(automake-1.16.info)Usage of Conditionals

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20.1 Usage of Conditionals
==========================

Before using a conditional, you must define it by using ‘AM_CONDITIONAL’
in the ‘configure.ac’ file (Note: Macros).

 -- Macro: AM_CONDITIONAL (CONDITIONAL, CONDITION)
     The conditional name, CONDITIONAL, should be a simple string
     starting with a letter and containing only letters, digits, and
     underscores.  It must be different from ‘TRUE’ and ‘FALSE’ that are
     reserved by Automake.

     The shell CONDITION (suitable for use in a shell ‘if’ statement) is
     evaluated when ‘configure’ is run.  Note that you must arrange for
     _every_ ‘AM_CONDITIONAL’ to be invoked every time ‘configure’ is
     run.  If ‘AM_CONDITIONAL’ is run conditionally (e.g., in a shell
     ‘if’ statement), then the result will confuse ‘automake’.

   Conditionals typically depend upon options that the user provides to
the ‘configure’ script.  Here is an example of how to write a
conditional that is true if the user uses the ‘--enable-debug’ option.

     AC_ARG_ENABLE([debug],
     [  --enable-debug    Turn on debugging],
     [case "${enableval}" in
       yes) debug=true ;;
       no)  debug=false ;;
       *) AC_MSG_ERROR([bad value ${enableval} for --enable-debug]) ;;
     esac],[debug=false])
     AM_CONDITIONAL([DEBUG], [test x$debug = xtrue])

   Here is an example of how to use that conditional in ‘Makefile.am’:

     if DEBUG
     DBG = debug
     else
     DBG =
     endif
     noinst_PROGRAMS = $(DBG)

   This trivial example could also be handled using ‘EXTRA_PROGRAMS’
(Note: Conditional Programs).

   You may only test a single variable in an ‘if’ statement, possibly
negated using ‘!’.  The ‘else’ statement may be omitted.  Conditionals
may be nested to any depth.  You may specify an argument to ‘else’ in
which case it must be the negation of the condition used for the current
‘if’.  Similarly you may specify the condition that is closed on the
‘endif’ line:

     if DEBUG
     DBG = debug
     else !DEBUG
     DBG =
     endif !DEBUG

Unbalanced conditions are errors.  The ‘if’, ‘else’, and ‘endif’
statements should not be indented, i.e., start on column one.

   The ‘else’ branch of the above two examples could be omitted, since
assigning the empty string to an otherwise undefined variable makes no
difference.

   In order to allow access to the condition registered by
‘AM_CONDITIONAL’ inside ‘configure.ac’, and to allow conditional
‘AC_CONFIG_FILES’, ‘AM_COND_IF’ may be used:

 -- Macro: AM_COND_IF (CONDITIONAL, [IF-TRUE], [IF-FALSE])
     If CONDITIONAL is fulfilled, execute IF-TRUE, otherwise execute
     IF-FALSE.  If either branch contains ‘AC_CONFIG_FILES’, it will
     cause ‘automake’ to output the rules for the respective files only
     for the given condition.

   ‘AM_COND_IF’ macros may be nested when m4 quotation is used properly
(Note: (autoconf)M4 Quotation).

   Here is an example of how to define a conditional config file:

     AM_CONDITIONAL([SHELL_WRAPPER], [test "x$with_wrapper" = xtrue])
     AM_COND_IF([SHELL_WRAPPER],
                [AC_CONFIG_FILES([wrapper:wrapper.in])])