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Copying vs. Renaming
You can make backup files by copying the old file or by renaming it.
This makes a difference when the old file has multiple names. If you
rename the old file into the backup file, the alternate names become
names for the backup file. If you copy the old file instead, the
alternate names remain names for the file that you are editing, and the
contents accessed by those names will be the new contents.
How you make a backup file may also affect the file's owner and
group. If you use copying, they 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 is always the person who last edited the file. Occasionally
there is a file whose owner should not change. Since most files should
change owners, it is a good idea to use local variable lists to set
`backup-by-copying-when-mismatch' for the special cases where the owner
should not change (Note: File Variables.).
Three variables control the choice of renaming or copying.
Normally, renaming is done. If the variable `backup-by-copying' is
non-`nil', copying is used. Otherwise, if the variable
`backup-by-copying-when-linked' is non-`nil', copying is done for files
that have multiple names, but renaming may still be done when the file
being edited has only one name. If the variable
`backup-by-copying-when-mismatch' is non-`nil', copying is done if
renaming would cause the file's owner or group to change.
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