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UPDATE controls which existing files in the destination are replaced. ’all’ is the default operation when an --update option is not specified, and results in all existing files in the destination being replaced. ’none’ is like the --no-clobber option, in that no files in the destination are replaced, and skipped files do not induce a failure. ’none-fail’ also ensures no files are replaced in the destination, but any skipped files are diagnosed and induce a failure. ’older’ is the default operation when --update is specified, and results in files being replaced if they’re older than the corresponding source file.