So cron or at can schedule our commands to run at  the exact time we need them to, but can we schedule commands to run when systems are inactive?Something like:echo some_commands | when 'cpu < 15%' 
Scheduling commands by system inactivity
command line;scheduling
hmmm... I don't think so... but what you could do is cron a script to run like every 5 minutes and check the load average to see if it's acceptably low. I wouldn't check the current because you could get the cpu in between 2 really high peaks. This is just thoughts on what I'd do to accomplish this, but there might be a better way.