ImageMagick (and GraphicsMagick)

• https://en.wikipedia.org/wiki/ImageMagick
• http://www.imagemagick.org/script/index.php
• https://en.wikipedia.org/wiki/GraphicsMagick
• http://www.graphicsmagick.org/

Batch-optimise a folder of JPEGs for the web

for i in *.jpg; do
    convert -strip -interlace Plane -gaussian-blur 0.05 -quality 75% -resize 1800x $i output/$i
done

Walks every .jpg in the current directory and writes a lighter copy into output/ (create it first with mkdir -p output). -strip drops EXIF and other metadata, -interlace Plane makes the JPEG progressive, -gaussian-blur 0.05 smooths noise so it compresses better, -quality 75% sets the JPEG quality, and -resize 1800x caps the width at 1800px while keeping the aspect ratio. Verified on ImageMagick 6.9.11: a 2400x1600 source comes out at 1800x1200 and about half the size.

On ImageMagick 7 the convert command is deprecated in favour of magick, so use magick "$i" ... "output/$i".