1  ON
   Performs a specified action when a command or program executed
   within a command procedure encounters an error condition or
   is interrupted by Ctrl/Y. The specified actions are performed
   only if the command interpreter is enabled for error checking or
   Ctrl/Y interrupts (the default conditions). Use the ON command
   only in a command procedure.

   Format

     ON  condition THEN [$] command
 

2  Parameters
 

condition

   Specifies either the severity level of an error or a Ctrl/Y
   interrupt. Specify one of the following keywords, which may be
   abbreviated to one or more characters:

   WARNING        Return status of warning occurs ($SEVERITY equals
                  0).

   ERROR          Return status of error occurs ($SEVERITY
                  equals 2).

   SEVERE_ERROR   Return status of error occurs ($SEVERITY
                  equals 4).

   CONTROL_Y      Ctrl/Y character occurs on SYS$INPUT.

   The default error condition is ON ERROR THEN EXIT.
 

command

   Specifies the DCL command line to be executed. Optionally, you
   can precede the command line with a dollar sign ($).

   If you specified an error condition as the condition parameter,
   the action is taken when errors equal to or greater than the
   specified level of error occur.
 

2  Examples

   1.
     A command procedure that contains this statement continues
     to execute normally when a warning or error occurs during
     execution. When a severe error occurs, the ON statement signals
     the procedure to execute the next statement anyway. Once
     the statement has been executed as a result of the severe
     error condition, the default action (ON ERROR THEN EXIT) is
     reinstated.

   2.$ ON ERROR THEN GOTO BYPASS
     $ RUN A
     $ RUN B
        .
        .
        .
     $ EXIT
     $ BYPASS:
     $      RUN C

     If either program A or program B returns a status code with a
     severity level of error or severe error, control is transferred
     to the statement labeled BYPASS and program C is run.

   3.$ ON WARNING THEN EXIT
        .
        .
        .
     $ SET NOON
     $ RUN [SSTEST]LIBRA
     $ SET ON
        .
        .
        .

     The ON command requests that the procedure exit when any
     warning, error, or severe error occurs. Later, the SET NOON
     command disables error checking before executing the RUN
     command. Regardless of any status code returned by the program
     LIBRA.EXE, the procedure continues. The next command, SET ON,
     reenables error checking and reestablishes the most recent ON
     condition.

   4.$ ON CONTROL_Y THEN GOTO CTRL_EXIT
        .
        .
        .
     $ CTRL_EXIT:
     $ CLOSE INFILE
     $ CLOSE OUTFILE
     $ EXIT

     The ON command specifies action to be taken when Ctrl/Y is
     pressed during the execution of this procedure: the GOTO
     command transfers control to the line labeled CTRL_EXIT. At
     CTRL_EXIT, the procedure performs cleanup operations (in this
     example, closing files and exiting).