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

  - one command following the THEN keyword if the test is true
  - multiple commands following the $THEN statement if the expression is
    true
  - one or more commands following the $ELSE statement if the expression
    is false











  Format:

    $ IF  expression THEN [$] command         $ IF  expression
                                              $   THEN [command]
                                        or    $         command
                                              $            .
                                              $   [ELSE [command]
                                              $          command
                                              $            .    ]
                                              $   ENDIF

Additional information available:

ParametersRestrictionsExamples

  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
  the  execution  of  the  command.  All  character  strings  beginning
  with alphabetic characters that are  not enclosed in quotation  marks
  are assumed  to be symbol names or  lexical  functions,  and  the  IF
  command replaces them 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, you  can  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.

 command

  Defines  the action to  take if the result of the expression is true.

  The command parameter can be any valid DCL command, whether  THEN  is
  used as a keyword (after IF and not preceded by a dollar sign) in the
  "single line" IF syntax or as a verb (a DCL  command  preceded  by  a
  dollar sign) in the block-structured IF syntax.

  The following restrictions apply to the block-structured IF command:

  - IF statements can be nested to 16 levels.

  - A command  block  started by a THEN statement must be terminated by
    either an ELSE or ENDIF statement.

  - A THEN  statement must  be the first executable statement following
    an IF statement.

  - THEN,  ELSE,  and  ENDIF  statements cannot be abbreviated to fewer
    than four characters.

  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 F$MODE() .EQS. "INTERACTIVE"
       $   THEN
       $     SET PROCESS/NAME="Tom Jones"
       $     GOTO INTERACTIVE_SETUP
       $   ELSE
       $     SET PROCESS/NAME="Jones Batch"
       $     GOTO BATCH_SETUP
       $   ENDIF
       $ INTERACTIVE_SETUP:
          .
          .
          .
       $ BATCH_SETUP:
          .
          .
          .

       $ EXIT


  This example shows how the IF command executes one of  two  paths  of
  code  based  on  the  result  of  a  call  to  the  F$MODE()  lexical
  function.  The IF statement determines whether the processing mode is
  interactive.  If  the mode is interactive, the commands following the
  $THEN are executed to establish  a  process  name  for  the  session;
  control is then transfered to  a  labeled  region  where  the  user's
  interactive environment is established. If the processing mode is not
  interactive, the commands following the $ELSE are executed to set  up
  a process name and environment for the non-interactive session.