Batch converting PNG to JPG in linux

Question

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.

Solution 1

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

which will keep the original as well as creating the converted image.

As for batch conversion, I think you need to use the Mogrify tool which is part of ImageMagick.

Keep in mind that this overwrites the old images.

The command is:

mogrify -format jpg *.png

Solution 2

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 CPU cores 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

Solution 3

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
    filename=${img%.*}
    convert "$filename.png" "$filename.jpg"
done

Solution 4

tl;dr

For those who just want the simplest commands:

Convert and keep original files:

mogrify -format jpg *.png

Convert and remove original files:

mogrify -format jpg *.png && rm *.png

Batch Converting Explained

Kinda late to the party, but just to clear up all of the confusion for someone who may not be very comfortable with cli, here's a super dumbed-down reference and explanation.

Example Directory

bar.png
foo.png
foobar.jpg

Simple Convert

Keeps all original png files as well as creates jpg files.

mogrify -format jpg *.png

Result

bar.png
bar.jpg
foo.png
foo.jpg
foobar.jpg

Explanation

• mogrify is part of the ImageMagick suite of tools for image processing.
  • mogrify processes images in place, meaning the original file is overwritten, with the exception of the -format option. (From the site: This tool is similar to convert except that the original image file is overwritten (unless you change the file suffix with the -format option))
• The - format option specifies that you will be changing the format, and the next argument needs to be the type (in this case, jpg).
• Lastly, *.png is the input files (all files ending in .png).

Convert and Remove

Converts all png files to jpg, removes original.

mogrify -format jpg *.png && rm *.png

Result

bar.jpg
foo.jpg
foobar.jpg

Explanation

• The first part is the exact same as above, it will create new jpg files.
• The && is a boolean operator. In short:
  • When a program terminates, it returns an exit status. A status of 0 means no errors.
  • Since && performs short circuit evaluation, the right part will only be performed if there were no errors. This is useful because you may not want to delete all of the original files if there was an error converting them.
• The rm command deletes files.

Fancy Stuff

Now here's some goodies for the people who are comfortable with the cli.

If you want some output while it's converting files:

for i in *.png; do mogrify -format jpg "$i" && rm "$i"; echo "$i converted to ${i%.*}.jpg"; done

Convert all png files in all subdirectories and give output for each one:

find . -iname '*.png' | while read i; do mogrify -format jpg "$i" && rm "$i"; echo "Converted $i to ${i%.*}.jpg"; done

Convert all png files in all subdirectories, put all of the resulting jpgs into the all directory, number them, remove original png files, and display output for each file as it takes place:

n=0; find . -iname '*.png' | while read i; do mogrify -format jpg "$i" && rm "$i"; fn="all/$((n++)).jpg"; mv "${i%.*}.jpg" "$fn"; echo "Moved $i to $fn"; done

Solution 5

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

View more solutions

Comments

[nedned]

According to the man page for convert: "The convert program is a member of the ImageMagick(1) suite of tools."

[nedned]

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.

[Marcin]

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.

[Vishnu Pradeep]

png images with transparent background does not convert properly to jpg.

[hyperknot]

To convert PNG's with transparent background, use the following command: mogrify -format jpg -background black -flatten *.png

[evilsoup]

This has got to be one of the ugliest, most convoluted command-lines I've ever seen

[evilsoup]

You can use bash expansion to improve that command like: for f in *.png; do convert "$f" "${f/%png/jpg}"; done

[evilsoup]

+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)

[Raza]

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.

[evilsoup]

...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]

(though those on distros that don't package GNU parallel can install it from source quite easily, using the instructions here)

[Raza]

I think I should change it back then so that it works with as many versions as possible.

[neurino]

I wonder how can this overwrite original files if the filename changes... in fact jpg files keep untouched ;) Ah, +1

[Raza]

@neurino The file name isn't changed in the mogrify example. The files still have a .png extension but are jpeg images. Try using file img.png (after running mogrify) and it will tell you that it is a jpeg image.

