sips downscale image by percentage in Mac OS X

downscale.sh

#!/bin/bash

if [ $1 ]
then
   echo Processing file $1;
else
   for var in `find *.png`;
   do
       echo Processing file $var;
       sips -Z $(($(sips -g pixelWidth "$var" | cut -s -d ':' -f 2 | cut -c 2-) / 2)) "$var" --out "resized_$var" &> /dev/null
   done
fi

Good work!

Fixed mess with spaces in path:

#!/bin/bash

if [ $1 ]
then
   echo Processing file $1;
else
   find "*.png" -print0 | while IFS= read -r -d '' file; do
       echo Processing file "$file";
       sips -Z $(($(sips -g pixelWidth "$file" | cut -s -d ':' -f 2 | cut -c 2-) / 2)) "$file" --out "$file" &> /dev/null
   done
fi

Also here is recursive version:

#!/bin/bash

if [ $1 ]
then
   echo Processing file $1;
else
   find . -name "*.png" -print0 | while IFS= read -r -d '' file; do
       echo Processing file "$file";
       sips -Z $(($(sips -g pixelWidth "$file" | cut -s -d ':' -f 2 | cut -c 2-) / 2)) "$file" --out "$file" &> /dev/null
   done
fi

Warning! These two scripts will override your files! So make sure that you've made your backups.