Postgres password authentication fails

Question

I've installed PostgreSQL 9.1after that I tried to connect to the default PostgreSQL template database:     sudo -u postgres psql template1but login failed with this error message:       psql: FATAL:  password authentication failed for user "postgres"

Linux Help · Answer

You've locked yourself out, because you enabled md5 authentication for database user postgres without setting a password for the database user postgres.
Open the filevim /etc/postgresql/10/main/pg_hba.conf
Add a new line to the top of pg_hba.conf: local    postgres     postgres     peer
Then restart/reload PostgreSQL:  /etc/init.d/postgresql reload\
and run:  sudo -u postgres psql
From the resulting prompt:  ALTER USER postgres PASSWORD 'my_postgres_password';
Remove the line you added to pg_hba.conf and restart Pg again. You can now use the password you set above to connect toPostgreSQL as the postgres user.