I'm using Ubuntu 10.04 LTS server, with the default security model (root locked, using sudo to elevate privileges). I occasionally enjoy using sudo -i when I'll need to run a series of commands with elevated privileges, or when I need to rummage around in directories with root-only privileges.

Sometimes, when setting up software that'll run as its own non-privileged system account (adduser --system --group --no-create-home --disabled-login some-daemon-user) I find that I need to run a sequence of commands as that user, rather than myself or root. I've tried using sudo -i -u some-daemon-user, but it just returns a 1 status without any error message.

I've checked the syslog, messages, auth, and debug log files in /var/log and none of them include any messages that reference sudo or the account in question.

So, is it possible to become another non-root user, sudo-style without just setting a password and logging in (as them)? Is my system 'broken' in some way or am I just doing it wrong?