Shell Programming and Scripting

The following Script takes each extension and determine what category it
belongs and then moves it into a directory based on the extension.
(for eg. 1.sh, 5.sh, 9.sh together; 4.csh, 120.csh, 6.csh together and 7.ksh, 2.ksh, 59.ksh together) and moves them to their respective directories viz. sh, csh and ksh...

#!/usr/bin/ksh
for ext in ` ls -d *.* | awk -F. '{print $NF}' | sort -u `
do
mkdir -p $ext
mv *."$ext" "$ext"/.
done

The above Script sees for all similar extension, creates the directories and moves Files to their corresponding directories.
I am trying to do it per extension only and not all at a time, like take all files with ksh only and try to move them to ksh directory while dont do anything to files with Extension sh and csh. That is the Shell parameter should be taken.

Can anyone give some inputs.

Thanks a lot in advance.

#!/usr/bin/ksh
if [[ $1 != "" ]]; then
ext=$1
else
echo "Not enough input arguments."
echo "Usage: basename $0 <extension_name>
exit
fi

if [[ ! -d $ext ]]; then
mkdir -p $ext
fi

mv *.$ext $ext/.

# untested... should work...

What is the difference in 'ls -d' and 'ls -d *.*'?