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(emacs)Backup

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Backup Files
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   On most operating systems, rewriting a file automatically destroys
all record of what the file used to contain.  Thus, saving a file from
Emacs throws away the old contents of the file--or it would, except that
Emacs carefully copies the old contents to another file, called the
"backup" file, before actually saving.

   For most files, the variable `make-backup-files' determines whether
to make backup files.  On most operating systems, its default value is
`t', so that Emacs does write backup files.

   For files managed by a version control system (Note: Version
Control), the variable `vc-make-backup-files' determines whether to
make backup files.  By default it is `nil', since backup files are
redundant when you store all the previous versions in a version control
system.  Note: General VC Options.

   The default value of the `backup-enable-predicate' variable prevents
backup files being written for files in the directories used for
temporary files, specified by `temporary-file-directory' or
`small-temporary-file-directory'.

   At your option, Emacs can keep either a single backup file or a
series of numbered backup files for each file that you edit.

   Emacs makes a backup for a file only the first time the file is saved
from one buffer.  No matter how many times you save a file, its backup
file continues to contain the contents from before the file was visited.
Normally this means that the backup file contains the contents from
before the current editing session; however, if you kill the buffer and
then visit the file again, a new backup file will be made by the next
save.

   You can also explicitly request making another backup file from a
buffer even though it has already been saved at least once.  If you save
the buffer with `C-u C-x C-s', the version thus saved will be made into
a backup file if you save the buffer again.  `C-u C-u C-x C-s' saves
the buffer, but first makes the previous file contents into a new
backup file.  `C-u C-u C-u C-x C-s' does both things: it makes a backup
from the previous contents, and arranges to make another from the newly
saved contents if you save again.

Names

Backup Names. How backup files are named;

choosing single or numbered backup files.

Deletion

Backup Deletion. Emacs deletes excess numbered backups.

Copying

Backup Copying. Backups can be made by copying or renaming.