#!/usr/bin/env bash
# Greatest Common Divisor in POSIX shell using Euclid's algorithm.  On
# return, variable gcd_value is set and is the gcd of parameters $1
# and $2. The GCD of a negative number is the same as the GCD of its
# absolute value, since a negative number is -1 times its positive
# value.  Negative numbers are set when there is an error; -1 is set
# when the wrong number of parameters are given.
proc gcd {
    typeset -i a=$1
    typeset -i b=$2
    if sh-expr ' a > b )' {
	setglobal a = $b
	setglobal b = $1
    }
   if sh-expr ' a == 1 || (b-a) == 0)' {
       setglobal gcd_value = $a
       return 0
   }
   typeset -i c
   sh-expr 'c=b-a'
   gcd $c $a
}

gcd $1 $2
echo $gcd_value