Elapsed time in shell scripting

Today I was looking at some building tools and they often print the time they take to do something. I wanted to add this feature to the configure-script.

It turns out to be very easy, just using the date command bring us the elapsed time. There’s just a little detail, BSD date behaves a bit differently from GNU date. More precisely BSD doesn’t have the %N format for nanoseconds. We just need to check if the system uses a BSD date or a GNU one

#! /bin/bash
# Save the current time
if man date | grep BSD >/dev/null 2>/dev/null; then
  IS_BSD="YES"
fi
if [ "${IS_BSD}" ]; then
  LOG_START=`date +%s`
else
  LOG_START=`date +%s%N`
fi

# Do stuff
...

# Get the time and make the difference
if [ "${IS_BSD}" ]; then
  LOG_END=`date +%s`
  ELAPSED=$(( $LOG_END - $LOG_START ))
else
  LOG_END=`date +%s%N`
  ELAPSED=`echo "scale=8; ($LOG_END - $LOG_START) / 1000000000" | bc`
fi

This is more like a note to myself but it may be usefull for someone one day :)