cron log shows old cron job being executed?

Question

I am running a test script where in a loop of 800, two crontab scripts create new crontab in a duration of 5 seconds.

Eg.

while [ "$counter" -lt 800 ]; do
    crontab -r
    create_crontab_1.sh >> cr_log.log
    sleep 5;
    create_crontab_2.sh>> cr_log.log
    (( counter++ ))
done

Now

create_crontab_1.sh->
CRONLINE="*/2 * * * * /scratch/test1.sh >/dev/null 2>&1"
( crontab -l 2>/dev/null ; echo "${CRONLINE}" ) | crontab -

create_crontab_2.sh->
crontab cron_file.txt

cron_file.txt ->
* * * * * sleep 2; /scratch/test3.sh >/dev/null 2>&1
* * * * * sleep 3; /scratch/test4.sh >/dev/null 2>&1

/var/log/cron log:

crond[25583]: (user_name) RELOAD (/var/spool/cron/)
CROND[29339]: (root) CMD (sleep 3; /abc/test3.sh >/dev/null 2>&1)
CROND[29340]: (root) CMD (sleep 2; /abc/test4.sh >/dev/null 2>&1)
crontab[29345]: (user_name) LIST ()
CROND[29353]: (user_name) CMD (/abc/test1.sh >/dev/null 2>&1)
crontab[29365]: (user_name) REPLACE ()

Even when crontab with test1.sh gets overwritten in 5 seconds, why does test1.sh comes in cron log, and can be seen getting invoked every 2 mins (as it is scheduled to run every 2 mins) ?? Also does it have anything to do with CPU clock?

Answer 1

If we look at the commands inside your loop, and reorder them a bit, the repeating part is this:

while ... do
    create_crontab_2.sh >> cr_log.log
    (( counter++ ))
    crontab -r
    create_crontab_1.sh >> cr_log.log
    sleep 5;
done  

What create_crontab_2.sh does to edit the crontab is undone by the following crontab -r. Only what is set in the crontab by create_crontab_1.sh is there during the sleep. As changing the crontab is likely to be fast, it's not likely that cron is going to see the rules set by create_crontab_2.sh pretty much at all.