シェルスクリプトで再帰を用いて階乗計算

recursive.sh

expr $1 + 1 >/dev/null 2>&1

if [ $? -ne 1 -a $? -ne 0 ]; then
    echo "must be numeric"
    exit
elif [ $1 -lt 0 ]; then
    echo "must be more than 0"
    exit
elif [ $1 -eq 1 -o $1 -eq 0 ]; then
    echo 1;
else
    echo $(expr $1 \* $($0 $(expr $1 - 1)))
fi

ex)
./recursive.sh 6
→ 720