For long-running processes it is sometimes useful to know what the
output is before you get the email, I use this instead:
ps to find the process ID of your running program (
lsof to find the file the output is being written to:
lsof -p PID
In the output, look for the
2u lines under the
FD column header.
These lines will tell you what temporary files the process output is
going into before it is sent by mail. Typically, for
at, the file is
/var/spool/cron/atjobs/. Finally, display that file
and you will find your process' (current) output.