The standard definition of this function is as follows:
(defun make-auto-save-file-name ()
"Return file name to use for auto-saves \
of current buffer@enddots{}"
(if buffer-file-name
(concat
(file-name-directory buffer-file-name)
"#"
(file-name-nondirectory buffer-file-name)
"#")
(expand-file-name
(concat "#%" (buffer-name) "#"))))
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 auto-save-file-name-p in a corresponding way.
nil, Emacs auto-saves buffers in
the files they are visiting. That is, the auto-save is done in the same
file that you are editing. Normally, this variable is nil, so
auto-save files have distinct names that are created by
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 auto-save-mode is called again.
t if the current buffer has been
auto-saved since the last time it was read in or saved.
nil.
nil, buffers that are visiting files
have auto-saving enabled by default. Otherwise, they do not.
Normally, if any buffers are auto-saved, a message that says
`Auto-saving...' is displayed in the echo area while auto-saving is
going on. However, if no-message is non-nil, the message
is inhibited.
If current-only is non-nil, only the current buffer
is auto-saved.
delete-auto-save-files is non-nil. It is called every
time a buffer is saved.
delete-auto-save-file-if-necessary. If it is non-nil,
Emacs deletes auto-save files when a true save is done (in the visited
file). This saves disk space and unclutters your directory.
If it is -1, that means auto-saving is temporarily shut off in this buffer due to a substantial deletion. Explicitly saving the buffer stores a positive value in this variable, thus reenabling auto-saving. Turning auto-save mode off or on also alters this variable.
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.
If 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 recover-session command uses
these files.
The default name for this file is in your home directory and starts with `.saves-'. It also contains the Emacs process ID and the host name.
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 revert-buffer command. See section `Reverting a Buffer' in The GNU Emacs Manual.
By default, if the latest auto-save file is more recent than the visited
file, revert-buffer asks the user whether to use that instead.
But if the argument ignore-auto is non-nil, then only the
the visited file itself is used. Interactively, ignore-auto is
t unless there is a numeric prefix argument; thus, the
interactive default is to check the auto-save file.
Normally, revert-buffer asks for confirmation before it changes
the buffer; but if the argument noconfirm is non-nil,
revert-buffer does not ask for confirmation.
Reverting tries to preserve marker positions in the buffer by using the
replacement feature of 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; then 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.
You can customize how revert-buffer does its work by setting
these variables--typically, as buffer-local variables.
revert-buffer
reverts the file without asking the user for confirmation, if the file
has changed on disk and the buffer is not modified.
nil, it is called as a function with no arguments to do
the work of reverting. If the value is 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, give this variable a buffer-local value that is a function to regenerate the contents.
nil, is 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, t if
the user has asked to read the auto-save file.
revert-buffer before actually
inserting the modified contents--but only if
revert-buffer-function is nil.
Font Lock mode uses this hook to record that the buffer contents are no longer fontified.
revert-buffer after actually inserting
the modified contents--but only if revert-buffer-function is
nil.
Font Lock mode uses this hook to recompute the fonts for the updated buffer contents.
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