Shell script with ImageMagick: hangs forever?

Question

I've generated a shell script that uses ImageMagick to convert and crop around 18000 images. Here's a sample entry (so there are 18000 of these):

if [ ! -f ./cropped/16333-1.png ]
then
convert -crop 724x118+876+1989 ./lin/34.png ./cropped/16333-1.png
echo cropping 16333-1
fi
if [ ! -f ./cropped/16333-1_thumb.png ]
then
convert -define jpeg:size=400x100 ./cropped/16333-1.png -thumbnail '400x100>' -background transparent -gravity center -extent 400x100 ./cropped/16333-1_thumb.png
echo thumbing 16333-1
fi

The script only runs for about 2000 images before hanging forever. Am I missing something, or leaking memory somewhere?

Thanks for your help!

UPDATE

I tried running the script with the -x flag as suggested, and the script hung after this command:

+ '[' '!' -f ./cropped/8967-1_thumb.png ']'
+ convert -define jpeg:size=400x100 ./cropped/8967-1.png -thumbnail '400x100>' -background transparent -gravity center -extent 400x100 ./cropped/8967-1_thumb.png

However, when I interrupt the script and then copy and paste the command above, it runs just fine. No hanging. I can then resume the script, and it sets off again fine.

Mysterious...

Answer 1

My guess is that there's some shell meta character in there somewhere which is tripping it up.

To diagnose and solve:

1. Identify exactly where it stops
2. Look at the ten lines around it
3. Look for any > < ! $ which aren't escaped

Alternately, add -x to the first line of the script so that you can see each command as it's run. It should look something like:

#! /bin/bash -x