gwire · April 22, 2023 16:23 · Apr 22, 2023
diff --git a/sorting_arpa_list.md b/sorting_arpa_list.md
@@ -0,0 +1,33 @@
+I have a process that outputs a list of `.in-addr.arpa` values. These consist of names with 3 to 6 labels.
+
+```
+161.187.42.143.in-addr.arpa
+18.139.243.162.in-addr.arpa
+38.51.19.58.in-addr.arpa
+136.67.34.in-addr.arpa
+16.134.243.162.in-addr.arpa
+18.240.203.159.in-addr.arpa
+240.54.in-addr.arpa
+230.157.in-addr.arpa
+```
+
+I wanted to sort this list, where the most significant number is the third label from the right hand side, and the order is numeric.
+
+I assumed I could do this with a simple command line unix tool, eg `sort`, but I couldn't figure out how to specify the columns as relative to the right hand side.
+
+My hacky solution: 
+* reverse the components into a 6 column, space-separated, list
+* remove any empty columns at the beginning, such that column one is always `arpa`
+* Use numeric sort, keyed on columns 3,4,5,6
+* reverse the components into a 6 column, dot-separated, list
+* remove any empty columns at the beginning
+
+```sh
+./process \
+| awk -F. '{print $6,$5,$4,$3,$2,$1}' \
+| sed 's/^[\ ]*//' \
+| sort -k 3,3 -k 4,4 -k 5,5 -k 6,6 -g \
+| awk '{print $6,$5,$4,$3,$2,$1}' OFS=. \
+| sed 's/^[\.]*//' \
+| uniq
+```