(make.info.gz) Conditional Syntax

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 7.2 Syntax of Conditionals
 ==========================
 
 The syntax of a simple conditional with no `else' is as follows:
 
      CONDITIONAL-DIRECTIVE
      TEXT-IF-TRUE
      endif
 
 The TEXT-IF-TRUE may be any lines of text, to be considered as part of
 the makefile if the condition is true.  If the condition is false, no
 text is used instead.
 
    The syntax of a complex conditional is as follows:
 
      CONDITIONAL-DIRECTIVE
      TEXT-IF-TRUE
      else
      TEXT-IF-FALSE
      endif
 
    or:
 
      CONDITIONAL-DIRECTIVE
      TEXT-IF-ONE-IS-TRUE
      else CONDITIONAL-DIRECTIVE
      TEXT-IF-TRUE
      else
      TEXT-IF-FALSE
      endif
 
 There can be as many "`else' CONDITIONAL-DIRECTIVE" clauses as
 necessary.  Once a given condition is true, TEXT-IF-TRUE is used and no
 other clause is used; if no condition is true then TEXT-IF-FALSE is
 used.  The TEXT-IF-TRUE and TEXT-IF-FALSE can be any number of lines of
 text.
 
    The syntax of the CONDITIONAL-DIRECTIVE is the same whether the
 conditional is simple or complex; after an `else' or not.  There are
 four different directives that test different conditions.  Here is a
 table of them:
 
 `ifeq (ARG1, ARG2)'
 `ifeq 'ARG1' 'ARG2''
 `ifeq "ARG1" "ARG2"'
 `ifeq "ARG1" 'ARG2''
 `ifeq 'ARG1' "ARG2"'
      Expand all variable references in ARG1 and ARG2 and compare them.
      If they are identical, the TEXT-IF-TRUE is effective; otherwise,
      the TEXT-IF-FALSE, if any, is effective.
 
      Often you want to test if a variable has a non-empty value.  When
      the value results from complex expansions of variables and
      functions, expansions you would consider empty may actually
      contain whitespace characters and thus are not seen as empty.
      However, you can use the `strip' function ( Text Functions)
      to avoid interpreting whitespace as a non-empty value.  For
      example:
 
           ifeq ($(strip $(foo)),)
           TEXT-IF-EMPTY
           endif
 
      will evaluate TEXT-IF-EMPTY even if the expansion of `$(foo)'
      contains whitespace characters.
 
 `ifneq (ARG1, ARG2)'
 `ifneq 'ARG1' 'ARG2''
 `ifneq "ARG1" "ARG2"'
 `ifneq "ARG1" 'ARG2''
 `ifneq 'ARG1' "ARG2"'
      Expand all variable references in ARG1 and ARG2 and compare them.
      If they are different, the TEXT-IF-TRUE is effective; otherwise,
      the TEXT-IF-FALSE, if any, is effective.
 
 `ifdef VARIABLE-NAME'
      The `ifdef' form takes the _name_ of a variable as its argument,
      not a reference to a variable.  The value of that variable has a
      non-empty value, the TEXT-IF-TRUE is effective; otherwise, the
      TEXT-IF-FALSE, if any, is effective.  Variables that have never
      been defined have an empty value.  The text VARIABLE-NAME is
      expanded, so it could be a variable or function that expands to
      the name of a variable.  For example:
 
           bar = true
           foo = bar
           ifdef $(foo)
           frobozz = yes
           endif
 
      The variable reference `$(foo)' is expanded, yielding `bar', which
      is considered to be the name of a variable.  The variable `bar' is
      not expanded, but its value is examined to determine if it is
      non-empty.
 
      Note that `ifdef' only tests whether a variable has a value.  It
      does not expand the variable to see if that value is nonempty.
      Consequently, tests using `ifdef' return true for all definitions
      except those like `foo ='.  To test for an empty value, use
      `ifeq ($(foo),)'.  For example,
 
           bar =
           foo = $(bar)
           ifdef foo
           frobozz = yes
           else
           frobozz = no
           endif
 
      sets `frobozz' to `yes', while:
 
           foo =
           ifdef foo
           frobozz = yes
           else
           frobozz = no
           endif
 
      sets `frobozz' to `no'.
 
 `ifndef VARIABLE-NAME'
      If the variable VARIABLE-NAME has an empty value, the TEXT-IF-TRUE
      is effective; otherwise, the TEXT-IF-FALSE, if any, is effective.
      The rules for expansion and testing of VARIABLE-NAME are identical
      to the `ifdef' directive.
 
    Extra spaces are allowed and ignored at the beginning of the
 conditional directive line, but a tab is not allowed.  (If the line
 begins with a tab, it will be considered a command for a rule.)  Aside
 from this, extra spaces or tabs may be inserted with no effect anywhere
 except within the directive name or within an argument.  A comment
 starting with `#' may appear at the end of the line.
 
    The other two directives that play a part in a conditional are `else'
 and `endif'.  Each of these directives is written as one word, with no
 arguments.  Extra spaces are allowed and ignored at the beginning of the
 line, and spaces or tabs at the end.  A comment starting with `#' may
 appear at the end of the line.
 
    Conditionals affect which lines of the makefile `make' uses.  If the
 condition is true, `make' reads the lines of the TEXT-IF-TRUE as part
 of the makefile; if the condition is false, `make' ignores those lines
 completely.  It follows that syntactic units of the makefile, such as
 rules, may safely be split across the beginning or the end of the
 conditional.
 
    `make' evaluates conditionals when it reads a makefile.
 Consequently, you cannot use automatic variables in the tests of
 conditionals because they are not defined until commands are run (
 Automatic Variables).
 
    To prevent intolerable confusion, it is not permitted to start a
 conditional in one makefile and end it in another.  However, you may
 write an `include' directive within a conditional, provided you do not
 attempt to terminate the conditional inside the included file.
 

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(make.info.gz) Conditional Example

(make.info.gz) Conditionals

(make.info.gz) Testing Flags