Spack shell detection in setup-env.sh was originally based on
examining the executable name of $$ (from "ps"). In some cases this
does not actually give the name of the shell used, for example when
setup-env.sh was invoked from a script using "#!". To make shell
detection more robust, this adds a preliminary check for shell
variables which indicate that the shell is bash or zsh; the
executable name of $$ is retained as a fallback if those variables
are not defined.
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