Tracking time with one-shot funcion

trace-time.sh

function __trace_time () {
	# Call this function to trace how much time spends your code
	# Ex:
	#
	# #!/bin/bash
	#
	# source /path/to/time-track.sh
	#
	# __trace_time # First call initalizes some variables and checks,
	#              # this is T-0 point
	#
	# some_stuff
	# __trace_time # Check how much time `some_stuff` spent running
	#
	# other_stuff
	# __trace_time # Check how much time `other_stuff` spent running
	#              # (since previous call and since T0)

	local S="${1:-}"
	local __TRACE_TIME_DATE_CMD="${__TRACE_TIME_DATE_CMD:-date}"

	# First call
	if [ -z "${__TRACE_TIME_T0:-}" ]; then
		__TRACE_TIME_LOG_FILE="${__TRACE_TIME_LOG_FILE:-${BASH_SOURCE}.time-track}"

		# Check if shell can handle large integers
		[ $((1506939639262915000-1506939633986741000)) -eq 5276174000 ] || {
			echo "Math error in shell, aborting" >&2
			return
		}

		# Check if date command can use %N format for nanoseconds
		__TRACE_TIME_T0="$("$__TRACE_TIME_DATE_CMD" +%s%N)"
		if ! [ "$__TRACE_TIME_T0" -eq "$__TRACE_TIME_T0" ] 2>/dev/null; then
			echo "Support for nanoseconds missings, aborting" >&2
			unset __TRACE_TIME_T0
			return
		fi

		export __TRACE_TIME_T0
		export __TRACE_TIME_TPREV=$__TRACE_TIME_T0

		echo -e "delta (previous)\tdelta (from T-0)\tMessage" > "$__TRACE_TIME_LOG_FILE"
		return
	fi

	local NOW="$("$__TRACE_TIME_DATE_CMD" +%s%N)"
	local PREV_DELTA=$(($NOW-$__TRACE_TIME_TPREV))
	local TO_DELTA=$(($NOW-$__TRACE_TIME_T0))

	echo -e "${PREV_DELTA}\t${TO_DELTA}\t${S}" >> "$__TRACE_TIME_LOG_FILE"

	export __TRACE_TIME_TPREV=$NOW
}