Boolean Difference with object files

if [ -f $OUTPUT_FILE ]; then
    rm -i $OUTPUT_FILE
fi

if [ -f $OUTPUT_FILE ]; then
    echo "Must remove ${OUTPUT_FILE}"
    exit
fi

rm -rf ./$INPUT_LIB_DIR
mkdir $INPUT_LIB_DIR
cd $INPUT_LIB_DIR

ar x $INPUT_LIB

cd ..

rm -rf ./$SUBTRACT_LIB_DIR
mkdir $SUBTRACT_LIB_DIR
cd $SUBTRACT_LIB_DIR

ar x $SUBTRACT_LIB

cd ..

rm -rf ./$OUTPUT_DIR
mkdir $OUTPUT_DIR

cd $INPUT_LIB_DIR

for a in ./*.o
do
    if [ -f ../$SUBTRACT_LIB_DIR/${a} ]; then
        echo "Skipping " ${a} > /dev/null
    else
        echo "Including " ${a}
        cp ${a} ../$OUTPUT_DIR
    fi
done

cd ..

for a in ./$OUTPUT_DIR/*.o
do
    ar r ./$OUTPUT_FILE $a 
done
ranlib ./$OUTPUT_FILE

rm -rf ./$INPUT_LIB_DIR
rm -rf ./$SUBTRACT_LIB_DIR
rm -rf ./$OUTPUT_DIR

Note: Code Formatting