[MacPorts] #24318: Document how to get the result of a program from within a Portfile

[MacPorts]

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

 In my opinion, `exec` should only be used under special circumstances. It
 is probably not a good idea not to expose it that widely in the guide.

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Ticket URL: <https://trac.macports.org/ticket/24318#comment:6>
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