[neurino]

@KevinCox on my linux box after mogrify -format jpeg img.png I have 2 files and file img.* reports one png, the original untouched, and a new jpeg one. So mogrify does not overwrite original files in this case.

[Raza]

@neurino Interesting. From the mogrify man page: "mogrify - resize an image, blur, crop, despeckle, dither, draw on, flip, join, re-sample, and much more. Mogrify overwrites the original image file, whereas, convert(1) writes to a different image file." So I don't know why that happens on your box.

[Raza]

@neurino This happens on my box as well. Now I'm really confused, it this a bug? Or an undocumented feature?

[Max Howell]

@evilsoup honestly, this is elegant for shell scripts. Claiming it is convoluted isn't fair.

[evilsoup]

@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.

[Max Howell]

@evilsoup I stand corrected! Good job.

[janko-m]

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."

[Joel Purra]

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.

[don_crissti]

Probably the best answer provided you get rid of the while read part (replace it or remove it all together)...

[Steven Jeffries]

@don_crissti, what's wrong with while read?

[don_crissti]

It's error prone (unless you're 100% sure you're dealing with sane file names) and slow (like in very, very, very slow).

[tsenapathy]

just remember it's case sensitive. my camera name it as *.JPG and didn't realize this in first instance.

[Flatron]

If you need to compress your png's more use mogrify -quality 75 -format jpg *.png

[Nam G VU]

Me too on Ubuntu Desktop 16 today - original files remain

[Dan Dascalescu]

What is the default JPG quality, and how can file timestamps be preserved?

[Steven Jeffries]

@DanDascalescu The methods above (except the last one) will preserve filenames, but replace their extension, so timestamped files should be okay (always make a copy and test first). According to ImageMagick, "The default is to use the estimated quality of your input image if it can be determined, otherwise 92" (imagemagick.org/script/command-line-options.php#quality) Quality can be specified with -quality <number> where <number> is 1 to 100.

[Dan Dascalescu]

Thanks. mogrify -format jpg foo.png created foo.jpg with the current timestamp though. I wanted to copy the timestamp of foo.png into foo.jpg, and I don't see anything of that nature in the options.

[Ahi Tuna]

If you have a directory with more than 10,000 png images in it... the ls command will likely fail. So, this command works in those situations: find . -type f -name '*.png' | parallel --eta convert '{}' '{.}.jpg'

[naim5am]

Thanks. Last comment but saved my from going down a rabbit hole figuring out why ? >> mogrify: unable to open image 'jpg:': No such file or directory @ error/blob.c/OpenBlob/3496.

[Mahesh Jamdade]

is it possible to compress the image, jpg to jpg or jpeg to jpeg?

[ubershmekel]

Should work. Please try this and let me know. Just change the line's .png to .jpg.

[Mahesh Jamdade]

just to confirm is it this command ffmpeg -i input.jpg output.jpg

[ubershmekel]

That command you mentioned should recompress a single jpg.

[Nae]

I've learnt so much from this answer.

[Raymond]

Be careful in using any of above command as it will mess up the background. I tried few commands and none worked. Always have a backup of your folder. @KevinCox you should test commands before posting as I messed up my images.

[Raza]

@Raymond all of these commands are testing and work fine. You would have to share more information for any help. What is "the background" and how is it messed up? One first step would be to check that you don't have any aliases or other custom commands that may be running instead of the commands intended by the answer.

[Raymond]

It's the transparency, you should include it in the command or mention it.

[Raza]

JPEG images don't support transparency so no method of converting will preserve that. By default convert uses a black background but you can change the color using -background '#FF0000' -alpha remove.

[Admin]

How is this better than accepted answer given 13 years ago?

[Admin]

To elaborate on @Toto's answer: doesn't mogrify (as mentioned in that accepted answer) do this for you? It's a nice effort though :)