Information
As with any server daemon that is accessible to the outside world, it is advisable to run PostgreSQL under a separate user account. This user account should only own the data that is managed by the server, and should not be shared with other daemons. (For example, using the user nobody is a bad idea.) It is not advisable to install executables owned by this user because compromised systems could then modify their own binaries.
Solution
To add a Unix user account to your system, look for a command useradd or adduser.