Google Site SearchFN Site Search FN Blog Login FN Blog Login
Site Navigation:

FUN WITH SHELLS: Do you want to calculate the amortization on a loan?

by Andrew Newman (CREATED: 2004-02-25)

This is the second, in a series, called "Fun with Shells".

This contribution is a simple bash shell that I wrote for calculating your car payments, home mortgage or whatever you get on a loan. If you look into the nuts and bolts of this program there are two fundamental things to be learned here about shells.
First: You can call in variables, and have it output a simple help if you forget them.
And second: You can code up a simple mathematical equation into a shell to have it perform a quick calculation, such as amortization.

Here is the script
# program myamort
# version 1.0  
# written by Andrew V. Newman
# last modified Wed Feb 25 17:37:04 MST 2004

# program to calculate monthly payment on a loan

# This is a default response if you do not give any input variables 
if [ ${#} -lt "2" ]; then
   echo "Usage: myamort 'LOAN' 'APR' [TRM] [CMP] [PPY]"
   echo "   where LOAN is amount in dollars"
   echo "       APR is in fractions of 100 (i.e. 6.5% is 6.5)"
   echo "     (following are optional:)"
   echo "       TRM is term in years [30 is default]"
   echo "       CMP is compounds per year [12 is default--monthly]"
   echo "       PPY is payments per year [12 is default]"
   echo "   Example:"
   echo "     myamort 150000 6.5 30 12 12"
   exit 1
# These need to be input every time
  LOAN=312000  # Amount of loan 
  APR=0.06     # Annual Percentange Rate 
  LOAN=$1      # Amount of loan 
  APR=`echo $2 | awk '{print $1/100}' ` # Annual Percentange Rate 

# These may or may not be input 
  TRM=30  # Term of loan in years 
    if [ $3 ]; then  TRM=$3 ; fi
  CMP=12  # Number of comounds/year 
    if [ $4 ]; then  CMP=$4 ; fi
  PPY=12  # Number of payments/year 
    if [ $5 ]; then  PPY=$5 ; fi

# Calculate Monthly Payment 
  MPMT=` echo $APR | awk '{
	END {printf "%8.2f", LOAN*(R^(TRM*PPY)*(R-1))/(R^(TRM*PPY)-1)
LOANK=`echo $LOAN | awk '{print "\$" $1/1000 "K"}'`
APR100=`echo $APR | awk '{print $1*100}'`
printf "For:\n LOAN: $LOANK\n APR $APR100 over $TRM years\n $PPY pmnt/yr at: \$%7.2f\n" $MPMT

# exit cleanly
  exit 0

It may be easier to just download a copy (right click).

Once you have a copy on your system, all you need to do is follow the instructions at the bottom of myforecast to get it up and running on your machine.

In the program, you will notice that there are several variables that are taken as a default value. These are the default values that I found necessary when calculating my mortgage and do not necessarily represent the values that you need (note: that amount in the example is not what I paid for my own house!).
Ok, now that you have the program in the right spot and permission set, give it a shot. Try running the following commands: (Do not type in comments in blue)
% myamort # will give you the usage
% myamort 150000 6.125 # $150K at 6.125 APR
% myamort 150000 6.125 15 # $150K at 6.125 APR over 15 years
% myamort 150000 6.125 15 1e10 # $150K at 6.125 APR over 15 years compounded continuously
% myamort 150000 6.125 15 12 26.125 # $150K at 6.125 APR over 15 years compounded monthly, but payments made biweekly

Please email me if you have any comments or suggestions!