hostsort

#!/bin/sh
# hostsort (part of ossobv/vcutil) // wdoekes/2023 // Public Domain
#
# Sort input lines by hostname (top-down, right-to-left).
#
# USAGE
#
#     $ printf '%s\n' \
#         node3.example.com node2.domain.tld \
#         node1.domain.tld master.example.com | hostsort
#     master.example.com
#     node3.example.com
#     node1.domain.tld
#     node2.domain.tld
#
# EXPLANATION
#
#     Sorts the input hostnames top-down, from right to left.
#     This is done by translating "node2.domain.tld" into
#     "tld.domain.node2." before sorting.
#
# WHY
#
#     "I have to say that now I regret that the syntax is so clumsy. I
#     would like http://www.example.com/foo/bar/baz to be just written
#     http:com/example/foo/bar/baz where the client would figure out
#     that www.example.com existed and was the server to contact."
#     --Tim Berners-Lee
#
#     But, because we're stuck with a bottom-up hostname we'll have to
#     accept it and live with it.
#
#     Using hostsort, you'll get easier groupings.
#
# TODO
#
#     We'll probably want to add the possibility of skipping over parts
#     before sorting (-k<N>) and leave other non-hostname parts untouched.
#     E.g. to sort a CSV by hostname. Right now it only works if you put
#     the hostname in the last field.
#
#     Also, we don't support numeric/version sort, so node11 will sort
#     before node2.
#
#     mawk does not have asorti. Maybe switch to perl instead?
#     https://github.com/decklin/bin/blob/master/hostsort
#
exec gawk -F. '
  {r="";for(i=NF;i>=1;--i)r=r$i".";m[r]=$0}
  END {n=asorti(m,o);for(i in o)print m[o[i]]}
'