How populate : variables dynamically?

I can't find a way to do something in PostgreSQL that I am used to in
Oracle:
How can I populate a variable in psql that can be used in subsequent
places in the same session, by SELECTing a value?

Here's an example in Oracle:
SET VERIFY OFF LINESIZE 132
COLUMN OSID            NEW_VALUE ORACLE_SID NOPRINT;
COLUMN SCHEMA_VALUE    NEW_VALUE SCHEMA NOPRINT;
COLUMN INSTANCEVERSION NEW_VALUE ORACLE_VERSION NOPRINT;
-- COLUMN MON_YY          NEW_VALUE MONYY NOPRINT;
SELECT SYS_CONTEXT('USERENV','DB_NAME') AS OSID            FROM DUAL;
SELECT USER                             AS SCHEMA_VALUE    FROM DUAL;
SELECT VERSION                          AS INSTANCEVERSION FROM V
$INSTANCE;
DEFINE OWNER=&&SCHEMA

PROMPT ORACLE_SID     IS &&ORACLE_SID
PROMPT SCHEMA         IS &&SCHEMA
PROMPT ORACLE_VERSION IS &&ORACLE_VERSION

SELECT '&&ORACLE_SID' AS ORASID, '&&ORACLE_VERSION' AS ORAVER,
'&&SCHEMA' AS CURRUSER FROM DUAL;


Here's the output from running it:
ORACLE_SID     IS NC9I
SCHEMA         IS DBMAINT
ORACLE_VERSION IS 9.2.0.6.0

ORASID                           ORAVER
CURRUSER
-------------------------------- --------------------------------
--------------------------------
NC9I                             9.2.0.6.0
DBMAINT

The point is to populate session variables dynamically using a select
statement
and then be able to use them more than once during the same session.
Some of these could be put in the Oracle login.sql so the values are
always available.

What I can not find is how to populate ':' variables in PostgreSQL
like this.  Is this supported, and I just haven't found the
documentation
that describes how to do it?

Thanks for your help.
--doug