Use the date Command to Measure Elapsed Time

When running bash scripts that take a long time to run it's often useful to know how long it took for the script to run. In addition to the overall run time, it's also often useful to know how long certain parts of the script took to run. The time command doesn't really help because it's meant to time a single command, not a sequence of commands. By using the %s format of date the script described here allows you to create as many timers as you want and time whatever part of a script you want.

The %s format of the date command outputs the number of seconds since Unix time began:

  $ date +'%s'
  1227068222

Using two of these values you can determine the elapsed time.

The following script defines a single bash function: timer. If called with no arguments it outputs the current second count. If called with an argument it assumes the argument is a value previously obtained by calling timer with no arguments and it outputs the time that has elapsed since the first value was obtained.

#!/bin/bash
#
# Elapsed time.  Usage:
#
#   t=$(timer)
#   ... # do something
#   printf 'Elapsed time: %s\n' $(timer $t)
#      ===> Elapsed time: 0:01:12
#
#
#####################################################################
# If called with no arguments a new timer is returned.
# If called with arguments the first is used as a timer
# value and the elapsed time is returned in the form HH:MM:SS.
#
function timer()
{
    if [[ $# -eq 0 ]]; then
        echo $(date '+%s')
    else
        local  stime=$1
        etime=$(date '+%s')

        if [[ -z "$stime" ]]; then stime=$etime; fi

        dt=$((etime - stime))
        ds=$((dt % 60))
        dm=$(((dt / 60) % 60))
        dh=$((dt / 3600))
        printf '%d:%02d:%02d' $dh $dm $ds
    fi
}

# If invoked directly run test code.
if [[ $(basename $0 .sh) == 'timer' ]]; then
    t=$(timer)
    read -p 'Enter when ready...' p
    printf 'Elapsed time: %s\n' $(timer $t)
fi

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To use the function first obtain a starting timer value in the following manner:

  tmr=$(timer)

Then when you want to know how much time has elapsed, pass the original timer value and print the result. For example, to print the timer obtained above:

  printf 'Elapsed time: %s\n' $(timer $tmr) 

Running the timer.sh script directly runs it in test mode. It obtains a timer, waits for you to hit enter, then prints the elapsed time:

  $ sh timer.sh
  Enter when ready...
  # Wait a while here
  Elapsed time: 0:01:12