A setup like this:

$bar = "Foo"
Cactus::terminate = "$bar"

will complain

WARNING[L1,P0] (Cactus): Major error in parameter file 'par/tov.par' line 2: Range error setting parameter 'Cactus::terminate' to 'Foo'

instead of the expected “$bar”. Double quotes should protect $ signs so that is possible to pass strings containing $ to thorns (for whatever reason).

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2333/piraha-expands-variables-inside-of-strings