I have a directory of images that might not have downloaded, and need to delete the duds.

This is too slow: Import /* ImageQ /@ fileNames

In bash, I use the file command to inspect the metadata of an image file's header, but ImageQ only works on in-memory images not paths or File objects.

Is there some way to do this efficiently, without importing them? There's a function for this I forgot.


FileFormat has a problem - it checks all known filetypes! e.g. you can't just check if the file is one of a list {"JPEG","PNG","TIFF"}.

  • $\begingroup$ If you don't want to import them, then all you have is the file's extension, right? Or what other property would you like to check without looking inside the file? $\endgroup$
    – MarcoB
    Aug 12, 2020 at 20:30
  • 1
    $\begingroup$ Make an image called "myfile.jpg" but then remove the extension. FileFormat["myfile"] will still return JPEG. Presumably it inspects the file's magic number FFD8FF and determines it looks like a JPEG. Of course it could be a coincidence or a corrupt file. Only parsing the headers would determine it was a valid JPEG file which is tantamount to Import-ing it $\endgroup$
    – flinty
    Aug 12, 2020 at 21:30
  • $\begingroup$ @flinty Yup FileFormat was the symbol I was forgetting, thanks! $\endgroup$
    – M.R.
    Aug 12, 2020 at 23:07
  • 1
    $\begingroup$ What is the problem in the update? Couldn't you just check MemberQ[{"JPEG", "PNG", "TIFF"}, FileFormat[f]] or is it too slow? $\endgroup$
    – flinty
    Aug 17, 2020 at 20:56
  • $\begingroup$ FileFormat checks all these: is a pdf, is it HTML, is it bson... That's inefficient $\endgroup$
    – M.R.
    Aug 17, 2020 at 22:31

1 Answer 1


Maybe this helps you already:

Lets take the filenames as a provided list as you suggest.

fileNames = {} (*your list here*)

First, lets time the code you provided:

AbsoluteTiming[Import /* ImageQ /@ fileNames]

{0.204095, {True, True, True, True}}

Speed up by a factor of 2 gives this code:

AbsoluteTiming[MemberQ[Import[#, "Elements"], "Image"] & /@ fileNames]

{0.0822613, {True, True, True, True}}

Speed up by a factor of 20 seems to give this:

Extensions = {‰PNG, ‰JPG} (*or other extensions*)
AbsoluteTiming[MemberQ[Extensions, First@ReadList[#, Expression, 1]] & /@ fileNames]

{0.00788099, {True, True, True, True}}

Note that the last code also works if you delete the .png extension from the file name. So "image.png" and "image" give both True.


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