sudo(8) -u postgres psql(1) -c 'SHOW config_file'
execute a command as another user
-u user     The -u (user) option causes sudo to run the specified command as a user other than root.  To
            specify a uid instead of a user name, use #uid.  When running commands as a uid, many shells
            require that the '#' be escaped with a backslash ('\').  Security policies may restrict uids
            to those listed in the password database.  The sudoers policy allows uids that are not in the
            password database as long as the targetpw option is not set.  Other security policies may not
            support this.
PostgreSQL interactive terminal
-c command

--command command
       Specifies  that  psql  is to execute one command string, command, and then exit. This is useful in
       shell scripts.

       command must be either a command string that is  completely  parsable  by  the  server  (i.e.,  it
       contains  no  psql  specific features), or a single backslash command. Thus you cannot mix SQL and
       psql meta-commands with this option. To achieve that, you could pipe the string  into  psql,  like
       this: echo '\x \\ SELECT * FROM foo;' | psql.  (\\ is the separator meta-command.)

       If  the command string contains multiple SQL commands, they are processed in a single transaction,
       unless there are explicit BEGIN/COMMIT commands included in the string to divide it into  multiple
       transactions.  This  is different from the behavior when the same string is fed to psql's standard
source manpages: sudopsql