When a script specifies the shell option “nounset” as part of the shebang (e.g.,
via “#!/usr/bin/env -S zsh -u”), our initialization scripts would produce error
messages of the form:
__ETC_FOO_SOURCED: parameter not set
These messages could probably be confusing to users when running such scripts.
By providing a fall-back in the parameter expansion, we can avoid them.
This patch does not address interactive shell start-up, where such messages may
(or may not) be less problematic.
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