Trouble with postgresql

I have uploaded a question about this but it hasn't been solved so I am re-posting it.

I am experiencing a problem in installing Postgresql on Ubuntu 14.04. What I did was:

1. sudo su -
2. sudo apt-get install postgresql postgresql-contrib
3. update-rc.d postgresql enable
4. service postgresql start

Doing #3, I got:

    update-rc.d: warning:  start runlevel arguments (none) do not match postgresql Default-Start values (2 3 4 5)
    update-rc.d: warning:  stop runlevel arguments (none) do not match postgresql Default-Stop values (0 1 6)
    Enabling system startup links for /etc/init.d/postgresql ...
    Removing any system startup links for /etc/init.d/postgresql ...
    /etc/rc0.d/K21postgresql
    /etc/rc1.d/K21postgresql
    /etc/rc2.d/S19postgresql
    /etc/rc3.d/S19postgresql
    /etc/rc4.d/S19postgresql
    /etc/rc5.d/S19postgresql
    /etc/rc6.d/K21postgresql
    Adding system startup for /etc/init.d/postgresql ...
    /etc/rc0.d/K21postgresql -> ../init.d/postgresql
    /etc/rc1.d/K21postgresql -> ../init.d/postgresql
    /etc/rc6.d/K21postgresql -> ../init.d/postgresql
    /etc/rc2.d/S19postgresql -> ../init.d/postgresql
    /etc/rc3.d/S19postgresql -> ../init.d/postgresql
    /etc/rc4.d/S19postgresql -> ../init.d/postgresql
    /etc/rc5.d/S19postgresql -> ../init.d/postgresql

In doing #4, I am getting this error:

    The PostgreSQL server failed to start. Please check the log output:
    2015-07-17 21:54:24 PHT LOG:  could not open secondary authentication file "@admins" as "/etc/postgresql/9.3/main/admins": No such file or directory
    2015-07-17 21:54:24 PHT LOG:  could not open secondary authentication file "@admins" as "/etc/postgresql/9.3/main/admins": No such file or directory
    2015-07-17 21:54:24 PHT LOG:  could not open secondary authentication file "@demodbs" as "/etc/postgresql/9.3/main/demodbs": No such file or directory
    2015-07-17 21:54:24 PHT LOG:  end-of-line before authentication method
    2015-07-17 21:54:24 PHT CONTEXT:  line 75 of configuration file "/etc/postgresql/9.3/main/pg_hba.conf"
    2015-07-17 21:54:24 PHT FATAL:  could not load pg_hba.conf

I thought I installed Postgresql correctly. What is wrong?

Copyright Notice：Content Author:「David Kim」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/31477677/trouble-with-postgresql