如何创建秒表Bash脚本以持续显示已用时间？

... working [hh:mm:ss]  00:07:58

片段：

#===  FUNCTION  ================================================================
#          NAME:  progressIndicatorTime
#   DESCRIPTION:  Display a progress indicator with seconds passed.
#    PARAMETERS:  [increment]   between 1 and 60 [sec],default is 2 [sec]
#===============================================================================
function progressIndicatorTime ()
{
  declare default=2                                       # default increment [sec]
  declare increment="${1:-$default}"                      # 1. parameter or default
  declare format='bbbbbbbb%02d:%02d:%02d'         # time format hh:mm:ss
  declare timepassed=0
  declare seconds minutes hours

  [[ ! "$increment" =~ ^([1-9]|[1-5][0-9]|60)$]] && increment=$default
  printf " ... working [hh:mm:ss]  00:00:00"
  while : ; do                                            # infinite loop 
    ((seconds=timepassed%60))
    ((minutes=timepassed/60))
    ((hours=minutes/60))
    ((minutes=minutes%60))
    printf "$format" $hours $minutes $seconds
    sleep $increment || break                             # sleep ...
    ((timepassed+=increment))
  done
}    # ----------  end of function progressIndicatorTime  ----------

progressIndicatorTime &                                   # run progress indicator
declare progressIndicatorPid=${!}                         # save process ID
trap  "kill $progressIndicatorPid" INT TERM               # trap keyboard interrupt

#
# run long command
#

kill -s SIGTERM $progressIndicatorPid                     # terminate progress indicator