OSX MacPorts install of postgres
Following their instructions to initialise the database, you get a complaing about /dev/null.
This is because the default postgres user is installed with a shell of /dev/null.
To change it, forget all the Unix stuff you know about administering users and do this
sudo dscl . -change /Users/postgres UserShell /dev/null /bin/sh
Or if you want to check first, check that the user exists
sudo dscl . -list /Users UniqueID
And then check the postgres user
sudo dscl . read /users/postgres