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GNU Emacs Lisp Reference Manual

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26.1.1 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 ()
                      (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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