Here is a simple script for converting raw NEF files from a Nikon to PNG format. It uses the convert utility which can handle a large variety of file types and has various options for the conversion. This is just a simple script that will generate a PNG that is either 1024 px wide or high.
#!/bin/bash
##################################
# Convert files to png #
##################################
files=`ls ./`;
if [ ! -d png ]
then
mkdir png;
fi
for file in $files
do
in=./$file;
out=./${file%.*}".png";
if [ ! -f $out ]
then
echo "converting $file";
convert "$in" -resize 1024x1024 "$out" ;
mv $out png/;
fi
done
The script will create a folder in the directory its run in and process all the files saving the converted PNG into the created folder.