how to execute a command as another user in one command?

["Star Liu" <minxinjianxin@gmail.com>]

currently i'm the root user, but I need to execute a command as
another user named postgres, I can do it in 2 commands, and it works
fine.
-----------
su postgres
postgres -D ~/data > ~/log/logfile 2>&1 &
-----------
but i hope I can do it in one command, so I tried this
---------------
su -c "postgres -D ~/data > ~/log/logfile 2>&1 &" postgres
----------------
but it doesn't work. what's wrong with it? or any other command to
realize it? thanks



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