amortize.awk

#!/usr/bin/env awk


BEGIN {
    print "principal", "interest_rate", "term", "monthly_payment", "total_paid", "interest_paid"
}

NR == 1 && NF == 0 {
    print "use:"
    print "\tawk -f amortize"
    print "\t\tprincipal, interest-rate, term (months), [down-payment]"
    print
    print "\t\tlike: 10000 7.0 48 1000"
    print
    print "outputs:"
    print "\tprincipal, interest rate, term, monthly payment, total paid, interest paid"
    exit 1
}

{
    principal = $1
    annual_interest_rate = $2
    months = $3
    down_payment = $4

    if(down_payment) {
        principal = principal - $4
    }

    monthly_interest_percentage = annual_interest_rate / 12.0 / 100

    num = monthly_interest_percentage * (1 + monthly_interest_percentage) ^ months

    denom = (1 + monthly_interest_percentage) ^ months - 1

    monthly_payment = principal * (num / denom)
    total_paid = principal * (num / denom) * months
    interest_paid = total_paid - principal

    print $1, $2, $3, monthly_payment, total_paid, interest_paid
}