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I've read through man pngcrush and it seems that there is no way to crush a PNG file and save it over the original. I want to compress several folders worth of PNGs so it would be useful to do it all with one command!

Currently I am doing pngcrush -q -d tmp *.png then manually cut-pasting the files from the tmp directory to the original folder. So I guess using mv might be the best way to go? Any better ideas?

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up vote 16 down vote accepted

(All one line:)

for file in *.png ; do pngcrush "$file" "${file%.png}-crushed.png" && mv "${file%.png}-crushed.png" "$file" ; done

should do it.

(THough so far in my own tests, less than half of the pngs I tested pngcrush on were smaller afterwards, so color me unimpressed.)

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Thanks! The amount you can compress the PNGs can depend on how they were made. I believe Photoshop's "Save for Web" has some form of PNG crusher built in. – DisgruntledGoat Feb 17 '11 at 10:40
Most of mine were created with ImageMagick's import command. I avoid commercial software like Photoshop whenever possible. – frabjous Feb 17 '11 at 17:45
use -brute for better compression. – Collin Anderson Aug 7 '12 at 3:00
The other answer is now better with newer pngcrush. – Hugo Oct 19 '15 at 7:58

Since version 1.7.22, pngcrush has an overwrite option.


pngcrush -ow file.png

See Changelog for more information:

Version 1.7.22  (built with libpng-1.5.6 and zlib-1.2.5)
  Added "-ow" (overwrite) option.  The input file is overwritten and the
    output file is just used temporarily and removed after it is copied
    over the input file..  If you do not specify an output file, "pngout.png"
    is used as the temporary file. Caution: the temporary file must be on
    the same filesystem as the input file.  Contributed by a group of students
    of the University of Paris who were taking the "Understanding of Programs"
    course and wished to gain familiarity with an open-source program.
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