Please verify the script below:
#/bin/bash
D=`date +%Y%m%d`
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