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What are these __MACOSX folders I keep seeing in zip files made by people on OSX? Some take as much as 30% of the file.

What program are producing these __MACOSX folder and how can mac users avoid this mistake?

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    They are super-irritating, yes, and usually pointless as the resource forks are so often empty. But at least they're harmless, unlike the non-standard approach Apple have taken to >4GB archive sizes with the built-in OS X stuff, which will confuse any other tool and break again for sufficiently large files. And hey, it could be worse, it could be storing two copies of each file with the same name, one for the data and one for the resource fork, often making it impossible to access either, like pre-OSX Mac used to. Oh Apple, why do you hate standard file formats? – bobince Feb 3 '10 at 23:01
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    @bobince: actually, resource forks were a very good idea ... at the time. These days, the same effect is achieved by storing resources as individual files, most of which look pretty much like standard file formats. – Duncan Feb 3 '10 at 23:20
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    Nothing wrong with metadata as such, it's just that Apple have such a knack of making up their own formats and messing up existing formats with gratuitously incompatible extensions! Having the content-type data as metadata is in itself a great thing and it saddens me that OS X is moving towards the Windows hack of file extensions as an alternative. Although this isn't as bad as on Linux, where the filesystem supports storing Content-Type metadata, but no desktops use it, preferring a thoroughly broken mixture of file-extension/name-patterns and content-sniffing (urgh!). Sigh, OSes eh? – bobince Feb 3 '10 at 23:33
  • @bobince: But yes, at least the format they made up for this does not do any real harm, other than slightly cluttering directory listings and wasting essentially 1 inode and 1 block per empty resource fork extracted, unless you use something like NTFS (which will store the file contents in the MFT for such small files), in which case it just wastes the "inode" (MFT entry). – SamB Feb 15 '11 at 20:52
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    Can be fixed after the fact by zip -d filename.zip __MACOSX/\* – Chris Johnson Dec 2 '12 at 1:31
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Apple provides built-in capability to ZIP files in OS X 10.3 and higher, and these files are the result of Apple storing Resource Forks safe manner. You would never see these files running OS X 10.3 or higher, but since Windows and other operating systems do not understand this special form of Resource Forks they will appear as you see them.

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    It's not just resource forks, anything beyond basic file contents gets put in the AppleDouble file. Apple's moving away from resource forks, but toward things like extended attributes that'll also get stored in the AppleDouble container. – Gordon Davisson Feb 5 '10 at 2:02
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    You make it sound like a feature. .zip files are intentionally lacking in the metadata department. If you want metadata, use a different format, not a Mac. Example of proper zip + metadata implementation: .jar – Zenexer Aug 22 '13 at 4:11
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    dead link, please fix. – Lucas Feb 2 '14 at 3:24
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    "since Windows and other operating systems do not understand"--Ugh. I just hate this kind of terminology – Joe Plante Apr 23 '14 at 2:38
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    Just discovered: if you're on a Mac, using the command line, unzip filename.zip will unpack the __MACOSX/ directory, which you don't want, but open filename.zip will do the right thing. – Edward Falk Jun 22 '16 at 18:40
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Here's a link that explains it pretty well. I suppose it is a bit late to help Yada, but for posterity.

Explanation of resource fork at Wikipedia

The rest is my opinion:

@nickf: Never seeing these files is not a FEATURE of those OS X versions it is a FLAW.

People produce data, wrap it up, store it on different mediums and so on. They need to know what is needed or what is not needed. Hiding it keeps them in the dark.

The age old bad idea of hiding things from users:

A programmer, concerned with expediency of accomplishing his/her own work, abuses something in the domain of the end user, to make it easy for himself/herself.

In this case he/she stored meta data in the user's data space, he/she then hid it from the user. He/she missed the big picture: The user won't become aware of the hidden details. When he/she packages his/her data and ships it somewhere unanticipated by the programmer, missing parts won't get shipped or unknown parts will arrive which neither the user nor the recipient can explain.

