ok, so i figured out why this path stuff is chaffing me.  it is analogous 
to the 'what do we put in the shebang?' question for python.

for the weewx source code, we want '#!/usr/bin/env python'

when you use python to install weewx, those get replaced with the full path 
to whatever python was actually used to install the weewx code.

so if you run weewx from a clone of the git repo, you can easily change 
which python you use by changing how '/usr/bin/env python' resolves.  this 
is really handy for switching pythons during testing, btw.  or for running 
multiple weewx instances from a single tree using different pythons.  (yes, 
these are corner cases, but very useful ones!)

the package installers retain the '/usr/bin/env python' so that the 
/etc/default/weewx file can dictate which python will be used.  that also 
lets you easily switch between the system's python or some locally 
installed python (from anaconda or brew or build-it-yourself).

anyway, as soon as you replace the shebang, you make the installation very 
brittle.  in many cases that is ok, but in many cases it is not.

i think there is a similar pattern in the use of the weewx parameters for 
where things are located.  as soon as you replace them, you have a very 
brittle installation.  that makes upgrades more difficult, and makes more 
complication for maintaining backward compatibility.

defer replacement as long as possible.

m

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