Tests the value of an expression and executes the command  following
  the THEN keyword if the test is true.

  Format:


    IF  expression THEN [$] command

Additional information available:

ParametersExamples

 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.   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.

  For  a  summary  of  operators  and  details  on  how   to   specify
  expressions, see Section 5.7 of the VAX/VMS DCL Concepts Manual.

 command

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

  You can specify any valid DCL command following  the  THEN  keyword.
  Optionally, you can precede the command with a dollar sign.

  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.   $ SET NOON
          .
          .
          .

       $ LINK CYGNUS,DRACO,SERVICE/LIBRARY
       $ IF .NOT. $STATUS THEN EXIT
       $ RUN CYGNUS

  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  LINK  command  returns  an  error  status  value,  the  command
  procedure exits.