But that won't work because it throws away the output of the at command itself. at just saves your job in a file to be run later by a system program. The commands you want quiet are the commands stored in that file. One way to keep at quiet, if you use the C shell, is:
>& /dev/nullat> [CTRL-d]
The Bourne shell makes it easier:
Some versions of at have a -s option that runs your job with the Bourne shell.
Not all versions of at prompt you with
at> as I showed above.