How to read/write files from PL/SQL?

The UTL_FILE database package can be used to read and write operating system files.
A DBA user needs to grant you access to read from/ write to a specific directory before using this package.
Here is an example:

CONNECT / AS SYSDBA
CREATE OR REPLACE DIRECTORY mydir AS '/tmp';
GRANT read, write ON DIRECTORY mydir TO scott;

Provide user access to the UTL_FILE package (created by catproc.sql):

GRANT EXECUTE ON UTL_FILE TO scott;

Copy and paste these examples to get you started:

Write File
----------
DECLARE
fHandler UTL_FILE.FILE_TYPE;
BEGIN
fHandler := UTL_FILE.FOPEN('MYDIR', 'myfile', 'w');
UTL_FILE.PUTF(fHandler, 'Look ma, Im writing to a file!!!\n');
UTL_FILE.FCLOSE(fHandler);
EXCEPTION
WHEN utl_file.invalid_path THEN
raise_application_error(-20000, 'Invalid path. Create directory or set UTL_FILE_DIR.');
END;
/
Read File
---------
DECLARE
fHandler UTL_FILE.FILE_TYPE;
buf varchar2(4000);
BEGIN
fHandler := UTL_FILE.FOPEN('MYDIR', 'myfile', 'r');
UTL_FILE.GET_LINE(fHandler, buf);
dbms_output.put_line('DATA FROM FILE: '||buf);
UTL_FILE.FCLOSE(fHandler);
EXCEPTION
WHEN utl_file.invalid_path THEN
raise_application_error(-20000, 'Invalid path. Create directory or set UTL_FILE_DIR.');
END;
/
NOTE: UTL_FILE was introduced with Oracle 7.3. Before Oracle 7.3 the only means of writing a file was to use DBMS_OUTPUT with the SQL*Plus SPOOL command.