Shell——计算器升级版

#!/bin/bash
#no.1

[ $# -ne 2 ] && {
    echo $"USAGE $0 NUM1 NUM2"
    exit 1
}

#no.2
a=$1
b=$2

expr $a + 1 &>/dev/null
RET1=$?

expr $b + 1 &>/dev/null
RET2=$?

if [ $RET1 -ne 0 -o $RET2 -ne 0 ];
    then
        echo "You must input two nums"
        exit 2
fi

#no.3
echo "a+b=$(($a+$b))"
echo "a-b=$(($a-$b))"
echo "a*b=$(($a*$b))"
echo "a/b=$(($a/$b))"
echo "a%b=$(($a%$b))"
echo "a**b=$(($a**$b))"