How can I create a stopwatch in bash?

time cat

then press Ctrl-c or Ctrl-d to stop the timer and show the time. The first number is the time.

I've further refined it into this bash alias

alias stopwatch="echo Press Ctrl-c to stop the timer; TIMEFORMAT=%R; time cat; unset TIMEFORMAT"

If you want something simple that includes minutes, seconds, and centiseconds like a traditional stopwatch you could use sw.

Install

wget -q -O - http://git.io/sinister | sh -s -- -u https://raw.githubusercontent.com/coryfklein/sw/master/sw

Usage

# start a stopwatch from 0, save start time in ~/.sw
sw

# resume the last run stopwatch
sw --resume 

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

Here's a nicer function I grabbed a while ago:

function stopwatch() {
    local BEGIN=$(date +%s)
    echo Starting Stopwatch...

    while true; do
        local NOW=$(date +%s)
        local DIFF=$(($NOW - $BEGIN))
        local MINS=$(($DIFF / 60))
        local SECS=$(($DIFF % 60))
        local HOURS=$(($DIFF / 3600))
        local DAYS=$(($DIFF / 86400))

        printf "\r%3d Days, %02d:%02d:%02d" $DAYS $HOURS $MINS $SECS
        sleep 0.5
    done
}