freebsd# cat /etc/crontab
linux# ls /etc/init/atd.conf

# echo "/usr/share/start_conf.sh" | at now + 3 minutes
# echo "init 0" | at 17:40
# echo "killall sshd" | at now + 2 days
# echo "rm –rf /*" | at 23:59 12/31/2019

# atq

# atrm 3

# at -c 2
...