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.
- Names: How backup files are named;
choosing single or numbered backup files.
- Deletion: Emacs deletes excess numbered backups.
- Copying: Backups can be made by copying or renaming.
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