13.5 Returning Program Status
  A PROGRAM unit lets you return a status from a BASIC
  image by optionally including an integer expression with the
  END PROGRAM and EXIT PROGRAM statements. After
  executing a program, you can examine this status by check-
  ing the DCL symbol $STATUS. By default, BASIC returns
  a status of 1, indicating success. Success is signaled with an
  odd numbered status value, while an error is signaled with
  an even numbered value. $STATUS contains the same value
  as the integer expression for the exit status in the EXIT and
  END PROGRAM statements. Note that if a program is ter-
  minated with an EXIT PROGRAM statement, the expression
  on the EXIT PROGRAM statement overrides any expression
  on the END PROGRAM statement.

  In the following example, exit_status contains the status
  value returned by the program. After program execution,
  $STATUS has the value of exit_status . You can examine the
  value of $STATUS and display the corresponding message
  text with the lexical function F$MESSAGE at DCL level, as
  shown in the following example:
  EXAMPLE: Click to display example.

  After program execution, you can examine the status of the
  program at DCL level:
  $ SHOW SYMBOL $STATUS
  $ STATUS = "%X10"
  $ error_text = F$MESSAGE(%X10)
  $ SHOW SYMBOL error_text
  ERROR_TEXT = "SYSTEM-W-BADPARAM, bad parameter value"

  The PROGRAM statement is always optional; EXIT
  PROGRAM and END PROGRAM are legal without a
  matching PROGRAM statement. Without a PROGRAM
  statement, these statements still exit the main compilation
  unit. The EXIT PROGRAM and END PROGRAM state-
  ments are not valid within SUB, FUNCTION, or PICTURE
  subprograms.