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(emacs)Backup Names

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

   You can change this behavior by defining the variable
`make-backup-file-name-function' to a suitable function.  Alternatively
you can customize the variable `backup-directory-alist' to specify that
files matching certain patterns should be backed up in specific
directories.

   A typical use is to add an element `("." . DIR)' to make all backups
in the directory with absolute name DIR; Emacs modifies the backup file
names to avoid clashes between files with the same names originating in
different directories.  Alternatively, adding, say, `("." . ".~")'
would make backups in the invisible subdirectory `.~' of the original
file's directory.  Emacs creates the directory, if necessary, to make
the backup.

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

   If you choose to have a series of numbered backup files, 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 `backup-directory-alist' applies to numbered
backups just as usual.

   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'
     Never 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.  Note: Locals.

   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'.