Linux Usermanagement
Create default home directory for existing user
$ sudo mkdir /home/testinguser
$ sudo chown testinguser:testinguser /home/testinguser
$ sudo usermod -d /home/testinguser testinguser
sudo
Allow user to change to another user
Edit /etc/sudoers:
# visudo
ralf ALL=(postgres) ALL
Test it
ralf$ sudo -su postgres
or
ralf$ sudo -u postgres -i