This is something that might more easily be done on current Ubuntu systems using a systemd timer.

Following the monotonic timer example in the archlinux wiki:

1. create a service unit file to run your command (/usr/local/bin/foo in this example) as a oneshot service

   
   

   [Unit]
   
   Description=do some stuff on schedule defined by foo.timer
   
   
   
   [Service]
   
   Type=oneshot
   
   ExecStart=/usr/local/bin/foo
   
   

   
   



2. create a corresponding timer unit with the desired time specifications (0 seconds after boot, and then 4 hours after the service unit was last activated)

   
   

   [Unit]
   
   Description=Run foo on boot and every 4 hours thereafter
   
   
   
   [Timer]
   
   OnBootSec=0
   
   OnUnitActiveSec=4h
   
   
   
   [Install]
   
   WantedBy=timers.target
   
   

   
   



3. enable the timer

   
   

   sudo systemctl enable foo.timer
   
   

   
   

After reboot, you should be able to see the new timer along with its last and next activation times:

$ systemctl list-timers foo

NEXT                        LEFT          LAST                        PASSED      UNIT      ACTIVATES  

Thu 2023-01-12 19:37:39 EST 3h 58min left Thu 2023-01-12 15:37:39 EST 1min 5s ago foo.timer foo.service



1 timers listed.

Pass --all to see loaded but inactive timers, too.