Converting Unix executable photo files

One can obtain a reasonable idea (before guessing) of what the UNIX Executable file format is by its byte signature as illustrated in a list of file signatures. One checks the file for its byte signature like so:

xxd -u -c8 -l8 -g1 filename
00000000: 89 50 4E 47 0D 0A 1A 0A  .PNG....

xxd -u -c8 -l8 -g1 filename
00000000: FF D8 FF E0 00 10 4A 46  ......JF (jpg)

xxd -u -c8 -l8 -g1 filename
00000000: 00 00 00 0C 6A 50 20 20  ....jP (jp2)

xxd -u -c8 -l8 -g1 filename
00000000: 4D 4D 00 2A 00 75 30 08  MM.*.u0. (tif)



FibreMan_85_125 wrote:

Is there a batch process to restore the jpg file extension?

If all the files are using the same internal format (see above) and can be bulk-selected in a Finder search-and-rename, sure. (info)

Otherwise, writing a shell script that uses find (or maybe mdfind) and calls the file magic command for each file (see above) and processes and renames the files accordingly is very likely feasible.

While news to me, it seems the oft-useful exiftool has a mechanism for this, too: https://superuser.com/a/1125652

A few dozen files probably isn’t worth custom automation, though. Not past a Finder bulk rename, or exiftool, etc. Or just renaming the files manually.

Is there a way to prevent this from reoccuring?

This mess should not happen in general, but—given the varied potential causes discussed above, and given the surprising design and coding practices of some apps—is probably not possible to completely prevent this.

App developers have occasionally been botching basic file operations and variously violating common expectations and practices for aeons, too.

You can also ask the app developers to use libmagic or such as part of their file open validation, too.

Be sure that you have the files on a disk that is properly formatted for Mac and not for Windows.

Append .jpg to the file name

Open in Preview

That will normally only work if the file actually is a JPEG to start with. If it's a TIFF or other raster format and you add .jpg to the end, that won't make it a JPEG. It will still be a TIFF or other file type with an incorrect extension.

However, I did test this just now. Photoshop does exactly what you would expect, and can't figure out how to open a .tif I intentionally misnamed as a .jpg file.

Affinity Designer, the same thing. Also as expected since the file is not actually a .jpg.

However, Preview must have simply ignored the extension and figured out what file type is what by the header information, and opened it anyway.

That usually means a file system or file name corruption of some sort. If the image files involved are undamaged and the sole issue is the file type, then renaming the file in Finder or at the command line is enough.

Keith Barkley wrote:

I thought it happened when the extension is stripped. Since the Mac no longer has creator codes, it has no way of knowing what kind of file it is, so it makes generically makes it a unix executable.

Yep. That’s one of the file name corruptions, yes. I’ve seen similar messes caused by various issues and file name and file system corruptions, by default app changes, by missing or deprecated apps (e.g. some old 32-bit photo app gets vaporized by a macOS upgrade), and by missing or renamed file types. There have been several app packages that did their own and creative things with their image file names, seemingly trying to keep the images tied to the vendor’s apps.

There have been several app packages that did their own and creative things with their image file names, seemingly trying to keep the images tied to the vendor’s apps.

That was an issue (of sorts) with EPS files back in the OS 9 and earlier world. Unlike Windows, where only one app could be associated with the .eps extension, no matter which app actually created it, at least the Mac had type and creator codes.

On all of them, the type was EPSF, but apps and Mac OS were still able to keep track of who owned which EPS by the creator. 8*** for Photoshop, ART5 for Illustrator, another for Quark XPress (which I can't recall right now). And so on.

But without a file extension, and the current Mac OS no longer paying attention to type/creator codes, the OS no longer knows what to do with them, so they just get tagged as a generic UNIX file.

Along with MrHoffman's links, a while back I put together a tiny compiled AppleScript that will tell you what the Type and Creator codes are for a file.

https://www.macupdate.com/app/mac/62161/typecreator

Run that on any of these mislabeled Unix files and (if it has them) it will tell you what the file is. Such as, anything with 8*** as the creator is a Photoshop file. The type should then be readily apparent JPEG, TIFF, EPFS, etc.

I thought it happened when the extension is stripped. Since the Mac no longer has creator codes, it has no way of knowing what kind of file it is, so it makes generically makes it a unix executable.

My reference to “creative” includes using no extension and a directory full of images, and includes apps that are using magic to identify files. Apple deprecated their own creator codes stuff a while back (older OSX, IIRC) but magic is still around (see man file), and app implementors will probably want to use the BSD magic version if implementing an app with magic. BSD rewrote and hardened their magic a while back. I’ve written various apps that auto-adapt to what gets opened, even if it is misnamed.

Kind of makes you wonder why more apps don't use that function. But then, places like Adobe and Affinity probably don't like the idea of extensions being meaningless. Especially if you send those files to a Windows user, where they will definitely be flagged as being unable to open if you did something like name a .png as a .tif.

Kurt Lang wrote:

Kind of makes you wonder why more apps don't use that function…

We can continue to chat about this over in the lounge, if you’re interested.

This ties into other app and system development topics, and support-related topics.

A few dozen .jpg files from ca. 2003 - 2004 have lost the file extension, but many others - older and newer -have not. They were screen captures, re-arranged and finesses by GraphicConverter. All remained on my main mac computer, though it and the OS were updated.

Last they were used was probably ~ 2018 when they worked fine, but being used sporadically, I don't know when that changed.

Two questions:

Is there a batch process to restore the jpg file extension?

and

Is there a way to prevent this from reoccuring?

Call me "an unidentified developer" 😉

MrHoffman wrote:

…

App developers have occasionally been botching basic file operations and variously violating common expectations and practices for aeons, too.

You can also ask the app developers to use libmagic or such as part of their file open validation, too.

There are reasons to not do this recognition, too. For some contexts, polyglot files — files that present as two (or more) different formats — can be a bit of a nightmare for some apps and for security. The oldest I’ve met (and one of many) was the so-called gifar file, which is both a gif image file and a Java jar file. Together. Websites might not mind somebody uploading a probably-harmless gif file, but probably don’t want somebody uploading a jar (executable) file.

Related:

… https://github.com/ansemjo/truepolyglot

… https://arxiv.org/abs/2203.07561

Ange Albertini has numerous clever polyglot file combinations, and has posters of common file formats, too. Apropos this discussion, here is the jpeg poster from that collection:

Messrs Hoffman and Lang; I appreciate your helpfulness!

Finder Rename tool worked perfectly... I think*. It's amazing how many things I either forgot or never knew as macOS evolved. Each release should have a study guide and quiz required to pass before being allowed to install.

*The files were in a Finder search window for 'unix', but they originated from several different folders. I wanted to check them to be sure they all worked, but after select all—right click—add '.jpg', they (of course) disappeared one by one. 😆

I'm not sure how to find them! A Finder search has nothing with changed date more recent than 2 days ago. Huh?

I appreciate your helpfulness in the manner you've learned and teach, but I prefer to stay out of the nuts and bolts. Thanks again!

FibreMan_85_125 wrote:

…

I'm not sure how to find them! A Finder search has nothing with changed date more recent than 2 days ago. Huh?

I appreciate your helpfulness in the manner you've learned and teach, but I prefer to stay out of the nuts and bolts. Thanks again!

Yet you simultaneously ask another involves-the-nuts-and-bolts question. 😉🤔🤭😉

I’d expect Spotlight will catch up with the name change shortly.

Everybody’s favorite “mds” processes are undoubtedly now active and are updating the Spotlight database.