Not really visudo is only to edit the sudoers file. sudoedit is a better way to edit system files.
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Temporary copies are made of the files to be edited with the owner. set to the invoking user.
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The editor specified by the policy is run to edit the temporary files. The sudoers policy uses the SUDO_EDITOR, VISUAL and EDITOR environment variables (in that order). If none of SUDO_EDITOR, VISUAL or EDITOR are set, the first program listed in the editor sudoers(5) option is used.
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If they have been modified, the temporary files are copied back to their original location and the temporary versions are removed.