18.3.2.1 Single or Numbered Backups

When Emacs makes a backup file, its name is normally constructed by appending ‘ ~’ to the file name being edited; thus, the backup file for eval.c would be eval.c~.

If access control stops Emacs from writing backup files under the usual names, it writes the backup file as ~/.emacs.d/%backup%~. Only one such file can exist, so only the most recently made such backup is available.

Emacs can also make numbered backup files. Numbered backup file names contain ‘ .~’, the number, and another ‘ ~’ after the original file name. Thus, the backup files of eval.c would be called eval.c.~1~, eval.c.~2~, and so on, all the way through names like eval.c.~259~ and beyond.

The variable version-control determines whether to make single backup files or multiple numbered backup files. Its possible values are:

nil

Make numbered backups for files that have numbered backups already. Otherwise, make single backups. This is the default.

t

Make numbered backups.

never

Never make numbered backups; always make single backups.

The usual way to set this variable is globally, through your init file or the customization buffer. However, you can set version-control locally in an individual buffer to control the making of backups for that buffer’s file (see Local Variables). You can have Emacs set version-control locally whenever you visit a given file (see Local Variables in Files). Some modes, such as Rmail mode, set this variable.

If you set the environment variable VERSION_CONTROL, to tell various GNU utilities what to do with backup files, Emacs also obeys the environment variable by setting the Lisp variable version-control accordingly at startup. If the environment variable’s value is ‘ t’ or ‘ numbered’, then version-control becomes t; if the value is ‘ nil’ or ‘ existing’, then version-control becomes nil; if it is ‘ never’ or ‘ simple’, then version-control becomes never.

If you set the variable make-backup-file-name-function to a suitable Lisp function, you can override the usual way Emacs constructs backup file names.