I'm assuming you are running recent version of ubuntu or a distribution based on upstart. You can check
/var/log/daemon.log for errors.
su takes the syntax
su [options] [username]. Checkout
man 1 su. You might want to try :
su -c "myCommand" anotheruser >> "myLogfile.log"
Also, a couple of things would happen (mostly not desirable)
myLogfile.log would be owned by
myLogfile.log would be created on
/ (root directory) if you don't use an absolute path like
/tmp/myLogfile.log (because upstart runs with pwd set to
If you want the file to be owned by
anotheruser you might switch the command to.
su -c "myCommand >> /tmp/myLogfile.log" anotheruser
This might cause problems if you have leftover
myLogfile.log owned by
root from earlier runs or if have not changed
myLogfile.log to something like
/tmp/myLogfile.log (normally, regular users can't create files on root dir