Extended attributes – xattrs – are almost completely hidden from the user’s eye, yet macOS uses them a lot, and for a wide range of purposes. The most obvious is Gatekeeper’s system of quarantine: download an app, and the first time that you run it, Gatekeeper will perform a full check of its signature. The macOS quarantine system uses a special xattr, which is automatically attached to downloaded files.
Making a screenshot and saving it as a JPEG document using GraphicConverter attaches a total of eight xattrs to the file, including one for quarantine, three different openmetainfo xattrs, Finder info, and two custom metadata info xattrs. Unless you go browsing with the Terminal command
xattr, you’ll never see any of these.
xattred is a lightweight extended attribute viewer and editor, which among its other features can generate authentic quarantine xattrs and attach them to apps, making it appear as if they have been freshly downloaded. The latter is useful to app developers, so that they can test that their apps pass Gatekeeper’s full checks.
If you wanted to, you could attach copyright information or other text metadata to files and folders using xattred.
I have just started a major overhaul of xattred, to give it a much better interface. This alpha release now includes a TableView listing of all xattrs for a given file or folder, and their size in bytes. It is much easier to use, as you only select a file or folder once to inspect and perform other actions on it.
The latest version is available from the Downloads menu item above.