How to get execution time of a script effectively?

Just use time when you call the script:

time yourscript.sh

If time isn't an option,

start=`date +%s`
stuff
end=`date +%s`

runtime=$((end-start))

Just call times without arguments upon exiting your script.

With ksh or zsh, you can also use time instead. With zsh, time will also give you the wall clock time in addition to the user and system CPU time.

To preserve the exit status of your script, you can make it:

ret=$?; times; exit "$ret"

Or you can also add a trap on EXIT:

trap times EXIT

That way, times will be called whenever the shell exits and the exit status will be preserved.

$ bash -c 'trap times EXIT; : {1..1000000}'
0m0.932s 0m0.028s
0m0.000s 0m0.000s
$ zsh -c 'trap time EXIT; : {1..1000000}'
shell  0.67s user 0.01s system 100% cpu 0.677 total
children  0.00s user 0.00s system 0% cpu 0.677 total

Also note that all of bash, ksh and zsh have a $SECONDS special variable that automatically gets incremented every second. In both zsh and ksh93, that variable can also be made floating point (with typeset -F SECONDS) to get more precision. This is only wall clock time, not CPU time.