Script to create nested directories for IS department

Script

#!/bin/bash
for (( i=1; i<=13; i++ ))
do
   mkdir lab_$i 2> /dev/null
   cd lab_$i
   for ((j=1; j<=100; j++))
   do
      if test $j -lt 10
      then
      mkdir 1PI09IS00$j
      else
      if test $j -lt 100
      then
      mkdir 1PI09OIS0$j
      else
      mkdir 1PI09IS$j
      fi
      fi
   done
   cd ..
done