#!/bin/bash
if [ "$1" = "" ]; then
    echo "usage: $0 \$times_to_flip"
    echo "shows all possible permutations of a cointoss"
    exit 1
fi

charset=(H T)
permute(){
  (($1 == 0)) && { echo "$2"; return; }
  for char in "${charset[@]}"
  do
    permute "$((${1} - 1 ))" "$2$char"
  done
}
permute "$1"