23.3.2 Backup Files
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 (see 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. See 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.
