Notes Copied from How-To Geek

#Resize 50%
     for i in $( ls *.jpg); do convert -resize 50% $i re_$i; done

#Resize 50% and rename to .gif
     for i in $( ls *.jpg); do convert -resize 50% $i $i.gif; done

#Resize to 800x600 and rename to .gif
     for i in $( ls *.jpg); do convert -resize 800x600 $i $i.gif; done

#Convert from .png to .jpg
     convert howtogeek.png howtogeek.jpg

#Specify compression level
     convert howtogeek.png -quality 95 howtogeek.jpg

#ImageMagick will try to preserve the aspect ratio if you use this command. It will alter the image to fit within a 200x100 area, but the image may not be exactly 200x100. If you want to force the image to become a specific size - even if it messes up the aspect ratio - add an exclamation point to the dimensions:
     convert example.png -resize 200x100! example.png

You can also specify a specific width or height and ImageMagick will resize the image to that width or height while preserving the aspect ratio. The following command will resize an image to a width of 200:
     convert example.png -resize 200 example.png

The following command will resize an image to a height of 100:
     convert example.png -resize x100 example.png

ImageMagick can quickly rotate an image. The following command takes an image named howtogeek.jpg, rotates it by 90 degrees and saves the rotated image as howtogeek-rotated.jpg:
     convert howtogeek.jpg -rotate 90 howtogeek-rotated.jpg

ImageMagick can apply a variety of effects to an image. For example, the following command applies the "charcoal" effect to an image:
     convert howtogeek.jpg -charcoal 2 howtogeek-charcoal.jpg

The implode effect makes it appear as if there's a black hole at the center of the image.
     convert howtogeek.jpg -implode 1 howtogeek-imploded.jpg

All these operations can be combined. With a single command, you could resize an image, rotate it, apply an effect, and convert it to another format:
     convert howtogeek.png -resize 400x400 -rotate 180 -charcoal 4 -quality 95 howtogeek.jp

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