### convert imagemagick

Batch:

for file in *.png; do convert $file -rotate 90 rotated-$file; done

for file in *.png; do convert $file -resize 20x20 small/$file; done

Variants:

convert howtogeek.png -resize 400x400 -rotate 180 -charcoal 4 -quality 95 howtogeek.jpg

convert howtogeek.png -quality 95 howtogeek.jpg

Convert book to little grayscale:

convert 1.pdf -grayscale -resize 50% 2.pdf

convert 1.pdf -monochrome  2.pdf

Since 7.14 version some pdf operations under policy.xml

convert-im6.q16: attempt to perform an operation not allowed by the security policy `PDF'

To solve look at /etc/ImageMagick-6/policy.xml or use previos versions.