Trick: Do not use convert but mogrify:

mogrify -resize 50% *

Rails professionals since 2007

Our laser focus on a single technology has made us a leader in this space. Need help?

• We build a solid first version of your product
• We train your development team
• We rescue your project in trouble

Read more Show archive.org snapshot

This overwrites the original image file.

In contrast, convert writes to a different image file. Here is an example if you need this:

cd /path/to/image/directory
for i in `ls -1 *jpg`; do convert -resize 50% $i "thumb_$i"; done