[Rpm-maint] [rpm-software-management/rpm] How to expand possibly non-existent user constructed variable name? (Discussion #3411)

[Michael Schroeder]

You just need to escape the first % in the expand macro:
```
rpm -D "name string-io" -D "string_io_prerelease .dev" --eval "%{expand:%%{?%{gsub %{name} - _}_prerelease}}"
.dev
```

Without the expand:
```
$ rpm -D "name string-io" -D "string_io_prerelease .dev" --eval "%%{?%{gsub %{name} - _}_prerelease}"
%{?string_io_prerelease}
$ rpm -D "name string-io" -D "string_io_prerelease .dev" --eval "%{?%{gsub %{name} - _}_prerelease}"
$
```
As you can see, the expand macro does not get anything to expand if you do not escape the `%`.

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