psql: error: FATAL: Peer authentication failed for user "postgres" code example

Example 1: psql: FATAL: Ident authentication failed for user "postgres"

$ sudo -u postgres psql
\password
Enter password: ...
...

Example 2: ubuntu psql: error: FATAL: Peer authentication failed for user

local   all             postgres                                md5

Example 3: db access postgres peer

local   all             postgres                                peer