Tests the value of an expression and, depending on the syntax
    specified, executes the following:

    o  One command following the THEN keyword if the expression is
       true

    o  Multiple commands following the $THEN command if the
       expression is true

    o  One or more commands following the $ELSE command if the
       expression is false

    Format

      $ IF  expression THEN [$] command

      or

      $ IF  expression

      $ THEN  [command]

      command

         .
         .
         .

      $ [ELSE]  [command]

      command

         .
         .
         .

      $ ENDIF

                                   NOTE

       VSI advises against assigning a symbolic name that is already
       a DCL command name. VSI especially discourages the assignment
       of symbols such as IF, THEN, ELSE, and GOTO, which can
       affect the interpretation of command procedures.

1  –  Parameters

 expression

    Defines the test to be performed. The expression can consist of
    one or more numeric constants, string literals, symbolic names,
    or lexical functions separated by logical, arithmetic, or string
    operators.

    Expressions in IF commands are automatically evaluated during
    the execution of the command. Character strings beginning with
    alphabetic characters that are not enclosed in quotation marks
    (" ")  are assumed to be symbol names or lexical functions. The
    command language interpreter (CLI) replaces these strings with
    their current values.

    Symbol substitution in expressions in IF commands is not
    iterative; that is, each symbol is replaced only once. However,
    if you want iterative substitution, precede a symbol name with an
    apostrophe (')  or ampersand (&).

    The command interpreter does not execute an IF command when it
    contains an undefined symbol. Instead, the command interpreter
    issues a warning message and executes the next command in the
    procedure.

    For a summary of operators and details on how to specify
    expressions, see the OpenVMS User's Manual.

 command

    Specifies the DCL command or commands to be executed, depending
    on the syntax specified, when the result of the expression is
    true or false.

2  –  Examples

    1.$ COUNT = 0
      $ LOOP:
      $ COUNT = COUNT + 1
         .
         .
         .
      $ IF COUNT .LE. 10 THEN GOTO LOOP
      $ EXIT

      This example shows how to establish a loop in a command
      procedure, using a symbol named COUNT and an IF statement.
      The IF statement checks the value of COUNT and performs an EXIT
      command when the value of COUNT is greater than 10.

    2.$ IF P1 .EQS. "" THEN GOTO DEFAULT
      $ IF (P1 .EQS. "A") .OR. (P1 .EQS. "B") THEN GOTO 'P1'
      $ WRITE SYS$OUTPUT "Unrecognized parameter option ''P1' "
      $ EXIT
      $ A:       !  Process option a
       .
       .
       .
      $ EXIT
      $ B:       !  Process option b
       .
       .
       .
      $ EXIT
      $ DEFAULT: !  Default processing
       .
       .
       .
      $ EXIT

      This example shows a command procedure that tests whether a
      parameter was passed. The GOTO command passes control to the
      label specified as the parameter.

      If the procedure is executed with a parameter, the procedure
      uses that parameter to determine the label to branch to. For
      example:

        @TESTCOM A

      When the procedure executes, it determines that P1 is not null,
      and branches to the label A. Note that the EXIT command causes
      an exit from the procedure before the label B.

    3.$  SET NOON
       .
       .
       .
      $  LINK CYGNUS,DRACO,SERVICE/LIBRARY
      $ IF $STATUS
      $ THEN
      $  RUN CYGNUS
      $ ELSE
      $   WRITE SYS$OUTPUT "LINK FAILED"
      $ ENDIF
      $ EXIT

      This command procedure uses the SET NOON command to disable
      error checking by the command procedure. After the LINK
      command, the IF command tests the value of the reserved global
      symbol $STATUS. If the value of $STATUS indicates that the LINK
      command succeeded, then the program CYGNUS is run. If the LINK
      command returns an error status value, the command procedure
      issues a message and exits.

    4.$ if 1 .eq. 1
      $ then
      $   if 2 .eq. 2
      $   then
      $     write sys$output  "Hello!"
      $   endif
      $ endif

      This example shows how to use a nested IF structure.