[MacPorts] #24318: Document how to get the result of a program from within a Portfile (was: assign variable value in variant)

[MacPorts]

#24318: Document how to get the result of a program from within a Portfile
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 Reporter:  mattiafrancescomoro@…          |       Owner:  markd@…           
     Type:  enhancement                    |      Status:  new               
 Priority:  Normal                         |   Milestone:                    
Component:  guide                          |     Version:                    
 Keywords:                                 |        Port:                    
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Changes (by ryandesign@…):

 * cc: ryandesign@… (added)


Comment:

 You're looking for the `exec` procedure. Also, you have to wait to call it
 until pre-configure time; any time before that, python is not guaranteed
 to have been installed yet.

 Also, you must revise this to call a specific python-config that was
 provided by a specific MacPorts python port (e.g. python26, python25,
 etc.), and declare a dependency on it. It's not ok to just call "python-
 config" and have it be "whatever" version of python the user has. So, the
 following would be acceptable:

 {{{
 variant python26 description {Include Python 2.6 support} {
     depends_lib-append      port:python26
     configure.args-delete   --disable-python
     configure.args-append   --enable-python \
                             --with-python-incdir=[exec
 ${prefix}/bin/python2.6-config --includes | sed -e 's/-I//' | awk '{print
 \$1}'] \
                             --with-python-libdir=${prefix}/lib
 }
 }}}

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Ticket URL: <http://trac.macports.org/ticket/24318#comment:2>
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