Compress using ditto vs zip

In versions of macOS up until 10.4 the zip command did not support resource forks at all. Which always was a bit strange given they are a remnant of the old Apple’s classic Mac OS. It was a feature of the MFS and HFS file system that allows storing meta data along side to files.

In order to zip up folders containing all data, you were meant to use ditto instead. Which is what the Finder uses to create archives, too.

ditto -c -k -X --rsrc some_folder some_folder.zip

Times have changed slightly though. The standard zip now does support resource forks and copying resource forks is now the default behavior for ditto. If you want to avoid copying the additional metadata --norsrc is your friend.

--rsrc

  Preserve resource forks and HFS meta-data. ditto will
  store this data in Carbon-compatible ._ AppleDouble
  files on filesystems that do not natively support
  resource forks. As of Mac OS X 10.4,
  --rsrc is default behavior.

--norsrc

  Do not preserve resource forks and HFS meta-data.
  If both --norsrc and --rsrc are passed, whichever
  is passed last will take precedence. Both options
  override DITTONORSRC. Unless explicitly specified,
  --norsrc also implies --noextattr and --noacl to
  match the behavior of Mac OS X 10.4.

And the idea of resource forks did not die with the introduction of APFS. APFS also supports, what is now called extended attributes. If you ever wondered how macOS remembers when you downloaded a file from the internet - that’s how. And Finder makes use of extended attributes to store things like tags for example. They are the quasi equivalent of resource forks in this new brave world.

The command line tool xattr can be used to read and write the attributes.

$ echo "foo" > foo
$ xattr -w attr_name attr_value foo
$ xattr -p attr_name foo

Checking the behaviour of zip and ditto on APFS, the special ._foo file (which holds the extended attributes data) is unfortunately only included in the zip created by ditto.

$ zip foo-zip.zip foo
$ ditto -ck foo foo-ditto.zip
% unzip -l foo-ditto.zip
Archive:  foo-ditto.zip
  Length      Date    Time    Name
---------  ---------- -----   ----
      408  01-17-2020 00:41   foo
      154  01-17-2020 00:41   ._foo
---------                     -------
      562                     2 files

In the end there are now resources forks, extended attributes and a new file system. And still you should use ditto over zip to create an archive to include all meta data. Somehow this feels like history repeating.

TLTR: Just keep using ditto.