Linux – How to restart atd without affecting running jobs

The VmPeak answer looks like a good one to me... You could either append vmpeak to a file every x seconds and then find the highest value. Or, every x seconds, run something like: VmPeak = curVmPeak if (curVmPeak > VmPeak).

Yes they are persistent across reboots (they're just files in a spool).

Regarding having access to them, as a regular user you won't have access to the files, but you could build a system to back them up. Maybe something like this:

MYAT=~/atjobs

/bin/rm -rf $MYAT/*

at -l >$MYAT/JOBS
for j in `cat $MYAT/JOBS | cut -f1`
do
  at -c $j >$MYAT/$i
done

If you needed to reload the job later:

for j in `cat $MYAT/JOBS | cut -f1`
do
  # make sure the job isn't defined
  atrm $j
  # reload it from the file
  at -f $MYAT/$j `grep ^$j $MYAT/JOBS | awk '{ print $3, $2 }'`
done

(this is all mostly untested. The basic command are right but there's sure to be a bug in the logic in there somewhere)

Having said all that though, I'm not sure I'd use at for the task you describe. I'd probably use a preexisting calendaring system. Failing that though, I would user a cron job that ran daily that checked a file to see if there were any messages to send. Much more portable than at jobs, and much more likely to be remembered if you switch machines...