The /etc/at.allow files determine which user can submit commands for later execution via at pr batch .
The format of the files is a list of usernames, one on each line. Whitespace is not permitted.
The superuser may always use at.
If the file /etc/at.allow exists, only usernames mentioned in it are allowed to use at.
If /etc/at.allow does not exist, /etc/at.deny is checked.