Where is the cron timer

Question

Is there any way to access the cron timers and get the seconds left until the next execution of scripts in the crontab or maybe the seconds since the last?

Answer 1

Not easily. crond waits for a signal from the kernel and goes to sleep. When it gets the signal it checks whether there is any cronjobs to execute in that minute, launches those, and goes back to sleep.

It is a very efficient design - cron doesn't use any CPU while it is sleeping. It doesn't have an awareness of time passing either. As it goes to sleep it sets a "timeout" based on how long it is before the next command from any "registered" cronjob must run.

On Solaris 10:

sol10-primary:/> # pflags  271  
271:    /usr/sbin/cron
        data model = _ILP32  flags = ORPHAN|MSACCT|MSFORK
 /1:    flags = ASLEEP  pollsys(0x8047c70,0x1,0x8047ce8,0x806ba00)

While tracing the cron daemon, you will see it goes to sleep with a timeout, as below:

waitid(P_ALL, 0, 0x08047CD0, WEXITED|WTRAPPED|WNOHANG) Err#10 ECHILD
time()                                          = 1361952435
pollsys(0x08047C70, 1, 0x08047CE8, 0x0806BA00) (sleeping...)
        fd=3  ev=POLLRDNORM rev=0
        timeout: 105.000000000 sec
        sigmask = 0 0 0 0

When you update any cron job, the sleeping process is also woken up but to update its own configuration, after which it will go back to sleep with the new timeout value.

It is possible to see the timeout that was set. Notice that cron called the time syscall just before when it went to sleep (which returns the seconds since the epoc), so if you have observed this (i.e. traced the process when it called the time() syscall, you will be able to subtract that time from current and compare it to the timeout set on the pollsys call.

So... as I said, not easily.