So today I needed to resize a photo archive of 3000 images spread out in sub-folders for every year and month… Fortunately, in linux with a bit of research I got it!
First off, you must download ImageMagick – if you are running Ubuntu, LinuxMint or a similar distro with APT you can simply do:
sudo apt-get install imagemagick
Then you can cd (change directory) to the folder containing the pictures that are to be resized. For example:
cd /home/username/Pictures/toShrink
Now for example, if you only need to resize the .jpgs in this folder to 800px wide do:
mogrify -resize 800 *.jpg
NOTE: This is case sensitive, .jpg is not .JPG. It is likely that you will have to run any of these commands for all file extention variations in your collection. For instance once with *.jpg, then *.JPG, then *.jpeg then *.JPEG
To resize them to a specific size (distorting their aspect ratio if necessary) do:
mogrify -resize 800x600! *.jpg
To resize them to 50% of their original size do:
mogrify -resize 50% *.jpg
To resize them to a specific pixel content (effectively scaling small images up and large images down to a uniform size while maintaining aspect ratio) of their original size do:
mogrify -resize 480000@ *.jpg
Basically, the above number is 800*600=480000px. Whether it be 800×600 or 600×800 or even 1200×400 pixels in size the image contains a total of 480000 pixels. This will scale the images so that they contain X number of pixels.
Since many of my photos were different aspect ratios, different sizes and different orientations, some even panoramas, I settled for the above option. I used 2000000@ which made all of my 4:3 photographs 1632×1224 and the panoramas similarly sized – not totaly precise but at least now all the pictures taken by different cameras at different resolutions are all roughly the same size!
Now RECURSIVE BATCH IMAGE RESIZING:
To resize all .jpgs in this folder and every sub folder contained within it (recursively) – FIRST DO A TEST RUN! – the following command will NOT change files, it will just output to console the commands that it would be performing:
find ./ -name "*.jpg" -exec echo mogrify -resize 800 {} \;
You should see outputted to console the mogrify command for ONLY the files that you want to resize.
mogrify -resize 800 ./subFolderName/fileName.jpg
This is an example output that I get:
mogrify -resize 800 ./2007/12_December/IMG_0172.jpg mogrify -resize 800 ./2007/12_December/IMG_0199.jpg mogrify -resize 800 ./2007/12_December/IMG_0166.jpg mogrify -resize 800 ./2007/04_April/2007-04-25_ph101_12.jpg mogrify -resize 800 ./2007/04_April/041207_13491.jpg mogrify -resize 800 ./2007/04_April/2007-04-25_ph101_13.jpg
Make sure that the options are correct and that it is modifying the correct files. Use caution, test it out on a few images first.. This overwrites the original file so make sure that you have a backup of everything just in case something goes wrong.
If it all looks good and you’re ready to go, simply remove the echo .
This is the final command that I used (resizing 4:3 images to around 1632×1224):
find ./ -name "*.jpg" -exec mogrify -resize 2000000@ {} \;
Remember, it is case sensitive. Run it again for *.JPG , *.jpeg, and *.JPEG!
ImageMagick can do a whole lot more, things such as image conversions.. like JPG to PNG 
convert *.jpg imageSeriesName.png
Check out the ImageMagick site to learn more about what you can do with this extremely powerful non-graphical graphics program! 
Good luck guys, hope this post helps make life easier for a fellow creature of the net!
