Keep DOI entries when using bibtool

.bibtoolrsc

@@ -7,8 +7,10 @@ print.parentheses = true
 
 
 % sort order for fields
-sort.order {* = author # title # school # type # year # month # day # journal # booktitle # volume # pages # publisher # adress # url # urlaccessdate}
+sort.order {* = author # title # school # type # year # month # day # journal # booktitle # volume # pages # publisher # adress # url # urlaccessdate # doi}
 
+% Keep DOI entries
+keep.field {doi}
 
 % Remove double braces around titles.
 rewrite.rule { title # "^\{[\{]\([^#]*\)[\}]\}$" # "{\1}" }