i="someparameter"

echo $D_$i

echo $D\_$i

echo $D

echo $i

echo _

//   my system is debian with coreutils 8.26-3

//
//   db=`date -d "-1day" +%Y%m%d`

//   i="0"

This is the result from my system

~$ echo "$db_$i"
0
~$ echo "$db\_$i"
20170413\_0
~$ echo "$db$i"
201704130

Thank you!