Print set of numbers as ranges

ranges.sh

#!/bin/sh
sort -n | awk '
    BEGIN     { last = "x" }
    /^[0-9]+$/ {
                if (last == "x") {
                    range = $1
                }
                else if (last + 1 != int($1)) {
                    if (range != last){
                        printf "%d-%d\n", range, last
                        range = int($1)
                    }
                    else {
                        printf "%d\n", last
                        range = int($1)
                    }
                }
                last = $1
    }
    END {
                if (range != last){
                    printf "%d-%d\n", range, last
                }
                else {
                    printf "%d\n", last
                }
    }'