23.3.2.2 Single or Numbered Backups

When Emacs makes a single 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~’.

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.