GNU Emacs Manual - Backup

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### 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 section 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 section VC Workfile Handling.

The default value of the `backup-enable-predicate` variable prevents backup files being written for files in ``/tmp'`.

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.

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