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

1  –  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).