Creating a user for postgresql

This took some time to figure out (found answer here)and it might come in handy sometime in the far future so I write it down here and now.
To create a user for the postgresql database on linux you have to do that from the linux user called postgres.

This would not work:

peterbe@computer:~$ createuser peterbe

What you need to do is this:

peterbe@computer:~$ su -
...log in as root...
computer:~# su postgres
computer:/root$ createuser peterbe

Now I know!