This has come up a few times so I thought I’d write it down so that Future Quinn™ could point folks at it.
If you have any questions or feedback, please start a new thread and tag it with Files and Storage so that I see it.
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—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"
Extended Attributes and Zip Archives
In a traditional Unix file system, a file consists of a flat sequence of bytes and a very limited set of metadata. In contrast, a traditional Mac OS file had a second sequence of bytes, the resource fork, and various other bits of Mac metadata. This caused an interoperability problem between Mac and Unix systems because the latter has no place to store the Mac metadata.
To resolve this problem Apple introduced the AppleDouble file format. This stores the Mac’s data fork in the Unix file and all the Mac metadata in a companion AppleDouble file. If the file name was foo, this companion file was called ._foo. The leading dot meant that this companion file was hidden by many tools.
macOS does not typically need AppleDouble files because Apple file systems, like APFS and HFS Plus, have built-in support for Mac metadata. However, macOS will create and use AppleDouble files when working with a volume format that doesn’t support Mac metadata, like FAT32.
Similarly macOS will create and use AppleDouble files when working with archive file formats, like zip, that don’t have native support for Mac metadata.
macOS 10.4 added support for extended attributes at the Unix layer. For more on this, see the getxattr man page. As with traditional Mac metadata, this is supported natively by Apple file systems but must be stored in an AppleDouble file elsewhere.
Note At the API level the original Mac metadata is modelled as two well-known extended attributes, XATTR_RESOURCEFORK_NAME and XATTR_FINDERINFO_NAME.
When creating a zip archive macOS supports two approaches for storing AppleDouble files:
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By default it stores the AppleDouble file next to the original file.
% ditto -c -k --keepParent root root-default.zip % unzip -t root-default.zip Archive: root-default.zip testing: root/ OK testing: root/nested/ OK testing: root/nested/FileWithMetadata OK testing: root/nested/._FileWithMetadata OK No errors detected in compressed data of root-default.zip. -
Alternatively, it can create a parallel hierarchy, rooted in
__MACOSX, that holds all AppleDouble files.% ditto -c -k --keepParent --sequesterRsrc root root-sequestered.zip % unzip -t root-sequestered.zip Archive: root-sequestered.zip testing: root/ OK testing: root/nested/ OK testing: root/nested/FileWithMetadata OK testing: __MACOSX/ OK testing: __MACOSX/root/ OK testing: __MACOSX/root/nested/ OK testing: __MACOSX/root/nested/._FileWithMetadata OK No errors detected in compressed data of root-sequestered.zip.
The latter is commonly used for mail attachments because it makes it easy for the recipient to discard all the Mac metadata.