man atd (8): run jobs queued for later execution

SYNOPSIS

atd [-l load_avg] [-b batch_interval] [-d] [-f] [-s]

atd runs jobs queued by at(1).

-l: Specifies a limiting load factor, over which batch jobs should not be run, instead of the compile-time choice of 1.5. For an SMP system with n CPUs, you will probably want to set this higher than n-1.
-b: Specify the minimum interval in seconds between the start of two batch jobs (60 default).
-d: Debug; print error messages to standard error instead of using syslog(3). This option also implies -f.
-f: Run atd in the foreground.
-s: Process the at/batch queue only once. This is primarily of use for compatibility with old versions of at; atd -s is equivalent to the old atrun command.

atd won't work if its spool directory is mounted via NFS even if no_root_squash is set.

/var/spool/cron/atjobs The directory for storing jobs; this should be mode 700, owner daemon.

/var/spool/cron/atspool The directory for storing output; this should be mode 700, owner daemon.

/etc/at.allow, /etc/at.deny determine who can use the at system.

The functionality of atd should be merged into cron(8).