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I'm cleaning up some files that have been handled by Mac OS X. This is also the destination for FTP transfers. I know about .DS_Store files. However I'm seeing some ._XXXX files, ie. files that are prefixed with a dot and an underscore. Is that some sort of Mac OS X backup / transfer file? Where would they come from?

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Yes, this is an OSX thing and is related to the AppleDouble File Format. When OSX writes to a non-native file system (so not HFS), that does not support resource forks, it writes extended info such as finder information in a "._" hidden file.

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Apple has a page about this : http://support.apple.com/kb/TA20578

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If the files are visible on a Mac, the dot_clean command line tool might help. There is a man page.

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Those files are indeed from Mac OS X. I imagine they're a metadata file of some kind, but I don't know what they do. I do know, however, that they can be successfully deleted without any negative repercussions .

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It's not always safe to delete the AppleDouble (._) files -- they contain, among other things, the resource fork and extended attributes of the original file. Apple has deprecated resource forks, but they're still used for some things (e.g. aliases and .webloc files store their target info in the resource fork). Extended attributes, on the other hand, seem to be getting more and more important... – Gordon Davisson Jun 29 '09 at 18:39
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