Measure elapsed time of process with bash

start=$(date +%s)

# processes you wanna measure
sleep 3

end=$(date +%s)
echo "elapsed time: $((end - start)) sec"

start=$(date +%s%N)

sleep 1

end=$(date +%s%N)
elapsed=$(( (end - start)/1000000 ))  # convert with msec
echo "elapsed time: ${elapsed} ms"