How connect Postgres to localhost server using pgAdmin on Ubuntu?

Modify password for role postgres:

sudo -u postgres psql postgres

alter user postgres with password 'postgres';

Now connect to pgadmin using username postgres and password postgres

Now you can create roles & databases using pgAdmin

How to change PostgreSQL user password?

You haven't created a user db. If its just a fresh install, the default user is postgres and the password should be blank. After you access it, you can create the users you need.

It helps me:

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1. Open the file pg_hba.conf

sudo nano /etc/postgresql/9.x/main/pg_hba.conf

and change this line:

Database administrative login by Unix domain socket
local   all             postgres                                md5

to

Database administrative login by Unix domain socket
local   all             postgres                                trust

2. Restart the server

   sudo service postgresql restart

3. Login into psql and set password

   psql -U postgres

ALTER USER postgres with password 'new password';

4. Again open the file pg_hba.conf and change this line:

Database administrative login by Unix domain socket
    local   all             postgres                                trust

to

    Database administrative login by Unix domain socket
    local   all             postgres                                md5

5. Restart the server

   sudo service postgresql restart

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It works.

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Helpful links
1: PostgreSQL (from ubuntu.com)