Another solution to the pipeline example, this time making use of a subprogram for the frequency/accumulation:

    < README.md \
      tr -c '[:alpha:]' '\n' \
    | tr '[:upper:]' '[:lower:]' \
    | awk '
        NF {
          if (!($0 in count)) order[++n] = $0
          count[$0]++
        }
        END {
          for (i = 1; i <= n; i++) {
            print count[order[i]], order[i]
          }
        }
    '

You can skip `tr` as well - works on BSD awk and GNU awk.

    {
        $0 = tolower($0)
        gsub(/[^[:alpha:]]/, "\n")
        for (i = 1; i <= NF; i++) {
            if (!($i in freq)) order[++n] = $i
            freq[$i]++
        }
    } END {
        for (i = 1; i <= n; i++)
            printf "%d %s\n", freq[order[i]], order[i]
    }

Nice to see more people getting nerdsniped by the sh code. ;-)

Yes, associative arrays work well. I think it should even be possible to use bash associative arrays. But at that point you're no longer doing classic sh - awk is basically halfway to Perl. (And pretty awesome.)