get previous month in shell script

prv_month.sh

#!/bin/bash

fn=$1

if [ "$2" == "" ] || [ "$3" == "" ]; then
  if [ -f last_${fn}.txt ]; then
    value=$(<last_${fn}.txt)
    _yy=${value:0:4}
    _mm=${value:4:2}
    echo "$_yy $_mm"
    mm=$(date --date="$(date +$_yy-$_mm-15) -1 month" +"%m")
    yy=$(date --date="$(date +$_yy-$_mm-15) -1 month" +"%Y")
    echo "$yy $mm"
  else
    echo 'Nothing to do `year` and `month` are required.'
    exit
  fi
else
  yy=$2
  mm=$3
fi

echo $yy$mm > ./last_${fn}.txt

ref: http://stackoverflow.com/questions/13168463/using-date-command-to-get-previous-current-and-next-month