make-pi - Experimental approach to calculate Pi with up to 18 decimal places using Euler method

#!/bin/sh

BASE=4
RES=0
RES_OLD=0
PREC_DEF=200000    # number of iterations with identical results bofore exiting
X=2

if [ -n "${1##*[!0-9]*}" ] && [ $1 -le 18 ]; then
    DIGIT=$1
else
    DIGIT=18
fi

while [ $((DIGIT=$DIGIT-1)) -ge 0 ]; do
    BASE=$(($BASE*10))
done
PREC=$PREC_DEF
while [ $PREC -ge 0 ]; do
    RES=$(($RES+($BASE/($X*$((X=$X+1))*$((X=$X+1))))-($BASE/($X*$((X=$X+1))*$((X=$X+1))))))
    if [ $RES_OLD -eq $RES ]; then
        PREC=$(($PREC-1))
    else
        ITERATION=$(($ITERATION+1))
        RES_OLD=$RES
        PREC=$PREC_DEF
    fi
    printf "\r3.$RES"
done
printf "\nUsed iterations: $ITERATION\n"