Bash countdown¶

This uses a few tricks to do what you want:

• The printf command does NOT append a linefeed automatically unless you tell it to.
• Setting IFS to something else lets the shell split things apart into arrays on whatever delimiter you want, in this case , :.
• The date command can be used to produce time in seconds.
• Carriage returns return the cursor to the beginning of the line without moving to the next line.

function countdown
{
  local OLD_IFS="${IFS}" 
  IFS=":" 
  local ARR=( $1 )
  local SECONDS=$((  (ARR[0] * 60 * 60) + (ARR[1] * 60) + ARR[2]  ))
  local START=$(date +%s)
  local END=$((START + SECONDS))
  local CUR=$START

  while [[ $CUR -lt $END ]]
  do
    CUR=$(date +%s)
    LEFT=$((END-CUR))

    printf "\r%02d:%02d:%02d" \
            $((LEFT/3600)) $(( (LEFT/60)%60)) $((LEFT%60))

            sleep 1
  done
  IFS="${OLD_IFS}" 
  echo "        " 
}

countdown "00:07:55"