psql from Linux script

Dear Postgresql Specialists

I am failing to update the password of the postgresql user from within
a Linux installation script run by root:

# su - postgres -c echo "ALTER USER postgres WITH PASSWORD
'newpassword';" | psql -U postgres template1
psql: FATAL:  Ident authentication failed for user "postgres"

In contrast, there is no problem with the same command issued in psql
interactively:

# su postgres
$ psql template1
template1=# ALTER USER postgres WITH PASSWORD 'newpassword';
ALTER USER
\q
$ exit
#

The client authentication configuration file pg_hba.conf is in its
original state.

I would appreciate your help very much.

Thanks.

Bernard