SQL> 
SQL> -- check the code of TEST_FN in my schema
SQL> select text from user_source where name='TEST_FN' order by line;

TEXT
--------------------------------------------------------------------------------
function test_fn
RETURN varchar2
IS
test_date VARCHAR2(100);
BEGIN

select to_char(sysdate,'DD-MON-YY') into test_date from dual;
return test_date ;
END test_fn;

9 rows selected.

SQL> 
SQL> -- check if there's a function (or any object for that matter) called "TEST_FN1"
SQL> select object_type, object_name, status from user_objects where object_name = 'TEST_FN1';

no rows selected

SQL> 
SQL> -- now try to call the (supposed) function "TEST_FN1"
SQL> declare
  2  x varchar2(20);
  3  begin
  4  x := test_fn1;
  5  end;
  6  /
x := test_fn1;
     *
ERROR at line 4:
ORA-06550: line 4, column 6:
PLS-00201: identifier 'TEST_FN1' must be declared
ORA-06550: line 4, column 1:
PL/SQL: Statement ignored


SQL> 
SQL> -- notice the "*" in the second line of the error message ?
SQL> -- now try another error - assign the return value of "TEST_FN" to a *NUMBER* instead of a *VARCHAR2*
SQL> 
SQL> declare
  2  x number;
  3  begin
  4  x := test_fn;
  5  end;
  6  /
declare
*
ERROR at line 1:
ORA-06502: PL/SQL: numeric or value error: character to number conversion error
ORA-06512: at line 4


SQL> 
SQL> -- notice the "*" in the second line of the error message ?
SQL> 
SQL>

$ 
$ # First, try out the error condition in the shell
$ echo "declare
> x varchar2(20);
> begin
> x := test_fn1;
> end;
> /
> " | sqlplus -s test/test
x := test_fn1;
     *
ERROR at line 4:
ORA-06550: line 4, column 6:
PLS-00201: identifier 'TEST_FN1' must be declared
ORA-06550: line 4, column 1:
PL/SQL: Statement ignored


$ # This was as expected. The text in blue is being assigned to shell
$ # variable "var" in your script. Let's try something like that -
$ 
$ my_variable=`echo "declare
> x varchar2(20);
> begin
> x := test_fn1;
> end;
> /
> " | sqlplus -s test/test`
$ 
$ # ok, so now the entire error message would've been assigned to "my_variable"
$ # Let's print it with double quotes
$ 
$ echo "Value of my_variable is ==>|$my_variable|<=="
Value of my_variable is ==>|x := test_fn1;
     *
ERROR at line 4:
ORA-06550: line 4, column 6:
PLS-00201: identifier 'TEST_FN1' must be declared
ORA-06550: line 4, column 1:
PL/SQL: Statement ignored|<==
$ 
$ # That's all fine and dandy. 
$ # Note 1 - newlines are printed as they are.
$ # Note 2 - the "*" is printed as it is.
$ # Now try without double quotes. I am printing 3 strings here
$ 
$ echo "Value of my_variable is ==>|" $my_variable "|<=="
Value of my_variable is ==>| x := test_fn1; calltest_fn.ksh create_function.sql file1 file2 file3 ERROR at line 4: ORA-06550: line 4, column 6: PLS-00201: identifier 'TEST_FN1' must be declared ORA-06550: line 4, column 1: PL/SQL: Statement ignored |<==
$ 
$ 
$ # Note 1 - newlines in 2nd string were removed by the *shell*
$ # Note 2 - the first and third strings were printed faithfully
$ # Note 3 - first string is "Value of my_variable is ==>|", third string is "|<=="
$ # Note 4 - the shell started printing the value of "my_variable" till "x := test_fn1", but then it encountered a "*", so it expanded it
$ # Note 5 - and after that it printed "ERROR at line 4:" and so on removing newlines as it went
$ 
$

$ 
$ cat -n calltest_fn.ksh
     1    #!/bin/ksh
     2    #set -x
     3    
     4    var=`sqlplus -s /nolog <<EOF
     5    connect test/test
     6    WHENEVER SQLERROR EXIT FAILURE
     7    WHENEVER OSERROR  EXIT FAILURE
     8    set serveroutput on
     9    set feedback off
    10    set linesize 250
    11    DECLARE
    12    test_var varchar2(100);
    13    BEGIN
    14    test_var:=test_fn1;
    15    dbms_output.put_line(test_var);
    16    END;
    17    /
    18    quit;
    19    EOF`
    20    
    21    echo "Value of var is $var"
    22    
$ 
$ 
$ ./calltest_fn.ksh
Value of var is test_var:=test_fn1;
          *
ERROR at line 4:
ORA-06550: line 4, column 11:
PLS-00201: identifier 'TEST_FN1' must be declared
ORA-06550: line 4, column 1:
PL/SQL: Statement ignored
$ 
$

$ 
$ 
$ cat -n calltest_fn.ksh
     1    #!/bin/ksh
     2    #set -x
     3    
     4    var=`sqlplus -s /nolog <<EOF
     5    connect test/test
     6    WHENEVER SQLERROR EXIT FAILURE
     7    WHENEVER OSERROR  EXIT FAILURE
     8    set serveroutput on
     9    set feedback off
    10    set linesize 250
    11    DECLARE
    12    test_var varchar2(100);
    13    BEGIN
    14    test_var:=test_fn;
    15    dbms_output.put_line(test_var);
    16    END;
    17    /
    18    quit;
    19    EOF`
    20    
    21    echo "Value of var is $var"
    22    
$ 
$ ./calltest_fn.ksh
Value of var is 15-MAY-10
$ 
$