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Does anyone know a good way to batch-convert a bunch of PNGs into JPGs in linux? (I'm using Ubuntu).

A png2jpg binary that I could just drop into a shell script would be ideal.

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7 Answers 7

up vote 121 down vote accepted

Your best bet would be to use Imagemagick

I am not an expert in the actual usage, but I know you can pretty much do anything image related with this!

An example is:

convert image.png image.jpg

and it will keep the original as well as creating the converted image. As for batch. I think you need to use the Mogrify tool (from the same command line when in imagemagick). Keep in mind that this overwrites the old images.

The command is:

mogrify -format jpg *.png  
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Awesome, that's exactly what I was after and will be using again. By the way, just to clarify as I didn't realise this is what you meant: convert is used to generate a separate output file, mogrify is used to modify the original image. –  humble coffee Nov 16 '09 at 4:11
Thanks +1, I wasn't very clear - hopefully a tiny bit better. –  William Hilsum Nov 16 '09 at 4:29
png images with transparent background does not convert properly to jpg. –  vishnu Nov 28 '11 at 4:33
To convert PNG's with transparent background, use the following command: mogrify -format jpg -background black -flatten *.png –  zsero Jun 26 '12 at 18:54
From mogrify documentation: "This tool is similiar to convert except that the original image file is overwritten (unless you change the file suffix with the -format option) with any changes you request." –  janko-m Sep 18 '14 at 17:07

I have a couple more solutions.

The simplest solution is like most already posted. A simple bash for loop.

for i in *.png ; do convert "$i" "${i%.*}.jpg" ; done

For some reason I tend to avoid loops in bash so here is a more unixy xargs approach, using bash for the name-mangling.

ls -1 *.png | xargs -n 1 bash -c 'convert "$0" "${0%.*}.jpg"'

The one I use. It uses GNU Parallel to run multiple jobs at once, giving you a performance boost. It is installed by default on many systems and is almost definitely in your repo (it is a good program to have around).

ls -1 *.png | parallel convert '{}' '{.}.jpg'

The number of jobs defaults to the number of processes you have. I found better CPU usage using 3 jobs on my dual-core system.

ls -1 *.png | parallel -j 3 convert '{}' '{.}.jpg'

And if you want some stats (an ETA, jobs completed, average time per job...)

ls -1 *.png | parallel --eta convert '{}' '{.}.jpg'

There is also an alternative syntax if you are using GNU Parallel.

parallel convert '{}' '{.}.jpg' ::: *.png

And a similar syntax for some other versions (including debian).

parallel convert '{}' '{.}.jpg' -- *.png
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+1 for correct bash string expansion in the for, if I could give you another upvote for mentioning parallel, I would. There's one typo, however - you need a done at the end of that for loop. Also, for the parallel stuff, you could avoid using that ls and pipe with a construct like: parallel -j 3 --eta convert '{}' '{.}.jpg' ::: *.png (see here) –  evilsoup Jan 28 '13 at 3:04
Fixed typo. That is a cool syntax that I didn't know of. I don't know which one I like better for probably the same reason I prefer not to use loops in bash. I put it the solution because it is probably the more "proper" way but I'll probably stick with the ls method for myself because it makes more sense to me. –  Kevin Cox Jan 28 '13 at 14:04
...although it should be noted that that syntax only works on GNU parallel. The parallel that's packaged in some linux distros (like Debian & Ubuntu) is actually a different version with a slightly different syntax (use -- rather than :::) - and even then, it frustratingly lacks some of the features of GNU parallel. –  evilsoup Jan 28 '13 at 14:17
(though those on distros that don't package GNU parallel can install it from source quite easily, using the instructions here) –  evilsoup Jan 28 '13 at 14:24
I think I should change it back then so that it works with as many versions as possible. –  Kevin Cox Jan 28 '13 at 18:16

The convert command found on many Linux distributions is installed as part of the ImageMagick suite. Here's the bash code to run convert on all PNG files in a directory and avoid that double extension problem:

for img in *.png; do
    convert "$filename.png" "$filename.jpg"
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According to the man page for convert: "The convert program is a member of the ImageMagick(1) suite of tools." –  humble coffee Nov 16 '09 at 4:06
You are correct. For some reason I thought it was part of a different library. Either way the code I posted above is the correct way to automate batch conversion within a directory. –  Marcin Nov 16 '09 at 4:17
You can use bash expansion to improve that command like: for f in *.png; do convert "$f" "${f/%png/jpg}"; done –  evilsoup Jan 28 '13 at 2:57

The actual "png2jpg" command you are looking for is in reality split into two commands called pngtopnm and cjpeg, and they are part of the netpbm and libjpeg-progs packages, respectively.

png2pnm foo.png | cjpeg > foo.jpeg
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my quick solution for i in $(ls | grep .png); do convert $i $(echo $i.jpg | sed s/.png//g); done

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This has got to be one of the ugliest, most convoluted command-lines I've ever seen –  evilsoup Jan 28 '13 at 2:56
@evilsoup honestly, this is elegant for shell scripts. Claiming it is convoluted isn't fair. –  Max Howell Nov 5 '13 at 17:31
@MaxHowell man. No. Here would be an elegant version of this: for f in ./*.png; do convert "$f" "${f%.*}.jpg"; done. That avoids the completely unnecessary ls, grep and sed calls (and echo, but IIRC that's a bash builtin and so will have no/very little performance impact), and gets rid of two pipes and two subshells, and involves less typing. It's even slightly more portable, since not all versions of ls are safe to parse. –  evilsoup Nov 6 '13 at 12:08
@evilsoup I stand corrected! Good job. –  Max Howell Nov 6 '13 at 14:58

For batch processing:

for img in *.png; do
  convert "$img" "$img.jpg"

You will end up with file names like image1.png.jpg though.

This will work in bash, and maybe bourne. I don't know about other shells, but the only difference would likely be the loop syntax.

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find . -name "*.png" -print0 | xargs -0 mogrify -format jpg -quality 50
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Thanks for a deep/recursive directory one-line solution which leaves the resulting *.jpg files next to the original *.png files, shows how to reduce file size/quality and doesn't break because of any odd characters in directory or file name. –  Joel Purra Dec 28 '14 at 15:21

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