How to use Bash to create a folder if it doesn’t already exist?

First, in Bash [ is just a command, which expects string ] as a last argument, so the whitespace before the closing bracket (as well as between ! and -d which need to be two separate arguments too) is important:

if [ ! -d /home/mlzboy/b2c2/shared/db ]; then
  mkdir -p /home/mlzboy/b2c2/shared/db;
fi

Second, since you are using -p switch for mkdir, this check is useless, because this is what it does in the first place. Just write:

mkdir -p /home/mlzboy/b2c2/shared/db;

and that’s it.