linux operators

#! /bin/bash
print_usage(){
printf "please enter an integer\n"
exit 5
}

read -p "please input first number: " firstnum
if [ -n "echo $firstnum | sed 's/[0-9]//g'" ];then
print_usage
fi

read -p "please input operators: " operators
if [ "${operators}" != "+" ] && [ "${operators}" != "-" ] && [ "${operators}" != "" ] && [ "${operators}" != "/" ];then
echo "please input {+|-||/}"
exit 10
fi

read -p "please input send number: " secondnum
if [ -n "echo $secondnum | sed 's/[0-9]//g'" ];then
print_usage
fi
echo "${firstnum}${operators}${secondnum}=$((${firstnum}${operators}${secondnum}))"