GNU Info
(elisp)Making Backups
Making Backup Files
-------------------
- Function: backup-buffer
This function makes a backup of the file visited by the current
buffer, if appropriate. It is called by `save-buffer' before
saving the buffer the first time.
- Variable: 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-`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-all-local-variables' does not alter it.
- User Option: make-backup-files
This variable determines whether or not to make backup files. If
it is non-`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 init file.)
(add-hook 'rmail-mode-hook
(function (lambda ()
(make-local-variable
'make-backup-files)
(setq make-backup-files nil))))
- Variable: backup-enable-predicate
This variable's value is a function to be called on certain
occasions to decide whether a file should have backup files. The
function receives one argument, a file name to consider. If the
function returns `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 `normal-backup-enable-predicate', which checks
for files in `temporary-file-directory' and
`small-temporary-file-directory'.
- Variable: backup-inhibited
If this variable is non-`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.
- Variable: backup-directory-alist
This variable's value is an alist of filename patterns and backup
directory names. Each element looks like
(REGEXP . DIRECTORY)
Backups of files with names matching REGEXP will be made in
DIRECTORY. DIRECTORY may be relative or absolute. If it is
absolute, so that all matching files are backed up into the same
directory, the file names in this directory will be the full name
of the file backed up with all directory separators changed to `!'
to prevent clashes. This will not work correctly if your
filesystem truncates the resulting name.
For the common case of all backups going into one directory, the
alist should contain a single element pairing `"."' with the
appropriate directory name.
If this variable is `nil', or it fails to match a filename, the
backup is made in the original file's directory.
On MS-DOS filesystems without long names this variable is always
ignored.
- Variable: make-backup-file-name-function
This variable's value is a function to use for making backups
instead of the default `make-backup-file-name'. A value of nil
gives the default `make-backup-file-name' behaviour.
This could be buffer-local to do something special for specific
files. If you define it, you may need to change
`backup-file-name-p' and `file-name-sans-versions' too.
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