IBM i commands can be used to override a parameter in the specified file description or to redirect a file at compilation time or run time. File redirection allows you to specify a file at run time to replace the file specified in the program (at compilation time):
In the preceding example, the CL command OVRDBF (Override With Database File) allows the program to run with an entirely different device file than was specified at compilation time.
To override a file at run time, you must make sure that record names in both files are the same. The RPG program uses the record-format name on the input/output operations, such as a READ operation where it specifies what record type is expected.
Not all file redirections or overrides are valid. At run time, checking ensures that the specifications within the RPG program are valid for the file being processed. The IBM i system allows some file redirections even if device specifics are contained in the program. For example, if the RPG device name is PRINTER, and the actual file the program connects to is not a printer, the IBM i system ignores the RPG print spacing and skipping specifications.
There are other file redirections that the IBM i system does not allow and that cause the program to end. For example, if the RPG device name is WORKSTN and the EXFMT operation is specified in the program, the program is stopped if the actual file the program connects to is not a display or ICF file.
In ILE, overrides are scoped to the activation group level, job level, or call level. Overrides that are scoped to the activation group level remain in effect until they are deleted, replaced, or until the activation group in which they are specified ends. Overrides that are scoped to the job level remain in effect until they are deleted, replaced, or until the job in which they are specified ends. This is true regardless of the activation group in which the overrides were specified. Overrides that are scoped to the call level remain in effect until they are deleted, replaced, or until the program or procedure in which they are specified ends.
The default scope for overrides is the activation group. For job-level scope, specify OVRSCOPE(*JOB) on the override command. For call-level scope, specify OVRSCOPE(*CALLLVL) on the override command.
For more detailed information on valid file redirections and file overrides, refer to the DB2 Universal Database for iSeries section of the Database and File Systems category in the i5/OS Information Center at this Web site - http://www.ibm.com/systems/i/infocenter/.
ILE Concepts also contains information about overrides and activation group vs. job level scope.