How can I create a stopwatch in bash?

Question

I created a simple stopwatch (bash function) for counting time, but for now it's showing current time with milliseconds.

The code:

function stopwatch() {
    date +%H:%M:%S:%N
    while true; do echo -ne "`date +%H:%M:%S:%N`\r"; done;
}

I tried to change it as explained in this answer, but it works only with second since Unix Epoch.

When I used date format +%s.%N the subtraction from the answer above stopped working due to the fact that bash subtraction takes only integer.

How can I solve it and have a terminal stopwatch that prints time like so:

0.000000000
0.123123123
0.435345345
(and so on..)

?

Answer 1

One possible (& hacky) mechanism that can work for a day:

$ now=$(date +%s)sec
$ while true; do
     printf "%s\r" $(TZ=UTC date --date now-$now +%H:%M:%S.%N)
     sleep 0.1
  done

Bonus: You can press enter at any time to get the LAP times. ;-)

Note: This is a quick fix. Better solutions should be available...

watch based variant (same logic):

$ now=$(date +%s)sec; watch -n0.1 -p TZ=UTC date --date now-$now +%H:%M:%S.%N