in reply to system command is not working

I agree with pvaldes, it seems like you are not using at correctly. In fact, I'm surprised at is not returning an error immediately. I have tried three Unix flavors and get an error when I do not specify a time.

me@myaixbox:/home/me/sandbox $ at -f at: 0481-110 Command usage error; a parameter is missing. me@myaixbox:/home/me/sandbox $ uname AIX
me@mylinuxbox:~/sandbox $ at -f Garbled time me@mylinuxbox:~/sandbox $ uname Linux
userid@somebox:/tmp/tmp.66 # at -f bad time specification userid@somebox:/tmp/tmp.66 # uname HP-UX