sorry for the delay, I've checked what you posted:
su userb <command>, and it works, thanks
related to sudo, is sudo a command for any particular distro or it's a general one? because I tried it in a server with this kernel:
Linux debian 2.2.20-idepci #1 Sat Apr 20 12:45:19 EST 2002 i686 GNU/Linux
maybe it's an old kernel version and "sudo" wasn't implemented yet or maybe "sudo" is a bash command feature, in this case the bash version implented in this machine is:
GNU bash, version 2.05b.0(1)-release (i386-pc-linux-gnu)
Copyright (C) 2002 Free Software Foundation, Inc.
thanks again for your comments.