sleep with countdown in bash

sleep-countdown

#!/bin/bash

sleep_time="$(( $1 - 1 ))" # subtract 1, since we wait on the 0th second.

start_s="$(date +%s)"
start_ns="$(date +%N)"

div_whole() {
  # whole-number division of integers.
  a="$1"
  b="$2"
  printf "%d" "$(( ( a - ( a % b) ) / b ))"
  }

countdown() {
  secs_total="$1"
  hours="$( div_whole "$secs_total" 3600 )"
  secs_total="$(( secs_total - ( 3600 * hours ) ))"
  mins="$( div_whole "$secs_total" 60 )"
  secs="$(( secs_total - 60 * mins ))"
  while [ "$hours" -ge 0 ]; do # count down hours
    while [ "$mins" -ge 0 ]; do # count down minutes in a single hour
      while [ "$secs" -ge 0 ]; do # count down seconds in a single minute
        printf "\033[0K\r%02d:%02d:%02d" "$hours" "$mins" "$secs"
        secs="$((secs - 1))"
        target_s="$(( sleep_time - (secs + 60 * mins + 3600 * hours) ))"
        target="$(( start_s + target_s ))"
        while [ "$(date +%s)" -lt "$target" ] # hi-res sleep loop
              || {
                   [ "$(date +%s)" -eq "$target" ]
                && [ "$(date +%N)" -lt "$start_ns" ]; };
              do
          sleep 0.01
          done # end hi-res sleep loop
        done # end count down seconds in a single minute
      secs=59
      mins="$((mins - 1))"
      done # end count down minutes in a single hour
      mins=59
      hours="$((hours - 1))"
    done # end count down hours
  printf "\033[0K\r" # clean up after we're done
  }

countdown "$sleep_time"