GNU Emacs Manual - Backup Copying

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#### Copying vs. Renaming

Backup files can be made by copying the old file or by renaming it. This makes a difference when the old file has multiple names. If the old file is renamed into the backup file, then the alternate names become names for the backup file. If the old file is copied instead, then the alternate names remain names for the file that you are editing, and the contents accessed by those names will be the new contents.

The method of making a backup file may also affect the file's owner and group. If copying is used, these do not change. If renaming is used, you become the file's owner, and the file's group becomes the default (different operating systems have different defaults for the group).

Having the owner change is usually a good idea, because then the owner always shows who last edited the file. Also, the owners of the backups show who produced those versions. Occasionally there is a file whose owner should not change; it is a good idea for such files to contain local variable lists to set `backup-by-copying-when-mismatch` locally (see section Local Variables in Files).

The choice of renaming or copying is controlled by three variables. Renaming is the default choice. If the variable `backup-by-copying` is non-`nil`, copying is used. Otherwise, if the variable `backup-by-copying-when-linked` is non-`nil`, then copying is used for files that have multiple names, but renaming may still be used when the file being edited has only one name. If the variable `backup-by-copying-when-mismatch` is non-`nil`, then copying is used if renaming would cause the file's owner or group to change. `backup-by-copying-when-mismatch` is `t` by default if you start Emacs as the superuser.

When a file is managed with a version control system (see section Version Control), Emacs does not normally make backups in the usual way for that file. But check-in and check-out are similar in some ways to making backups. One unfortunate similarity is that these operations typically break hard links, disconnecting the file name you visited from any alternate names for the same file. This has nothing to do with Emacs--the version control system does it.

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