Create a file (e.g., convert_text_to_macroman.sh) and put it somewhere (e.g., in ~/bin). Make the script executable: chmod a+x ~/bin/convert_text_to_macroman.sh
Now start FileMerge and go to the Preferences. Select the Filter tab and click in an empty row to start a new filter. Type txt in the Extension column, and put the following in the Filter column: ~/bin/convert_text_to_macroman.sh $(FILE)
http://hints.macworld.com/article.php?story=20091116060951378
#!/bin/bash
charset=`file -I "$1" | sed "s/^.*charset=//"`
case $charset in
iso-8859* | utf-8 )
iconv -c -f $charset -t MacRoman "$1"
;;
* )
cat "$1"
;;
esac