Hiding things from the user is bad.

It assumes the user is stupid, when more accurately it is the programmer being stupid, or lazy.

To be clear, this bad habit is not confined to MAC. It is everywhere. It's a consequence of programmers falling in love with their own schemes and vendors prioritizing their own goals ahead of the needs of the end user.

In brief.

__MACOSX:
weird smelling programmer droppings emerging from under the rug where they were swept.

Programmers and vendors: Please keep things in the open. When you hide them, you make yourself stupid and the user uninformed.

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    This answer would be better if it was just an answer. The extended ranting about stupid programmers and lazy users detracts from it. – Kristopher Johnson Jun 19 '15 at 13:15
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    @pbernatchez Actually, if you stopped to learn about how things looked like and how they evolved, probably you wouldn't leave here your own ranty, arrogant "droppings". Resource forks were a (really nice!) implementation detail pre-Mac OS X and users only found them when interfacing with other systems; since the 90s, Apple has increasingly moved to a world without resource forks, but still bending backwards to keep compatibility. "Store metadata in the user's data space"? WTF? How twistedly do you have to define "metadata" and "user's data space" to say that? You must hate filesystems! – hmijail mourns resignees Jan 20 '17 at 12:28
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    "Hiding things from the user is bad". Man, you must also hate all kind of user interfaces, and libraries, and abstraction, and... well, any kind of programming and computers more advanced than an abacus. – hmijail mourns resignees Jan 20 '17 at 12:39
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    This does not answer the question. After reading this answer I still have no idea what metadata is contained in the _MACOSX folder, or what resource forks are. I guess people are upvoting this just because they agree with the rant? – Atte Juvonen Feb 15 '17 at 13:25
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    While I agree that the emotionally-charged tone is not helpful, I also absolutely agree that creating hidden things that are occasionally needed is a really bad idea, precisely as stated in this answer: users will either (a) not ship hidden files that are needed, or (b) ship hidden files that the recipient can see and neither understand. These files should either be visible, or the data should be embedded in the files in some manner that is flagged as "skippable" by other OS's. Hidden files and directories should be used primarily to protect users, not keep them in the dark. – Mike Williamson Oct 25 '18 at 1:47
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To answer your final question:

how can mac users avoid this mistake?

macOS users can install a 3rd-party archiving utility like Keka, then tell it to not use Resource Forks, then set it as the default compressor.

How to do this with Keka

Tell Keka to not use Resource Forks

  1. Open Keka without a file (From Launchpad, Spotlight, etc.)
  2. Press ⌘ Cmd+, to open Preferences
  3. Select the Compression tab
    Keka "Compression" tab, selected
  4. Check "Exclude Mac resource forks (eg: .DS_Store)"
    A checkbox reading "Exclude Mac resource forks (ex: .DS_Store)"

Make Keka the default compressor

  1. In the same Keka Preferences window
  2. Select the General tab
    Keka "General" tab, selected
  3. Click "Set Keka as default compressor/uncompressor" [sic]
    enter image description here
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    This could be improved my adding some information on the last 2 steps, "tell it to not use Resource Forks, then set it as the default compressor." – Steven C. Howell Feb 16 '16 at 21:17
  • @stvn66 Done! Just FYI, though, that's usually outside the scope of these questions, which is why I didn't at first. – Ben Leggiero Feb 16 '16 at 21:52
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    Downvoting, sorry. I'm not a fan of installing 3rd-party software to fix a problem that can be fixed with the default software. As Chris Johnson pointed out above, zip -d will remove the resource forks from a zipfile. In fact, I think if you use zip in the first place, the resource forks don't get added in the first place. – Edward Falk Jun 22 '16 at 18:44
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    @EdwardFalk That's fair! This answer was meant to solve the "How do I get it to always behave this way?" problem, not the "How do I get it to behave this way once?" one. – Ben Leggiero Nov 9 '16 at 14:24

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