$ at time date

$ at 11 am may 20

# at 02:00 AM Fri
at> yum update glibc
at> echo "Executing scheduled task" | logger
at> shutdown -r +5 "Server is going to be rebooted in 5 minutes for scheduled task. Please save your work ASAP." 
at> <EOT>
job 2 at 2015-02-06 02:00

Ends using CTRL+D that generates the <EOT> bit.

Source: http://www.computerhope.com/unix/uat.htm

Check the processes in the queue

atq

Check content of a job

at -c <job number>

Delete job

atrm <job number>

Also, you can cat/modify the job in /var/spool/cron/atjobs/ or /var/spool/at (in Centos)

If not installed, on Centos, make sure to start also ‘atd‘ service.

# chkconfig atd on && service atd start