executando
$ strace at
setreuid32(1000, 1000) = 0
setregid32(1000, 1000) = 0
open("/etc/at.allow", O_RDONLY) = -1 ENOENT (No such file or directory)
setregid32(1000, 1000) = 0
setreuid32(1000, 1000) = 0
setreuid32(1000, 1000) = 0
setregid32(1000, 1000) = 0
open("/etc/at.deny", O_RDONLY) = -1 EACCES (Permission denied)
setregid32(1000, 1000) = 0
setreuid32(1000, 1000) = 0
write(2, "You do not have permission to us"..., 38You do not have permission to use at.
) = 38
exit_group(1) = ?
pode mostrar o motivo.