Mortgage calculator - bash script

This simply outputs the monthly mortgage payment.

Takes five arguments : principle years interest points fees

For instance,

bash mortgage.sh 100000 5 .075 0 0

Outputs

Principle + fees = 100000 , Years = 5 , Months = 60 , interest .006250000000
Points = 0 , 0
2003.794859563881

#!/bin/sh

if [[ $# -ne 5 ]]
then
 echo "enter 5 args : principle years interest points fees"
  exit
  fi

  prin=$1
  ((mon=$2 * 12 ))
  inter=`echo "scale=12;$3/12" | bc`
  pts=`echo "scale=12; $prin * $4" | bc`
  fee=$5
  prin=`echo "scale=12;$prin + $fee" | bc`


  echo "Principle + fees = $prin , Years = $2 , Months = $mon , interest $inter"
  echo "Points = $4 , $pts"

  prin=`echo "scale=12; $prin - $pts" | bc`


  echo `echo "scale=12;($prin*$inter)*((1+$inter)^$mon)/(((1+$inter)^$mon)-1)"|bc`
  echo "the monthly payment : $new_prin"