so i want to defrag my modem every hour. since it always fragments every time



is this the right syntax?

also when i typed cronjob -l



why is it null? is that normal?

i meant crontab -l and i have already entered that job

Don't worry about the /dev/null stuff. That is just to prevent your system log being filled with all sorts of bloat.
What you should be worried about is trying to defragment your environment every hour. Defragmenting is not a trivial exercise, and should only be done when necessary. You should not need to do it more than once a month - but if you really feel the need to do it regularly, then once a week is more than sufficient.

even if my router gets fragmented every time i reboot it or edit some settings?

still once a week?

Yes. Fragmentation is inevitable, especially with the adam2 bootloader. It only begins to become a problem when the fragmentation level is very high (e.g., more than 30 for adam2 and more than 50 for pspboot).

Your cronjob will actually not defragment anything until the default threshold is reached - but the hourly checks are totally unnecessary. Weekly checks are fine, unless you start getting problems before the week is done (which would be most unlikely).