GNU Emacs Manual - Backup Names

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#### Single or Numbered Backups

If you choose to have a single backup file (this is the default), the backup file's name is constructed by appending ``~'` to the file name being edited; thus, the backup file for ``eval.c'` would be ``eval.c~'`.

If you choose to have a series of numbered backup files, backup file names are made by appending ``.~'`, the number, and another ``~'` to the original file name. Thus, the backup files of ``eval.c'` would be called ``eval.c.~1~'`, ``eval.c.~2~'`, and so on, through names like ``eval.c.~259~'` and beyond.

If protection stops you from writing backup files under the usual names, the backup file is written as ``%backup%~'` in your home directory. Only one such file can exist, so only the most recently made such backup is available.

The choice of single backup or numbered backups is controlled by the variable `version-control`. Its possible values are

`t`
  Make numbered backups.
`nil`
  Make numbered backups for files that have numbered backups already. Otherwise, make single backups.
`never`
  Do not in any case make numbered backups; always make single backups.

You can set `version-control` locally in an individual buffer to control the making of backups for that buffer's file. For example, Rmail mode locally sets `version-control` to `never` to make sure that there is only one backup for an Rmail file. See section Local Variables.

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`.

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