Imagemagick 7.0.7.22 and above

-crop 3:2 works since January 6th, 2018.

JPG

magick convert in.jpg -gravity center -crop 3:2 out.jpg

Warning/reminder: if you don’t use -gravity center, you will get two output files:

PNG

As fmw42 points out, PNG files store the virtual canvas size. +repage is recommended.

magick convert in.png -gravity center -crop 3:2 +repage out+repage.png

GIMP, IrfanView, Chrome and Windows Explorer don’t show any difference, but Imagemagick knows:

magick identify out*png
out_stndrd.png PNG 252x168 314x168+31+0 8-bit sRGB 78557B 0.000u 0:00.000
out+repage.png PNG 252x168 252x168+0+0 8-bit sRGB 78529B 0.000u 0:00.000

Imagemagick 6.9.9-34 and above

JPG

convert in.jpg -gravity center -crop 3:2 out.jpg

PNG

convert in. -gravity center -crop 3:2 +repage out.png

Imagemagick 6.9.9-33 / 7.0.7.21 and below

Note: you need to add magick before any convert for v7.

1. Specific target resolution

If your goal at the end is to have a certain resolution (for example 1920×1080) then it’s easy, using -geometry, the circumflex/hat/roof/house symbol (^) and -crop:

convert in.jpg -geometry 1920x1080^ -gravity center -crop 1920x1080+0+0 out.jpg

To loop over multiple jpg files:

for i in *jpg
  do convert "$i" -geometry 1920x1080^ -gravity center -crop 1920x1080+0+0 out-"$i"
done

2. Aspect ratio crop only

If you want to avoid scaling, you have to calculate the new length of the cropped side outside of Imagemagick. This is more involved:

aw=16 #desired aspect ratio width...
ah=9 #and height
in="in.jpg"
out="out.jpg"

wid=`convert "$in" -format "%[w]" info:`
hei=`convert "$in" -format "%[h]" info:`

tarar=`echo $aw/$ah | bc -l`
imgar=`convert "$in" -format "%[fx:w/h]" info:`

if (( $(bc <<< "$tarar > $imgar") ))
then
  nhei=`echo $wid/$tarar | bc`
  convert "$in" -gravity center -crop ${wid}x${nhei}+0+0 "$out"
elif (( $(bc <<< "$tarar < $imgar") ))
then
  nwid=`echo $hei*$tarar | bc`
  convert "$in" -gravity center -crop ${nwid}x${hei}+0+0 "$out"
else
  cp "$in" "$out"
fi

I’m using 16:9 in the examples, expecting it to be more useful than 3:2 to most readers. Change both occurrences of 1920x1080 in solution 1 or the aw/ah variables in solution 2 to get your desired aspect ratio.

Photo credit: Anders Krusberg / Peabody Awards